Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

Everycell of the body needs nourish-

ment, yet most cells cannot leave

their position in the bodyand travel to

a food source, so the food mustbe con-

verted to a usable form and delivered.

The digestive system, with the help ofthe

circulatory system, acts like a gigantic

“meals on wheels,” providing nourishment to

over a hundred trillion “customer” cellsin the body. It

also hasits own quality control and waste disposal system.

The digestive system provides the bodywith water, electrolytes, and

other nutrients. To do this, the digestive system isspecialized to ingest food,

propelit through the digestive tract, digest it, and absorb water, electrolytes,

and other nutrients from the lumen of the gastrointestinaltract. Once these

usefulsubstances are absorbed, they are transported through the circulatory

system to cells, where theyare used. The undigested portion of the food is

moved through the digestive tractand eliminated through the anus.

Thischapter presents the general anatomyof the digestive system (860),

followed bydescriptions of the functions of the digestive system (860), the his-

tologyof the digestive tract (862), the regulation of the digestive system (863)

and the peritoneum(864). The anatomy and physiology of each section of the

digestive tractand its accessory structures are then presented: the oral cavity

(866), pharynx (870), esophagus (870), along with a section on swallowing

(872),stomach (872), small intestine (881), liver (884), gallbladder (889), pan-

creas (890), and large intestine (891). Digestion, absorption, and transport

(896) ofnutrients are then discussed, along with the effects of aging on the di-

gestive system(901).

Digestive

System

Colorized SEM of the interiorsurface of the small

intestine showing villi.

CHAPTER

Part 4 Regulationsand Maintenance

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

Anatomy of the Digestive System

Objective

■ Describe the general regionsof the digestive tract.

Thedigest ive system (ﬁgure 24.1) consists of the digestive

tract,a tube extending from the mouth to the anus, and its associ-

ated accessory organs, primarily glands, which secrete ﬂuids into

the digestive tract.The digestive tract is also called the alimentary

tract,or alimentary canal. The term gastrointestinal (gas⬘tro¯-in-

tes⬘tin-a˘l;GI) tracttechnically only refers to the stomach and in-

testines but is often used as a synonym for the digestive tract.

The regions ofthe digestive tract include

1. the mouthor oral cavity, which has salivary glands and

tonsils as accessory organs;

2. the pharynx,or throat, with tubular mucous glands;

3. the esophagus,with tubular mucous glands;

4. the stomach,which contains many tubelike glands;

5. the small intestine,consisting of the duodenum, jejunum,

and ileum,with the liver, gallbladder,and pancreas as major

accessory organs;

Part4 Regulationsand Maintenance860

6. the large intestine,including the cecum, colon, rectum, and

anal canal,with mucous glands;

7. the anus.

1. List the major regions of the digestive tract.

Functions of the Digestive System

Objective

■ Describe the processesinvolved in the functioning of the

digestive system.

The major functions of the digestive system are outlined as

follows (table 24.1):

1. Ingestionis the introduction of solid or liquid food into the

stomach.The normal route of ingestion is through the oral

cavity,but food can be introduced directly into the stomach

by a nasogastric,or stomach, tube.

2. Masticationis the process by which food taken into the

mouth is chewed by the teeth.Digestive enzymes cannot

easily penetrate solid food particles and can only work

effectively on the surfaces ofthe particles. It’s vital,

therefore,to normal digestive function that solid foods be

mechanically broken down into small particles.Mastication

breaks large food particles into many smaller particles,

which have a much larger total surface area than do a few

large particles.

3. Propulsionin the digestive tract is the movement of food

from one end ofthe digestive tract to the other. The total

time that it takes food to travel the length ofthe digestive

tract is usually about 24–36 hours.Each segment of the

digestive tract is specialized to assist in moving its contents

from the oral end to the anal end.Deglutition (de¯⬘gloo-

tish⬘u˘n),or swallowing, moves food and liquids, called a

bolus,from the oral cavity into the esophagus. Peristalsis

(per-i-stal⬘sis;ﬁgure 24.2) is responsible for moving

material through most ofthe digestive tract. Muscular

contractions occur in peristaltic(per-i-stal⬘tik) waves,

consisting ofa wave of relaxation of the circular muscles,

which forms a leading wave ofdistention in front of the

bolus,followed by a wave of strong contraction of the

Oral cavity

(mouth)

Liver

Gallbladder

Appendix

Rectum

Anus

Pharynx

(throat)

Salivary

glands

Esophagus

Stomach

Pancreas

Small

intestine

Large

intestine

Figure 24.1

The Digestive System

Wave of

relaxation

Bolus or chyme

Digestive tract

A wave of circular smooth muscle relaxation

moves ahead of the bolus of food or chyme

allowing the digestive tract to expand.

A wave of contraction of the circular smooth

muscles behind the bolus of food or chyme

propels it through the digestive tract.

Wave of

contraction

ProcessFigure 24.2

Peristalsis

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

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24. Digestive System

Companies, 2004

Chapter 24 Digestive System 861

circular muscles behind the bolus,which forces the bolus

along the digestive tube.Each peristaltic wave travels the

length ofthe esophagus in about 10 seconds. Peristaltic

waves in the small intestine usually only travel for short

distances.In some parts of the large intestine, material is

moved by mass movements,which are contractions that

extend over much larger parts ofthe digestive tract than

peristaltic movements.

4. Mixing.Some contractions don’t propel food (chyme) from

one end ofthe digestive tract to the other but rather move

the food back and forth within the digestive tract to mixit

with digestive secretions and to help break it into smaller

pieces.Segmental contractions (ﬁgure 24.3) are mixing

contractions that occur in the small intestine.

5. Secretion.As food moves through the digestive tract,

secretionsare added to lubricate, liquefy, and digest the

food.Mucus, secreted along the entire digestive tract,

lubricates the food and the lining ofthe trac t.The mucus

coats and protects the epithelial cells ofthe digestive tract

from mechanical abrasion,from the damaging effect of acid

Table 24.1

Organ Functions

Functions of the Digestive Tract

Oral cavity Ingestion. Solid food and fluids are taken into the digestive tract through the oral cavity.

Taste. Tastants dissolved in saliva stimulate taste buds in the tongue.

Mastication. Movement of the mandible bythe muscles of mastication cause the teeth to break food down into smaller pieces. The

tongue and cheeks help to place the food between the teeth.

Digestion. Amylase in saliva begins carbohydrate (starch) digestion.

Swallowing. The tongue forms food into a bolus and pushes the bolus into the pharynx.

Communication. The lips, cheeks, teeth, and tongue are involved in speech. The lips change shape as part of facial expressions.

Protection. Mucin and water in saliva provides lubrication, and lysozyme kills microorganisms.

Pharynx Swallowing. The involuntary phase of swallowing moves the bolus from the oral cavity to the esophagus. Materials are prevented

from entering the nasal cavity by the soft palate and from entering the lower respiratory tract by the epiglottis and

vestibular folds.

Breathing. Air passes from the nasal or oral cavity through the pharynx to the lower respiratory tract.

Protection. Mucus provides lubrication.

Esophagus Propulsion. Peristaltic contractions move the bolus from the pharynx to the stomach. The lower esophageal sphincter limits reflux of

the stomach contents into the esophagus.

Protection. Glands produce mucus that provides lubrication and protects the inferior esophagusfrom stomach acid.

Stomach Storage. Rugae allow the stomach to expand and hold food until it can be digested.

Digestion. Protein digestion begins as a result of the actions of hydrochloric acid and pepsin. Intrinsic factor prevents the

breakdown of vitamin B

by stomach acid.

Absorption. Except for a few substances (e.g., water, alcohol, aspirin) little absorption takes place in the stomach.

Mixing and propulsion. Mixing waves churn ingested materials and stomach secretions into chyme. Peristaltic waves move the

chyme into the small intestine.

Protection. Mucus provides lubrication and prevents digestion of the stomach wall. Stomach acid kills most microorganisms.

Small intestine Neutralization. Bicarbonate ions from the pancreas and bile from the liver neutralize stomach acid to form a pH environment

suitable for pancreatic and intestinal enzymes.

Digestion. Enzymes from the pancreas and the lining of the small intestine complete the breakdown of food molecules. Bile salts

from the liver emulsify fats.

Absorption. The circular folds, villi, and microvilli increase surface area. Most nutrients are actively or passively absorbed. Most of

the ingested water or the water in digestive tract secretions is absorbed.

Mixing and propulsion. Segmental contractions mix the chyme, and peristaltic contractions move the chyme into the large intestine.

Excretion. Bile from the liver contains bilirubin, cholestrol, fats, and fat-soluble hormones.

Protection. Mucus provides lubrication, prevents the digestion of the intestinal wall, and protects the small intestine from stomach

acid. Peyer’s patches protect against microorganisms.

Large intestine Absorption. The proximal half of the colon absorbs salts (e.g., sodium chloride), water, and vitamins (e.g., K) produced

by bacteria.

Storage. The distal half of the colon holds feces until it is eliminated.

Mixing and propulsion. Slight segmental mixing occurs. Mass movements propel feces toward the anus and defecation eliminates

the feces.

Protection. Mucus and bicarbonate ions protect against acids produced by bacteria.

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Part4 Regulationsand Maintenance862

Histology of the Digestive Tract

Objective

■ Outline the basic histologic characteristicsof the digestive

tract.

Figure 24.4 depicts a generalized view of the digestive tract

histology.The digestive tube consists of four major layers, or tu-

nics:an internal mucosa and an external serosa with a submucosa

and muscularis in between.These four tunics are present in all ar-

eas ofthe digestive tract from the esophagus to the anus. Three ma-

jor types of glands are associated with the intestinal tract:

(1) unicellular mucous glands in the mucosa, (2) multicellular

glands in the mucosa and submucosa,and (3) multicellular glands

(accessory glands) outside the digestive tract.

Mucosa

The innermost tunic,the mucosa (mu¯-ko¯⬘sa˘),consists of three lay-

ers: (1) the inner mucous epithelium, which is moist stratiﬁed

squamous epithelium in the mouth, oropharynx,esophagus, and

anal canal and simple columnar epithelium in the remainder ofthe

digestive tract;(2) a loose connective tissue called the lamina pro-

pria (lam⬘i-na˘ pro¯⬘pre¯-a˘);and (3) an outer thin smooth muscle

layer,the muscularis mucosae.

Submucosa

Thesubmucosa is a thick connective tissue layer containing nerves,

blood vessels,and small g lands that lies beneath the mucosa.The

plexus ofnerve cells in the submucosa form the submucosal plexus

(plek⬘su˘s; Meissner’s plexus), a parasympathetic ganglionic plexus

consisting ofaxons and many scattered cell bodies.

Muscularis

The next tunic is the muscularis,which consists of an inner layer

of circular smooth muscle and an outer layer of longitudinal

smooth muscle. Two exceptions are the upper esophagus,where

the muscles are striated,and the stomach, which has three layers of

smooth muscle. Another nerve plexus, the myenteric plexus

in the stomach,and from the digestive enzymes of the

digestive tract.The secretions also contain large amounts of

water,which liqueﬁes the food, thereby making it easier to

digest and absorb.Water also moves into the intestine by

osmosis.Liver secretions break large fat droplets into much

smaller droplets,which makes possible the digestion and

absorption offats. Enzymes secreted by the oral cavity,

stomach,intestine, and pancreas break large food molecules

down into smaller molecules that can be absorbed by the

intestinal wall.

6. Digestionis the breakdown of large organic molecules

intotheir component parts: carbohydrates into

monosaccharides,proteins into amino acids, and

triglycerides into fatty acids and glycerol.Digestion consists

ofmechanical digestion, which involves mastication and

mixing offood, and chemical digestion, which is

accomplished by digestive enzymes that are secreted along

the digestive tract.Digestion of large molecules into their

component parts must be accomplished before they can be

absorbed by the digestive tract.Minerals and water are not

broken down before being absorbed.Vitamins are also

absorbed without digestion and lose their function if their

structure is altered by digestion.

7. Absorptionis the movement of molecules out of the

digestive tract and into the circulation or into the lymphatic

system.The mechanism by which absorption occurs

depends on the type of molecule involved.Molecules pass

out ofthe digestive tract by simple diffusion, facilitated

diffusion,active transport, or cotransport (see chapter 3).

8. Eliminationis the process by which the waste products of

digestion are removed from the body.During this process,

occurring primarily in the large intestine,water and salts

are absorbed and change the material in the digestive tract

from a liqueﬁed state to a semisolid state.These semisolid

waste products,called feces, are then eliminated from the

digestive tract by the process ofdefecation.

2. Describe each of the processes involved in the normal

functionsof the digestive system.

1. A secretion introduced into

the digestive tract or chyme

within the tract begins in

one location.

2. Segments of the digestive

tract alternate between

contraction and relaxation.

3. Material (

brown

) in the intestine

is spread out in both directions

from the site of introduction.

4. The secretion or chyme is

spread out in the digestive

tract and becomes more

diffuse (

lighter color

)

through time.

Secretion or chyme

ProcessFigure 24.3

SegmentalContractions

Seeley−Stephens−Tate:

Anatomy and Physiology,

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IV. Regulations and

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24. Digestive System

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Chapter 24 Digestive System 863

(mı¯-en-ter⬘ik;Auerbach’s plexus),which also consists of axons and

many scattered neuron cell bodies,is between these two muscle

layers (see ﬁgure 24.4).

Together,the submucosal and myenteric plexuses constitute

the enteric plexus (en-te˘r⬘ik;relating to the intestine) or intra-

mural(in⬘tra˘-mu˘⬘ra˘l; within the walls) plexus. The enteric plexus

is extremely important in the control ofmovement and secretion.

Serosa or Adventitia

The fourth layer of the digestive tract is a connective tissue layer

called either the serosaor the adventitia (ad-ven-tish⬘a˘; foreign or

coming from outside), depending on the structure of the layer.

Parts ofthe digestive tract that protrude into the peritoneal cavity

have a serosa as the outermost layer.This serosa is called the vis-

ceral peritoneum.It consists of a thin layer of connective tissue and

a simple squamous epithelium.When the outer layer of the diges-

tive tract is derived from adjacent connective tissue,the tunic is

called the adventitia and consists of a connective tissue covering

that blends with the surrounding connective tissue.These areas in-

clude the esophagus and the retroperitoneal organs (discussed later

in relation to the peritoneum,p. 864).

3. What are the major layers of the digestive tract? How do the

serosa and adventitia differ?

4. Describe the enteric plexus. In what layers of the digestive

tractare the submucosal and myenteric plexuses found?

Regulation of the

Digestive System

Objective

■ Outline the nervous and chemical mechanismsthat

regulate the digestive system.

Elaborate nervous and chemical mechanisms regulate the

movement,secretion, absorption, and elimination processes.

Nervous Regulation ofthe Digestive System

Some of the nervous control is local,occurring as the result of lo-

cal reﬂexes within the enteric plexus, and some is more general,

mediated largely by the parasympathetic division of the ANS

through the vagus nerve.

Local neuronal control ofthe digestive tract occurs within

theenteric ner vous system (ENS). The ENS consists of the en-

teric plexus,made up of enteric neurons within the wall of the

digestive tract (see ﬁgure 24.4).There are three major types of

enteric neurons: (1) Enteric sensory neurons detect changes in

the chemical composition of the digestive tract contents or de-

tect mechanical changes such as stretch of the digestive tract

wall. (2) Enteric motor neurons stimulate or inhibit smooth

muscle contraction and glandular secretion in the digestive sys-

tem.(3) Enteric interneurons connect enteric sensory and motor

Lymphatic

nodule

Duct from

gland

Gland in

submucosa

Enteric

plexus

Myenteric

plexus

Submucosal

plexus

Blood vessels

Nerve

Mesentery

Serosa

Connective

tissue

layer

Peritoneum

Muscularis

Circular

muscle

layer

Longitudinal

muscle

layer

Mucosa

Mucous

epithelium

Lamina

propria

Muscularis

mucosae

Submucosa

Figure 24.4

Digestive TractHistology

The four tunicsare the mucosa, submucosa, muscularis, and serosa or adventitia. Glandsmay exist along the digestive tract as part of the epithelium, within the

submucosa, or aslarge glands that are outside the digestive tract.

Seeley−Stephens−Tate:

Anatomy and Physiology,

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IV. Regulations and

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24. Digestive System

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neurons.The ENS coordinates peristalsis and regulates local re-

ﬂexes, which control activities within speciﬁc, short regions of

the digestive tract.Although the enteric neurons are capable of

controlling the activities of the digestive tract independent of

the CNS,normally the two systems work together. For example,

autonomic innervation from the CNS inﬂuences the activity of

the ENS neurons.

General control of the digestive system by the CNS occurs

when reﬂexes are activated by stimuli originating in the digestive

tract.Action potentials are carried by sensory neurons in the vagus

nerves to the CNS,where the reﬂexes are integrated. In addition,

reﬂexes within the CNS may be activated by the sight,smell, or

taste offood, which stimulate the sensation of hunger. All of these

reﬂexes inﬂuence parasympathetic neurons in the CNS.Parasym-

pathetic neurons extend to the digestive tract through the vagus

nerves to control responses or alter the activity ofthe ENS and lo-

cal reﬂexes.Some sympathetic neurons inhibit muscle contraction

and secretion in the digestive system and decrease blood ﬂow to the

digestive system.

ChemicalRegulation of the Digestive System

The digestive tract produces a number ofhor mones,such as gas-

trin,secretin, and others, which are secreted by endocrine cells of

the digestive system and carried through the circulation to target

organs ofthe digestive system or to target tissues in other systems.

These hormones help regulate many gastrointestinal tract func-

tions as well as the secretions ofassociated glands such as the liver

and pancreas.

In addition to the hormones produced by the digestive sys-

tem,which enter the circulation, other paracrine chemicals,such as

histamine,are released locally within the digestive tract and inﬂu-

ence the activity of nearby cells. These localized chemical regula-

tors help local reﬂexes within the ENS control local digestive tract

environments,such as pH levels.

5. What are the nervous and chemical mechanisms that

regulate the digestive system?

Peritoneum

Objective

■ Describe the serous membranesfound in the abdominal

cavity.

The body walls and organs of the abdominal cavity are

lined with serous membranes. These membranes are very

smooth and secrete a serous ﬂuid that provides a lubricating ﬁlm

between the layers of membranes. These membranes and ﬂuid

reduce the friction as organs move within the abdomen. The

serous membrane that covers the organs is the visceral peri-

toneum(per⬘ i-to¯-ne¯⬘u¯m; to stretch over), and the one that cov-

ers the interior surface of the body wall is the parietal

peritoneum(ﬁgure 24.5).

Part4 Regulationsand Maintenance864

Peritonitis

Peritonitisis the inﬂammation of the peritoneal membranes. This

inﬂammation mayresult from chemical irritation bysubstances such as

bile thathave escaped from a damaged digestive tract; or it mayresult

from infection, again originating in the digestive tract, such aswhen the

appendixruptures. Peritonitis can be life-threatening. An accumulation of

excessserous ﬂuid in the peritoneal cavityis called ascites (a˘-sı¯⬘te¯z).

Ascitesmay accompany peritonitis, starvation, alcoholism, or liver cancer.

Connective tissue sheets called mesenteries (mes⬘ en-

ter⬘e¯z;middle intestine) hold many of the organs in place within

the abdominal cavity. The mesenteries consist of two layers of

serous membranes with a thin layer ofloose connective tissue be-

tween them. They provide a route by which vessels and nerves

can pass from the body wall to the organs.Other abdominal or-

gans lie against the abdominal wall,have no mesenteries, and are

referred to as retroperitoneal (re⬘tro¯-per⬘i-to¯-ne¯⬘a˘l; behind the

peritoneum; see chapter 1). The retroperitoneal organs include

the duodenum,the pancreas, the ascending colon,the descending

colon, the rectum,the kidneys, the adrenal glands, and the uri-

nary bladder.

Some mesenteries are given speciﬁc names.The mesenter y

connecting the lesser curvature of the stomach and the proximal

end of the duodenum to the liver and diaphragm is called the

lesser omentum(o¯-men⬘tu˘m; membrane of the bowels), and the

mesentery extending as a fold from the greater curvature and then

to the transverse colon is called the greater omentum (see ﬁgure

24.5).The greater omentum forms a long, double fold of mesen-

tery that extends inferiorly from the stomach over the surface of

the small intestine. Because of this folding, a cavity,or pocket,

called the omental bursa (ber⬘sa˘; pocket) is formed between the

two layers ofmesentery. A large amount of fat accumulates in the

greater omentum, and it is sometimes referred to as the “fatty

apron.”The greater omentum has considerable mobility in the

abdomen.

PREDICT

Ifyou placed a pin through the greater omentum, through how many

layersof simple squamous epithelium would the pin pass?

Thecoronary ligament attaches the liver to the diaphragm.

Unlike other mesenteries,the coronary ligament has a wide space

in the center,the bare area of the liver,where no peritoneum exists.

Thefalciform ligament attaches the liver to the anterior abdomi-

nal wall (see ﬁgure 24.5).

Although the term mesentery is a general term referring to

the serous membranes attached to the abdominal organs,it is also

used speciﬁcally to refer to the mesentery associated with the small

intestine,sometimes called the mesentery proper. The mesenter-

ies of parts of the colon are the transverse mesocolon, which ex-

tends from the transverse colon to the posterior body wall,and the

sigmoid mesocolon. The vermiform appendix even has its own

little mesentery called the mesoappendix.

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Anatomy and Physiology,

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Chapter 24 Digestive System 865

Rectum (retroperitoneal)

Mesentery proper

Transverse colon

Transverse mesocolon

Duodenum (retroperitoneal)

Pancreas (retroperitoneal)

Stomach

Lesser omentum

Liver

Coronary ligament

Urinary bladder

(retroperitoneal)

Small intestine

Omental bursa

Greater omentum

Parietal peritoneum

Visceral peritoneum

Falciform

ligament

Liver

Greater

omentum

Stomach

Liver

Transverse

colon

Small

intestine

Gallbladder

Figure 24.5

Peritoneum and Mesenteries

(a) Sagittalsection through the trunk showing the peritoneum and mesenteries associated with some abdominalorgans. (b) Photograph of the abdomen of a

cadaver with the greater omentum in place. (c) Photograph ofthe abdomen of a cadaver with the greater omentum removed to revealthe underlying viscera.

(a)

(b) (c)

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24. Digestive System

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6. Where are visceral peritoneum and parietal peritoneum

found? Whatis a retroperitoneal organ?

7. Deﬁne the term mesentery. Name and describe the location

of the mesenteriesfound in the abdominal cavity.

Oral Cavity

Objective

■ List and describe the major structuresand secretions of the

oral cavity.

Theoral cavity (ﬁgure 24.6), or mouth, is that part of the di-

gestive tract bounded by the lips anteriorly,the fauces (faw⬘se¯z;

throat;opening into the pharynx) posteriorly, the cheeks laterally,

the palate superiorly,and a muscular ﬂoor inferiorly.The or al cav-

ity is divided into two regions: (1) the vestibule(ves⬘ti-bool; en-

try),which is the space between the lips or cheeks and the alveolar

processes,which contain the teeth; and (2) the oral cavity proper,

which lies medial to the alveolar processes.The oral cavity is lined

with moist stratiﬁed squamous epithelium,which provides protec-

tion against abrasion.

Lipsand Cheeks

The lips, or labia (la¯⬘be¯-a˘) (see ﬁgure 24.6),are muscular struc-

tures formed mostly by the orbicularis oris (o¯r-bik⬘u¯-la¯⬘ris o¯r⬘is)

muscle (see ﬁgure 10.9a), as well as connective tissue. The outer

surfaces of the lips are covered by skin.The keratinized stratiﬁed

epithelium ofthe skin is thin at the margin of the lips and is not as

Part4 Regulationsand Maintenance866

highly keratinized as the epithelium of the surrounding skin (see

chapter 5); consequently,it is more transparent than the epithe-

lium over the rest ofthe body.The color from the underlying blood

vessels can be seen through the relatively transparent epithelium,

giving the lips a reddish pink to dark red appearance,depending on

the overlying pigment.At the internal margin of the lips, the ep-

ithelium is continuous with the moist stratiﬁed squamous epithe-

lium ofthe mucosa in the oral cavity.

One or more frenula (fren⬘u¯-la˘; bridle), which are mucosal

folds,extend from the alveolar processes of the maxilla to the upper

lip and from the alveolar process ofthe mandible to the lower lip.

The cheeks form the lateral walls of the oral cavit y.They

consist of an interior lining of moist stratiﬁed squamous epithe-

lium and an exterior covering ofskin. The substance of the cheek

includes the buccinator muscle (see chapter 10), which ﬂattens

the cheek against the teeth,and the buccal fat pad, which rounds

out the proﬁle on the side ofthe face.

The lips and cheeks are important in the processes ofmasti-

cation and speech.The y help manipulate food within the mouth

and hold it in place while the teeth crush or tear it.They also help

form words during the speech process.A large number of the mus-

cles offacial expression are involved in movement of the lips. They

are listed in chapter 10.

Palate and Palatine Tonsils

The palate (see ﬁgure 24.6) consists of two parts, an anterior

bony part,the hard palate (see chapter 7), and a posterior, non-

bony part,the soft palate, which consists of skeletal muscle and

Superior vestibule

Hard palate

Soft palate

Uvula

Cheek

Molars

Premolars

Canine

Incisors

Inferior vestibule

Upper lip (labium)

Frenulum of upper lip

Gingiva covering the

maxillary alveolar

process

Fauces

Palatine tonsil

Tongue

Frenulum of tongue

Openings of

submandibular ducts

Gingiva covering the

mandibular alveolar

process

Frenulum of lower lip

Lower lip (labium)

Figure 24.6

OralCavity

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Chapter 24 Digestive System 867

Teeth

Normal adults have 32 teeth,which are distributed in two dental

arches.One is called the maxillary arch and the other is called the

mandibular arch.The teeth in the right and left halves of each den-

tal arch are roughly mirror images of each other.As a result, the

teeth are divided into four quadrants:right upper, left upper, right

lower,and left lower. The teeth in each quadrant include one cen-

tral and one lateral incisor,one canine, ﬁrst and second premo-

lars, and ﬁrst, second, and third molars (ﬁgure 24.7a). The third

molars are called wisdom teeth because they usually appear in a

person’s late teens or early twenties,when the person is old enough

to have acquired some wisdom.

Impacted Wisdom Teeth

In some people with smalldental arches, the third molars maynot have

room to eruptinto the oral cavity and remain embedded within the jaw.

Embedded wisdom teeth are referred to asimpacted, and their surgical

removalis often necessary.

The teeth ofthe adult mouth are permanent, or secondary,

teeth.Most of them are replacements for primar y, or deciduous

(de¯-sid⬘u¯-u˘s;those that fall out; also called milk teeth), teeth that

are lost during childhood (ﬁgure 24.7b).The deciduous teeth erupt

(the crowns appear within the oral cavity) between about 6months

and 24 months of age (see ﬁgure 24.7b).The permanent teeth be-

gin replacing the deciduous teeth by about 5 years and the process

is completed by about 11 years.

Each tooth consists of a crown with one or more cusps

(points), a neck, and a root (ﬁgure 24.8). The clinical crown is

that part of the tooth exposed in the oral cavity.The anatomical

crownis the entire enamel-covered part of the tooth. The center of

the tooth is a pulp cavity,which is ﬁlled with blood vessels, nerves,

and connective tissue called pulp.The pulp cavity within the root

is called the root canal.The ner ves and blood vessels ofthe tooth

enter and exit the pulp through a hole at the point of each root

called the apical foramen.The pulp cavity is surrounded by a liv-

ing, cellular,and calciﬁed tissue called dentin. The dentin of the

tooth crown is covered by an extremely hard,nonliving, acellular

substance called enamel,which protects the tooth against abrasion

and acids produced by bacteria in the mouth.The surface of the

dentin in the root is covered with a cellular,bonelike substance,

calledcementum, which helps anchor the tooth in the jaw.

The teeth are set in alveoli (al-ve¯⬘o¯-lı¯; sockets) along the

alveolar processes ofthe mandible and maxilla. Dense ﬁbrous con-

nective tissue and stratiﬁed squamous epithelium, referred to as

the gingiva (jin⬘ji-va˘;gums) cover the alveolar processes (see ﬁg-

ure 24.6). Periodontal (per⬘e¯-o¯-don⬘ta˘l; around a tooth) liga-

mentssecure the teeth in the alveoli.

The teeth play an important role in mastication and a role in

speech.

8. Distinguish between the vestibule and the oral cavity

proper.

9. What are the functions of the lips and cheeks? What muscle

formsthe substance of the cheek?

10. What are the hard and soft palate? Where is the uvula

found?

connective tissue.The uvula (u¯⬘vu¯-la˘; a grape) is the projection

from the posterior edge ofthe soft palate. The palate is important

in the swallowing process;it prevents food from passing into the

nasal cavity.

Palatine tonsilsare located in the lateral wall of the fauces

(see chapter 22).

Tongue

The tongue is a large, muscular organ that occupies most of the

oral cavity proper when the mouth is closed.Its major attachment

in the oral cavity is through its posterior part.The anterior part of

the tongue is relatively free and is attached to the ﬂoor of the

mouth by a thin fold oftissue called the frenulum. The muscles as-

sociated with the tongue are divided into two categories:intrinsic

muscles, which are within the tongue itself; and extrinsic mus-

cles,which are outside the tongue but attached to it. The intrinsic

muscles are largely responsible for changing the shape of the

tongue,such as ﬂattening and elevating the tongue during drinking

and swallowing.The extrinsic tongue muscles protrude and retract

the tongue, move it from side to side,and change its shape (see

chapter 10).

Tongue-Tied

A person is“tongue-tied” in a more literal sense if the frenulum extends

too far toward the tip ofthe tongue, thereby inhibiting normal movement

ofthe tongue and interfering with normal speech. Surgically cutting the

frenulum can correctthe condition.

A groove called the terminal sulcusdivides the tongue into

two parts. The part anterior to the terminal sulcus accounts for

about two-thirds of the surface area and is covered by papillae,

some of which contain taste buds (see chapter 15).The posterior

one-third of the tongue is devoid of papillae and has only a few

scattered taste buds.It has, instead, a few small glands and a large

amount of lymphoid tissue, the lingual tonsil (see chapter 22).

Moist stratiﬁed squamous epithelium covers the tongue.

Lipid-Soluble Drugs

Certain drugsthat are lipid-soluble and can diffuse through the plasma

membranesof the oral cavity can be quicklyabsorbed into the

circulation. An example isnitroglycerin, which is a vasodilator used to

treatcases of angina pectoris. The drug isplaced under the tongue,

where, in lessthan 1 minute, it dissolves and passes through the very

thin oralmucosa into the lingual veins.

The tongue moves food in the mouth and,in cooperation

with the lips and gums,holds the food in place during mastication.

It also plays a major role in the mechanism of swallowing (dis-

cussed on p.872). It is a major sensory organ for taste (see chapter

15) and one ofthe primar y organs of speech.

Glossectomyand Speech

Patientswho have undergone glossectomies (tongue removal) as a result

ofglossal carcinoma can compensate for lossof the tongue’s function in

speech, and theycan learn to speak fairlywell. These patients, however,

have substantialdifﬁculty chewing and swallowing food.

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Part4 Regulationsand Maintenance868

11. List the functions of the tongue. Distinguish between

intrinsicand extrinsic tongue muscles.

12. What are deciduous and permanent teeth? Name the

differentkinds of teeth.

13. Describe the parts of a tooth. What are dentin, enamel,

cementum, and pulp?

DentalDiseases

Dentalcaries, or tooth decay, is caused bya breakdown of enamel by

acidsproduced by bacteria on the tooth surface. Because the enamelis

nonliving and cannotrepair itself, a dental ﬁlling is necessaryto prevent

further damage. Ifthe decay reaches the pulp cavity with itsrich supply

ofnerves, toothache pain may result. In some cases in which decayhas

reached the pulp cavity, itmaybe necessary to perform a dental

procedure called a “rootcanal,” which consistsof removing the pulp

from the tooth.

Periodontaldisease isthe inﬂammation and degradation of the

periodontalligaments, gingiva, and alveolar bone. This disease isthe

mostcommon cause of tooth loss in adults. Gingivitis (jin-ji-vı¯⬘tis) isan

inﬂammation ofthe gingiva, often caused by food deposited in gingival

crevicesand not promptly removed by brushing and ﬂossing. Gingivitis

mayeventually lead to periodontal disease. Pyorrhea(pı¯-o¯-re¯⬘a˘) isa

condition in which pusoccurs with periodontaldisease. Halitosis (hal-i-

to¯⬘sis), or bad breath, often occurswith periodontal disease and pyorrhea.

Mastication

Food taken into the mouth is chewed,or masticated, by the teeth.

The anterior teeth,the incisors, and the canines primarily cut and

tear food,whereas the premolars and molars primarily crush and

grind it.Mastication breaks large food particles into smaller ones,

which have a much larger total surface area.Because digestive en-

zymes digest food molecules only at the surface of the particles,

mastication increases the efﬁciency ofdigestion.

Four pairs ofmuscles move the mandible during mastication:

the temporalis, masseter, medial pterygoid, and lateral ptery-

goid(see chapter 10 and ﬁgure 10.9). The temporalis, masseter,and

medial pterygoid muscles close the jaw; and the lateral pterygoid

muscle opens it.The medial and lateral pterygoids and the masseter

muscles accomplish protraction and lateral and medial excursion of

Central incisor

Lateral incisor

Canine

First premolar

First molar

Second molar

Third molar

(wisdom tooth)

Second premolar

Central incisor

(erupts at 6–8 months;

lost at 5–7 years)

Lateral incisor

(erupts at 8–11 months;

lost at 6–8 years)

Canine

(erupts at 16–20 months;

lost at 8–11 years)

First molar

(erupts at 10–16 months;

lost at 9–11 years)

Second molar

(erupts at 20–24 months;

lost at 9–11 years)

Cusp

Enamel

Gingiva

Dentin

Pulp cavity

with nerves

and vessels

Periodontal

ligaments

Root canal

Cementum

Alveolar bone

Clinical

crown

Neck

Root

Anatomical

crown

Apical foramen

Figure 24.7

Teeth

(a) Permanentteeth. (b) Deciduous teeth.

(a)

(b)

Figure 24.8

Molar Tooth in Place in the Alveolar Bone

The tooth consistsof a crown and root. The root is covered with cementum,

and the tooth isheld in the socket by periodontal ligaments. Nerves and

vesselsenter and exit the tooth through the apical foramen.

Seeley−Stephens−Tate:

Anatomy and Physiology,

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IV. Regulations and

Maintenance

24. Digestive System

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Chapter 24 Digestive System 869

ithelium of the tongue (lingual glands), palate (palatine glands),

cheeks (buccal glands), and lips (labial glands). The secretions

from these glands help keep the oral cavity moist and begin the

process ofdigestion.

All ofthe major large salivary glands are compound alveolar

glands,which are branching glands with clusters of alveoli resem-

bling grapes (see chapter 4).The y produce thin serous secretions

or thicker mucous secretions. Thus,saliva is a combination of

serous and mucous secretions from the various salivary glands.

The largest salivary glands,the parotid (pa˘-rot⬘id;beside the

ear)glands, are serous glands, which produce mostly watery saliva,

and are located just anterior to the ear on each side of the head.

Each parotid duct exits the gland on its anterior margin, crosses

the lateral surface of the masseter muscle, pierces the buccinator

muscle,and enters the oral cavity adjacent to the second upper mo-

lar (see ﬁgure 24.9).

Saliva and the Second Molar

Because the parotid secretionsare released directly onto the surface of

the second upper molar, ittends to have a considerable accumulation of

mineral, secreted from the gland, on itssurface.

the jaw.The temporalis retracts the jaw.All these movements are in-

volved in tearing,crushing, and grinding food.

Thechewing, or mastication, reﬂex, which is integrated in

the medulla oblongata,controls the basic movements involved in

chewing.The presence of food in the mouth stimulates sensory re-

ceptors,which activate a reﬂex that causes the muscles of mastica-

tion to relax.The muscles are stretched as the mandible is lowered,

and stretch ofthe muscles activates a reﬂex that causes contraction

of the muscles of mastication. Once the mouth is closed,the food

again stimulates the muscles ofmastication to relax, and the cycle

is repeated.Descending pathways from the cerebrum strongly in-

ﬂuence the activity ofthe mastication reﬂex so that chewing can be

initiated or stopped consciously.The rate and intensity of chewing

movements can also be inﬂuenced by the cerebrum.

Salivary Glands

A considerable number of salivary glands are scattered through-

out the oral cavity.Three pairs of large multicellular glands exist:

the parotid,the submandibular, and the sublingual glands (ﬁgure

24.9).In addition to these large consolidations of glandular tissue,

numerous small,coiled tubular glands are located deep to the ep-

Duct

epithelium

Salivary

duct

Serous

alveolus

Serous cell

Mucous cell

Mucous

alveolus

Mixed

alveoli

Salivary

duct

Serous

alveoli

Masseter

muscle

Parotid

gland

Buccinator

muscle

Ducts of the

sublingual gland

Sublingual

gland

Submandibular

gland

Parotid duct

Mucous membrane

(cut)

Submandibular

duct

150x

Figure 24.9

SalivaryGlands

(a) The large salivaryglands are the parotid glands, the submandibular

glands, and the sublingualglands. The parotid duct extends anteriorly

from the parotid gland. (b) An idealized schematicdrawing of the

histologyof the large salivary glands. The ﬁgure is representative of all

the glandsand does not depict any one salivary gland.

(c)Photomicrograph of the parotid gland.

(a)

(b)

(c)

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Mumps

Inﬂammation ofthe parotid gland is called parotiditis. Mumps,which is

caused bya virus, is the most common type of parotiditis.

The submandibular (below the mandible) glands are

mixed glands with more serous than mucous alveoli.Each gland

can be felt as a soft lump along the inferior border ofthe posterior

half of the mandible. A submandibular duct exits each gland,

passes anteriorly deep to the mucous membrane on the ﬂoor ofthe

oral cavity,and opens into the oral cavity beside the frenulum of

the tongue (see ﬁgure 24.6). In certain people, if the mouth is

opened and the tip of the tongue is elevated, the submandibular

ducts are compressed and saliva may squirt out ofthe mouth from

the openings ofthese ducts.

Thesublingual (below the tongue) glands, the smallest of the

three large,paired salivary glands, are mixed glands containing some

serous alveoli but consisting primarily ofmucous alveoli. They lie im-

mediately below the mucous membrane in the ﬂoor of the mouth.

These glands do not have single,well-deﬁned ducts like those of the

submandibular and parotid glands. Instead,each sublingual g land

opens into the ﬂoor ofthe oral cavity through 10–12 small ducts.

Salivais secreted at the rate of about 1–1.5 L/day. The serous

part of saliva,produced mainly by the parotid and submandibular

glands, contains a digestive enzyme called salivary amylase

(am⬘il-a¯s; starch-splitting enzyme), which breaks the covalent

bonds between glucose molecules in starch and other polysaccha-

rides to produce the disaccharides maltose and isomaltose (tables

24.2 and 24.4).The release of maltose and isomaltose gives starches

a sweet taste in the mouth. Food spends very little time in the

mouth,however; therefore, only about 3%–5% of the total carbo-

hydrates are digested in the mouth.Most of the starches are cov-

ered by cellulose in plant tissues and are inaccessible to salivary

amylase.Cooking and thorough chewing of food destroy the cellu-

lose covering and increase the efﬁciency ofthe digestive process.

Saliva prevents bacterial infection in the mouth by washing

the oral cavity.Saliva also contains substances, such as lysozyme,

which has a weak antibacterial action, and immunoglobulin A,

which helps prevent bacterial infection.Any lack of salivary gland

secretion increases the chance of ulceration and infection of the

oral mucosa and ofcaries in the teeth.

The mucous secretions of the submandibular and sublingual

glands contain a large amount of mucin (mu¯⬘sin), a proteoglycan

that gives a lubricating quality to the secretions ofthe salivary glands.

Salivary gland secretion is stimulated by the parasympathetic

and sympathetic nervous systems, with the parasympathetic sys-

tem being more important. Salivary nuclei in the brainstem in-

crease salivary secretions by sending action potentials through

parasympathetic ﬁbers of the facial (VII) and glossopharyngeal

(IX) cranial nerves in response to a variety of stimuli,such as tac-

tile stimulation in the oral cavity or certain tastes,especially sour.

Higher centers of the brain also affect the activity of the salivary

glands. Odors that trigger thoughts of food or the sensation of

hunger can increase salivary secretions.

Part4 Regulationsand Maintenance870

14. List the muscles of mastication and the actions they

produce. Describe the mastication reﬂex.

15. Name and give the location of the three largest salivary

glands. Name the otherkinds of salivary glands.

16. What substances are contained in saliva?

17. What is the difference between serous and mucous saliva?

Pharynx

Objective

■ Describe the anatomy of the pharynxand esophagus.

Thepharynx was described in detail in chapter 23; thus, only

a brief description is provided here.The phar ynx consists of three

parts: the nasopharynx,the orophar ynx,and the lar yngopharynx.

Normally, only the oropharynx and laryngopharynx transmit

food.The oropharynx communicates with the nasopharynx supe-

riorly,the larynx and lar yngopharynx inferiorly, and the mouth

anteriorly. The laryngopharynx extends from the oropharynx to

the esophagus and is posterior to the larynx.The posterior walls of

the oropharynx and laryngopharynx consist of three muscles: the

superior,middle, and inferior pharyngeal constrictors, which are

arranged like three stacked ﬂowerpots,one inside the other. The

oropharynx and the laryngopharynx are lined with moist stratiﬁed

squamous epithelium, and the nasopharynx is lined with ciliated

pseudostratiﬁed columnar epithelium.

18. Name the three parts of the pharynx. What are the

pharyngeal constrictors?

PREDICT

Explain the functionalsigniﬁcance of the differencesin epithelial

typesamong the three pharyngeal regions.

Esophagus

Objective

■ Describe the esophagus, its layersand sphincters.

Theesophagus is that part of the digestive tube that extends

between the pharynx and the stomach.It is about 25 cm long and

lies in the mediastinum,anterior to the vertebrae and posterior to

the trachea. It passes through the esophageal hiatus (opening) of

the diaphragm and ends at the stomach.The esophagus transports

food from the pharynx to the stomach.

HiatalHernia

Ahiatal hernia is a widening of the esophageal hiatus. Hiatalhernias

occur mostcommonly in adults and allow part of the stomach to extend

through the opening into the thorax. The hernia can decrease the resting

pressure in the lower esophagealsphincter, allowing gastroesophageal

reﬂuxand subsequent esophagitis to occur. Hiatalherniation can also

compressthe blood vessels in the stomach mucosa, which can lead to

gastritisor ulcer formation. Esophagitis, gastritis, and ulceration can be

verypainful.

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Chapter 24 Digestive System 871

Table 24.2

Fluid or Enzyme Function

Functions of Major Digestive Secretions

Saliva

Serous (watery) Moistens food and mucous membrane; lysozyme kills bacteria

Salivary amylase Starch digestion (conversion to maltose and isomaltose)

Mucus Lubricates food; protects gastrointestinal tract from digestion by enzymes

Esophagus

Mucus Lubricates the esophagus; protects the esophagus lining from abrasion and allows food to move more

smoothly through the esophagus

Gastric Secretions

Hydrochloric acid Decreases stomach pH to activate pepsinogen

Pepsinogen Pepsin, the active form of pepsinogen, digests protein into smaller peptide chains

Mucus Protects stomach lining from digestion

Liver

Bile Bile salts emulsify fats, making them available to intestinal lipases; help make end products

Sodium glycocholate (bile salt) soluble and available for absorption by the intestinal mucosa; aid peristalsis. Many

Sodium taurocholate (bile salt) of the other bile contents are waste products transported to the intestine for disposal.

Cholesterol

Biliverdin

Bilirubin

Mucus

Fat

Lecithin

Cells and cell debris

Pancreas

Trypsin Digests proteins (breaks polypeptide chains at arginine or lysine residues)

Chymotrypsin Digests proteins (cleaves carboxyl links of hydrophobic amino acids)

Carboxypeptidase Digests proteins (removes amino acids from the carboxyl end of peptide chains)

Pancreatic amylase Digests carbohydrates (hydrolyzes starches and glycogen to form maltose and isomaltose)

Pancreatic lipase Digests fat (hydrolyzes fats—mostly triacylglycerols—into glycerol and fatty acids)

Ribonuclease Digests ribonucleic acid

Deoxyribonuclease Digests deoxyribonucleic acid (hydrolyzes phosphodiester bonds)

Cholesterol esterase Hydrolyzes cholesterol esters to form cholesterol and free fatty acids

Bicarbonate ions Providesappropriate pH for pancreatic enzymes

SmallIntestine Secretions

Mucus Protects duodenum from stomach acid, gastric enzymes, and intestinal enzymes; provides adhesion for

fecal matter; protects intestinal wall from bacterial action and acid produced in the feces

Aminopeptidase Splits polypeptidesinto amino acids (from amino end of chain)

Peptidase Splits amino acids from polypeptides

Enterokinase Activates trypsin from trypsinogen

Amylase Digests carbohydrates

Sucrase Splits sucrose into glucose and fructose

Maltase Splits maltose into two glucose molecules

Isomaltase Splits isomaltose into two glucose molecules

Lactase Splits lactose into glucose and galactose

Lipase Splits fats into glycerol and fatty acids

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The esophagus has thick walls consisting ofthe four tunics

common to the digestive tract:mucosa, submucosa, muscularis,

and adventitia. The muscular tunic has an outer longitudinal

layer and an inner circular layer,as is true of most parts of the di-

gestive tract,but it’s different because it consists of skeletal mus-

cle in the superior part of the esophagus and smooth muscle in

the inferior part. An upper esophageal sphincter and a lower

esophageal sphincter,at the upper and lower ends of the esoph-

agus,respectively, regulate the movement of materials into and

out of the esophagus. The mucosal lining of the esophagus is

moist stratified squamous epithelium. Numerous mucous

glands in the submucosal layer produce a thick,lubricating mu-

cus that passes through ducts to the surface of the esophageal

mucosa.

19. Where is the esophagus located? Describe the layers of the

esophageal wall and the esophageal sphincters.

Swallowing

Objective

■ Describe the process of swallowing.

Swallowing,or de glutition, is divided into three separate

phases: voluntary,phar yngeal,and esophageal. During the vol-

untary phase (ﬁgure 24.10a), a bolus of food is formed in the

mouth and pushed by the tongue against the hard palate,forcing

the bolus toward the posterior part of the mouth and into the

oropharynx.

Thephar yngeal phase (ﬁgure 24.10b–d) of swallowing is

a reﬂex that is initiated by stimulation oftactile receptors in the

area ofthe oropharynx. Afferent action potentials travel through

the trigeminal (V) and glossopharyngeal (IX) nerves to the

swallowing centerin the medulla oblongata. There, they initiate

action potentials in motor neurons, which pass through the

trigeminal (V), glossopharyngeal (IX), vagus (X),and accessory

(XI) nerves to the soft palate and pharynx. This phase of swal-

lowing begins with the elevation of the soft palate, which closes

the passage between the nasopharynx and oropharynx. The

pharynx elevates to receive the bolus of food from the mouth

and moves the bolus down the pharynx into the esophagus.The

superior,middle, and inferior pharyngeal constrictor muscles

contract in succession,forcing the food through the pharynx. At

the same time, the upper esophageal sphincter relaxes,the ele-

vated pharynx opens the esophagus,and food is pushed into the

esophagus.This phase of swallowing is unconscious and is con-

trolled automatically, even though the muscles involved are

skeletal. The pharyngeal phase of swallowing lasts about 1–2

seconds.

PREDICT

Whyis it important to close the opening between the nasopharynx

and oropharynxduring swallowing? What mayhappen if a person has

an explosive burstof laughter while trying to swallow a liquid?

Part4 Regulationsand Maintenance872

During the pharyngeal phase,the vestibular folds are moved

medially,the epiglottis (ep-i-glot⬘is; on the glottis) is tipped pos-

teriorly so that the epiglottic cartilage covers the opening into the

larynx,and the lar ynx is elevated.These movements of the lar ynx

prevent food from passing through the opening into the larynx.

PREDICT

Whathappens if you try to swallow and speakat the same time?

The esophageal phase (ﬁgure 24.10e) of swallowing takes

about 5–8 seconds and is responsible for moving food from the

pharynx to the stomach.Muscular contractions in the wall of the

esophagus occur in peristaltic waves.

The peristaltic waves associated with swallowing cause relax-

ation of the lower esophageal sphincter in the esophagus as the

peristaltic waves, and bolus of food, approach the stomach.This

sphincter is not anatomically distinct from the rest ofthe esopha-

gus,but it can be identiﬁed physiologically because it remains ton-

ically constricted to prevent the reﬂux ofstomach contents into the

lower part ofthe esophagus.

The presence offood in the esophagus stimulates the enteric

plexus,which controls the peristaltic waves. The presence of food in

the esophagus also stimulates tactile receptors,which send afferent

impulses to the medulla oblongata through the vagus nerves.Motor

impulses,in turn, pass along the vagal efferent ﬁbers to the striated

and smooth muscles within the esophagus, thereby stimulating

their contractions and reinforcing the peristaltic contractions.

Swallowing and Gravity

Gravityassists the movement of materialthrough the esophagus,

especiallywhen liquids are swallowed. The peristaltic contractionsthat

move materialthrough the esophagus are sufﬁciently forceful, however,

to allow a person to swallow, even while doing a headstand or ﬂoating in

the zero-gravityenvironment of space.

20. What are the three phases of deglutition? List sequentially

the processesinvolved in the last two phases, and describe

howthey are regulated.

Stomach

Objectives

■ List the anatomicand histologic characteristics of the

stomach thatare most important to its function.

■ Describe the stomach secretionsand their functions during

the cephalic, gastric, and intestinal phasesof stomach

secretion regulation.

■ Describe gastric ﬁlling, mixing, and emptying, and explain

theirregulation.

Thestomach is an enlarged segment of the digestive tract in

the left superior part of the abdomen (see ﬁgure 24.1). Its shape

and size vary from person to person;even within the same individ-

ual its size and shape change from time to time,depending on its

food content and the posture ofthe body. Nonetheless,several gen-

eral anatomic features can be described.

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Chapter 24 Digestive System 873

Tongue NasopharynxHard palate Soft palate

Superior pharyngeal

constrictor

Middle pharyngeal

constrictor

Inferior pharyngeal

constrictor

Upper esophageal

sphincter

Esophagus

Epiglottis

Soft palate

Epiglottis

Opening of larynx

Oropharynx

blue arrows

)

forces the bolus through the pharynx and into the esophagus. As this occurs, the epiglottis is

bent down over the opening of the larynx largely by the force of the bolus pressing against it.

(e) During the esophageal phase, the bolus is

moved by peristaltic contractions of the

esophagus toward the stomach (inwardly

directed

blue arrows

(d)3–4. As the inferior pharyngeal constrictor

contracts, the upper esophageal sphincter

relaxes (outwardly directed

blue arrows

allowing the bolus to enter the esophagus.

Bolus

Larynx

(a) During the voluntary phase, a bolus of food

(

yellow

) is pushed by the tongue against

the hard and soft palates and posteriorly

toward the oropharynx (

blue arrow

indicates tongue movement;

black arrow

indicates movement of the bolus).

Tan

bone,

purple

:cartilage,

red

: muscle.

(b)1. During the pharyngeal phase, the soft

palate is elevated, closing off the

nasopharynx. 2. The pharynx is elevated

(

blue arrows

indicate muscle movement).

ProcessFigure 24.10

Three Phasesof Swallowing (Deglutition)

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Part4 Regulationsand Maintenance874

Secretionsof the Stomach

Ingested food and stomach secretions, mixed together,form a

semiﬂuid material called chyme (kı¯m;juice). The stomach func-

tions primarily as a storage and mixing chamber for the chyme.Al-

though some digestion and absorption occur in the stomach,they

are not its major functions.

Stomach secretions include mucus, hydrochloric acid,gas-

trin,histamine, intrinsic factor, and pepsinogen. Pepsinogen is the

inactive form ofthe protein-digesting enzyme pepsin.

The surface mucous cells and mucous neck cells secrete a vis-

cous and alkaline mucus that covers the surface of the epithelial

cells and forms a layer 1–1.5 mm thick.The thick layer of mucus

lubricates and protects the epithelial cells ofthe stomach wall from

the damaging effect of the acidic chyme and pepsin. Irritation of

the stomach mucosa results in stimulation of the secretion of a

greater volume ofmucus.

Parietal cells in the gastric glands of the pyloric region se-

crete intrinsic factor and a concentrated solution of hydrochloric

acid.Intrinsic factor is a glycoprotein that binds with vitamin B

and makes the vitamin more readily absorbed in the ileum.Vita-

min B

is important in deoxyribonucleic acid (DNA) synthesis.

Hydrochloric acid produces the low pH of the stomach,

which is normally between 1 and 3.Although the hydrochloric acid

secreted into the stomach has a minor digestive effect on ingested

food,one of its main functions is to kill bacteria that are ingested

with essentially everything humans put into their mouths. Some

pathogenic bacteria may avoid digestion in the stomach,however,

because they have an outer coat that resists stomach acids.

The low pH of the stomach also stops carbohydrate digestion

by inactivating salivary amylase.Stomach acid also denatures many

proteins so that proteolytic enzymes can reach internal peptide bonds,

and it provides the proper pH environment for the function ofpepsin.

Hydrogen ions are derived from carbon dioxide and water,

which enter the parietal cell from its serosal surface,which is the

side opposite the lumen ofthe gastric pit (ﬁgure 24.12). Once inside

the cell,carbonic anhydrase catalyzes the reaction between carbon

dioxide and water to form carbonic acid.Some of the carbonic acid

molecules then dissociate to form hydrogen ions and bicarbonate

ions.The hydrogen ions are actively transported across the mucosal

surface of the parietal cell into the lumen of the stomach; some

potassium ions are moved into the cell in exchange for the hydrogen

ions. Although hydrogen ions are actively transported against a

steep concentration gradient,chloride ions diffuse with the hydro-

gen ions from the cell through the plasma membrane.Diffusion of

chloride ions with the positively charged hydrogen ions reduces the

amount of energy needed to transport the hydrogen ions against

both a concentration gradient and an electrical gradient.Bicarbon-

ate ions move down their concentration gradient from the parietal

cell into the extracellular ﬂuid. During this process,bicarbonate

ions are exchanged for chloride ions through an anion exchange

molecule,which is located in the plasma membrane, and the chlo-

ride ions subsequently move into the cell.

Anatomyof the Stomach

The opening from the esophagus into the stomach is the gastro-

esophageal,or cardiac (located near the heart), opening, and the

region of the stomach around the cardiac opening is the cardiac

region (ﬁgure 24.11). The lower esophageal sphincter,also called

thecardiac sphincter, surrounds the cardiac opening. Recall that

although this is an important structure in the normal function of

the stomach, it is a physiologic constrictor only and cannot be

seen anatomically.A part of the stomach to the left of the cardiac

region, the fundus (fu˘n⬘du˘ s; the bottom of a round-bottomed

leather bottle), is actually superior to the cardiac opening. The

largest part of the stomach is the body,which turns to the right,

thus creating a greater curvature and a lesser curvature. The

body narrows to form the pyloric(pı¯-lo¯r⬘ik; gatekeeper) region,

which joins the small intestine.The opening between the stomach

and the small intestine is the pyloric opening, which is sur-

rounded by a relatively thick ring ofsmooth muscle called the py-

loric sphincter.

HypertrophicPyloric Stenosis

Hypertrophicpyloric stenosis is a common defect ofthe stomach in

infants, occurring in 1 in 150 malesand 1 in 750 females, in which the

pylorusis greatly thickened, resulting in interference with normal

stomach emptying. Infantswith this defect exhibitprojectile vomiting.

Because the pylorusis blocked, little food entersthe intestine, and the

infantfails to gain weight. Constipation isalso a frequent complication.

Histology of the Stomach

The serosa, or visceral peritoneum, is the outermost layer of the

stomach.It consists of an inner layer of connective tissue and an

outer layer ofsimple squamous epithelium. The muscularis of the

stomach consists ofthree layers: an outer longitudinal layer,a mid-

dle circular layer, and an inner oblique layer (ﬁgure 24.11a).

Insome areas of the stomach, such as in the fundus, the three lay-

ers blend with one another and cannot be separated.Deep to the

muscular layer are the submucosa and the mucosa, which are

thrown into large folds called rugae(roo⬘ge¯;wrinkles) when the

stomach is empty.These folds allow the mucosa and submucosa to

stretch,and the folds disappear as the stomach volume increases as

it is ﬁlled.

The stomach is lined with simple columnar epithelium.The

epithelium forms numerous tubelike gastric pits, which are the

openings for the gastric glands (ﬁgure 24.11b). The epithelial cells

ofthe stomach are of ﬁve types. The ﬁrst type, surface mucous cells,

which produce mucus,is on the surface and lines the gastric pit. The

remaining four cell types are in the gastric glands.They are mucous

neck cells, which produce mucus; parietal (oxyntic) cells, which

produce hydrochloric acid and intrinsic factor;chief (zymogenic)

cells,which produce pepsinogen; and endocrine cells, which pro-

duce regulatory hormones.The mucous neck cells are located near

the openings ofthe glands; whereas the parietal, chief, and endocrine

cells are interspersed in the deeper parts ofthe glands.

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IV. Regulations and

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24. Digestive System

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Chapter 24 Digestive System 875

Fundus

Body

Serosa

Longitudinal muscle layer

Circular muscle layer

Oblique muscle layer

Submucosa

Mucosa

Muscularis

Location of lower

esophageal sphincter

Rugae

Esophagus

Gastroesophageal opening

Cardiac region

Pyloric sphincter

Pyloric region

Pyloric opening

Duodenum

Surface mucous

cells

Mucous

neck cells

Gastric

glands

Gastric

pit

Parietal

cells

Chief

cells

Endocrine

cells

Muscularis

mucosae

Blood vessels

Oblique muscle

layer

Circular muscle

layer

Longitudinal

muscle layer

Connective

tissue layer

Visceral

peritoneum

Serosa

Muscularis

Submucosa

Mucosa

Lamina

propria

Gastric pit

Surface

mucous

cell

Mucous

neck cell

LM 30x

Figure 24.11

Anatomyand Histology of the Stomach

(a) Cutawaysection reveals muscular layers and internal anatomy. (b) A section ofthe stomach wall that illustrates its histology, including several gastric pitsand

glands. (c) Photomicrograph ofgastric glands.

(a)

(b) (c)

Seeley−Stephens−Tate:

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IV. Regulations and

Maintenance

24. Digestive System

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PREDICT

Explain whya slight increase in the blood pH may occur following a

heavymeal. The elevated pH of blood, especiallyin the veins that carry

blood awayfrom the stomach, is called “the postentericalkaline tide.”

Chief cells within the gastric glands secrete pepsinogen

(pep-sin⬘o¯-jen). Pepsinogen is packaged in zymogen (zı¯-mo¯-jen;

related to enzymes) granules, which are released by exocytosis

when pepsinogen secretion is stimulated.Once pepsinogen enters

the lumen of the stomach, hydrochloric acid and previously

formed pepsin molecules convert it to pepsin.Pepsin exhibits op-

timum enzymatic activity at a pH of 3 or less.Pepsin catalyzes the

cleavage of some covalent bonds in proteins,thus breaking them

into smaller peptide chains.

Heartburn

Heartburn,or pyrosis (pı¯-ro¯ ⬘sis), is a painful or burning sensation in the

chestusually associated with reﬂux ofacidic chyme into the esophagus.

The pain isusually short-lived but may be confused with the pain of an

ulcer or a heartattack. Overeating, eating fatty foods, lying down

immediatelyafter a meal, consuming too much alcohol or caffeine,

smoking, or wearing extremelytight clothing can all cause heartburn. A

hiatalhernia can also cause heartburn, especially in older people.

Regulation of Stomach Secretion

Approximately 2–3 L ofgastr ic secretions (gastric juice) are pro-

duced each day.The amount and type of food entering the stom-

ach dramatically affects the secretion amount,but up to 700 mL is

secreted as a result ofa t ypical meal.Both ner vous and hormonal

mechanisms regulate gastric secretions. The neural mechanisms

involve reﬂexes integrated within the medulla oblongata and local

reﬂexes integrated within the enteric plexus of the GI tract. In

Part4 Regulationsand Maintenance876

addition,higher brain centers inﬂuence the reﬂexes. Chemical sig-

nals that regulate stomach secretions include the hormones gas-

trin,secretin, gastric-inhibitory polypeptide, and cholecystokinin,

as well as the paracrine chemical signal histamine (table 24.3).

Regulation ofstomach secretion is divided into three phases:

cephalic,gastric, and intestinal.

1. Cephalic phase. In the cephalic phase of gastric regulation, the

sensations ofthe taste and smell of food, stimulation of tactile

receptors during the process ofchewing and swallowing, and

pleasant thoughts offood stimulate centers within the medulla

oblongata that inﬂuence gastric secretions (ﬁgure 24.13a).

Action potentials are sent from the medulla along

parasympathetic neurons within the vagus (X) nerves to the

stomach.Within the stomach wall,the preganglionic neurons

stimulate postganglionic neurons in the enteric plexus.The

postganglionic neurons,which are primarily cholinergic,

stimulate secretory activity in the cells ofthe stomach mucosa.

Parasympathetic stimulation ofthe stomach mucosa

results in the release ofthe neurotransmitter acetylcholine,

which increases the secretory activity ofboth the parietal and

chiefcells and stimulates the secretion of gastrin (gas⬘trin)

and histamine from endocrine cells.Gastrin is released into

the circulation and travels to the parietal cells,where it

stimulates additional hydrochloric acid and pepsinogen

secretion.In addition, gastrin stimulates endocrine cells to

release histamine,which stimulates parietal cells to secrete

hydrochloric acid.The histamine receptors on the parietal

cells are called H

receptors,and are different from the H

receptors involved in allergic reactions.Drugs that block

allergic reactions do not affect histamine-mediated stomach

acid secretion and vice versa.Acetylcholine, histamine,and

1. Carbon dioxide (CO

) diffuses

into the cell.

is combined with water

O) in an enzymatic reaction

that is catalyzed by carbonic

anhydrase (CA) to form

carbonic acid (H

3. Carbonic acid dissociates

into a bicarbonate ion (HCO

–

)

and a hydrogen ion (H

4.HCO is transported back into

the bloodstream. An anion

exchange molecule in the

plasma membrane exchanges

HCO

5.The hydrogen ion (H

actively transported into the

duct of the gastric gland.

6. Chloride ions (CI

the charged hydrogen ions.

7. Some potassium ions (K

counter transported into the cell

in exchange for the hydrogen

ions.

–

for a chloride ion (Cl

–

)

) is

–

) diffuse with

) are

Parietal cell

+ H

–

HCO

–

Blood

vessel

ATP

ADP

Serosal

surface

To stomach

–

HCO

–

Duct of

gastric

gland

(counter transport).

ProcessFigure 24.12

HydrochloricAcid Production by Parietal Cellsin the Gastric Glands of the Stomach

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Chapter 24 Digestive System 877

gastrin working together cause a greater secretion of

hydrochloric acid than any ofthem does separately. Of the

three,histamine has the greatest stimulatory effect.

Inhibitorsof Gastric Acid Secretion

Cimetidine (Tagamet) and ranitidine (Zantac) are syntheticanalogsof

histamine thatcan bind to H

histamine receptorson parietal cells, and

preventhistamine binding, without stimulating the cell. These chemicals

are called histamine blockersand are extremelyeffective inhibitors of

gastricacid secretion. Cimetidine, one of the most commonly prescribed

drugs, isused to treat cases of gastric acid hypersecretion associated

with gastritisand gastric ulcers.

2. Gastric phase. The greatest volume of gastric secretions is

produced during the gastric phase ofgastr ic regulation.The

presence offood in the stomach initiates the gastric phase

(ﬁgure 24.13b).The primary stimuli are distention of the

stomach and the presence ofamino acids and peptides in

the stomach.

Distention ofthe stomach wall, especially in the body or

fundus,results in the stimulation of mechanoreceptors.

Action potentials generated by these receptors initiate

reﬂexes that involve both the CNS and enteric reﬂexes,

resulting in secretion ofmucus, hydrochloric acid,

pepsinogen,intrinsic factor, and gastrin. The presence of

partially digested proteins or moderate amounts ofalcohol

or caffeine in the stomach also stimulates gastrin secretion.

When the pH ofthe stomach contents falls below 2,

increased gastric secretion produced by distention ofthe

stomach is blocked.This negative-feedback mechanism

limits the secretion ofgastric juice.

Amino acids and peptides released by the digestive action

ofpepsin on proteins directly stimulate parietal cells of the

stomach to secrete hydrochloric acid.The mechanism by

which this response is mediated is not clearly understood.It

doesn’t involve known neurotransmitters,and,when the pH

drops below 2,the response is inhibited. Histamine also

stimulates the secretory activity ofparietal cells.

3. Intestinal phase. The entrance of acidic stomach contents

into the duodenum ofthe small intestine controls the

intestinal phase ofgastric regulation (ﬁgure 24.13c). The

presence ofchyme in the duodenum activates both neural

and hormonal mechanisms.When the pH of the chyme

entering the duodenum drops to 2 or below,or if the

chyme contains fat digestion products,gastric secretions

are inhibited.

Acidic solutions in the duodenum cause the release of

the hormone secretin(se-kre¯⬘tin) into the circulatory

system.Secretin inhibits gastric secretion by inhibiting both

parietal and chiefcells. Acidic solutions also initiate a local

enteric reﬂex,which inhibits gastric secretions.

Fatty acids and certain other lipids in the duodenum

and the proximal jejunum initiate the release oftwo

hormones:gastric inhibitor y polypeptide and

cholecystokinin(ko¯⬘le¯-sis-to¯-kı¯⬘nin).Gastr ic inhibitory

polypeptidestrongly inhibits gastr ic secretion,and

cholecystokinin inhibits gastric secretions to a lesser

degree.Hypertonic solutions in the duodenum and

jejunum also inhibit gastric secretions.The mechanism

appears to involve the secretion ofa hormone referred to as

enterogastrone(en⬘te r-o¯-gas⬘tro¯n), but the actual

existence ofthis hormone has never been established.

Table 24.3

Site of Production Method of Stimulation Secretory Effects Motility Effects

Functions of the GastrointestinalHormones

Gastrin

Stomach and Distention; partially digested Increases gastric secretion Increases gastric emptying

duodenum proteins, autonomic stimulation, by increasing stomach

ingestion of alcohol or caffeine motility and relaxing the

pyloric sphincter

Secretin

Duodenum Acidity of chyme Inhibits gastric secretion; Decreases gastric motility

stimulates pancreatic secretions high in

bicarbonate ions; increases the rate of bile

and increases intestinal

secretion; mucus secretion

Cholecystokinin

Intestine Fatty acids and other lipids Slightly inhibits gastric secretion; Decreases gastric motility

stimulates pancreatic secretions high in

digestive enzymes; and causes contraction

of the gallbladder and relaxation of the

hepatopancreatic ampullar sphincter

Gastric Inhibitory Polypeptide

Duodenum and proximal jejunum Fatty acids and other lipids Inhibits gastric secretions Decreases gastric motility

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Part4 Regulationsand Maintenance878

Medulla oblongata

Vagus nerves

Gastrin

Secretions

stimulated

Circulation

Taste or smell of food

Tactile sensation in mouth

1. The taste or smell of food, tactile sensations of food

in the mouth, or even thoughts of food stimulate the

medulla oblongata (

green

arrow

2. Parasympathetic action potentials are carried by the

vagus nerves to the stomach (

pink arrow

3. Preganglionic parasympathetic vagus nerve fibers

stimulate postganglionic neurons in the enteric

plexus of the stomach.

4. Postganglionic neurons stimulate secretion by

parietal and chief cells and stimulate gastrin

secretion by endocrine cells.

5. Gastrin is carried through the circulation back to the

stomach (

purple arrow

), where it stimulates

secretion by parietal and chief cells.

Cephalic Phase

Medulla

oblongata

Stomach

Secretions

stimulated

Distention

Local reflexes

stimulated by

stomach distention

Vagus nerves

1. Distention of the stomach activates a

parasympathetic reflex. Action potentials are carried

by the vagus nerves to the medulla oblongata

(

green arrow

2. The medulla oblongata stimulates stomach

secretions (

pink arrow

3. Distention of the stomach also activates local

reflexes that increase stomach secretions (

purple

arrow

Gastric Phase

Medulla oblongata

Vagus

nerves

Decreased

gastric

secretions

Circulation

Secretin, gastric inhibitory

polypeptide, cholecystokinin

1. Chyme in the duodenum with a pH less than 2 or

containing fat digestion products (lipids) inhibits

gastric secretions by three mechanisms (2–4).

2. Sensory vagal action potentials to the medulla

oblongata (

green arrow

) inhibit motor action

potentials from the medulla oblongata (

pink arrow

3. Local reflexes inhibit gastric secretion (

orange

arrows

4. Secretin, gastric inhibitory polypeptide, and

cholecystokinin produced by the duodenum (

brown

arrows

) inhibit gastric secretions in the stomach.

pH<2

or lipids

Vagus

nerves

Intestinal Phase

Local

reflexes

Stomach

ProcessFigure 24.13

The Three Phasesof Gastric

Secretion

(a) Cephalicphase. (b) Gastric phase. (c) Intestinalphase.

(a)

(b)

(c)

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Clinical Focus PepticUlcer

Approximately10% of the U.S. population

willdevelop peptic ulcers during their life-

time. Mostcases of peptic ulcer are appar-

ently due to the infection of a speciﬁc

bacterium, Helicobacter pylori. It’s also

thought that the bacterium is involved in

manycases of gastritis and gastric cancer.

Conventionalwisdom has focused for years

on the notion thatstress, diet, smoking, or

alcohol cause excessacid secretion in the

stomach, resulting in ulcers.

Antacidsremain very popular in treat-

ing ulcers, aswell as for the relief of tempo-

rary stomach problems. Close to $1 billion

is spent on antacids in the United States

annually. Antacid therapydoes relieve the

ulcer in most cases. A 50% incidence of

relapse occurs within 6 months with

antacid treatment, and a 95% incidence of

relapse occursafter 2 years. On the other

hand, studies using antibiotic therapy in

addition to bismuth and ranitidine have

demonstrated a 95% eradication ofgastric

ulcersand 74% healing of duodenal ulcers

within 2 months. Dramaticallyreduced re-

lapse rates have also been obtained. One

such studyreported a recurrence rate of 8%

following antibiotictherapy, compared with

a recurrence rate of86% in controls.

Other treatments include H

receptor

antagonists, which bind histamine recep-

tors and prevent histamine-stimulated HCl

secretion. Proton pump inhibitors directly

inhibit HCl secretion. Prostaglandins are

naturallyproduced by the mucosa of the GI

tractand help the mucosa resist injury. Syn-

thetic prostaglandins can supplementthis

resistance aswell as inhibit HCl secretion.

the infection rate from h. Pylori in the

united states population is about 1% per

year ofage: 30% of people that are 30 years

old have the bacterium, and 80% of those

age 80 are infected. In Third World countries,

asmany as 100% of people age 25 or older

are infected. This may relate to the high

rates of stomach cancer in some ofthose

countries. We stillhave much to learn before

we can understand thisbacterium. Verylittle

isknown concerning how people become in-

fected. Also, with such high ratesof infec-

tion, it’snot known why only a small fraction

ofthose infected actually develop ulcers. It

maybe that factors such as diet and stress

predispose a person who isinfected by the

bacterium to actuallydevelop an ulcer.

Peptic ulcer isclassically viewed as a

condition in which the stomach acidsand

pepsin digestthe mucosal lining of the GI

tractitself. The most common site of a pep-

ticulcer is near the pylorus, usually on the

duodenalside (i.e., a duodenal ulcer; 80%

ofpeptic ulcers are duodenal). Ulcers occur

lessfrequently along the lesser curvature of

the stomach or at the point at which the

esophagus enters the stomach. The most

common presumed cause ofpeptic ulcers is

the oversecretion ofgastric juice relative to

the degree ofmucous and alkaline protec-

tion ofthe small intestine. One reason that

bacterial involvement in ulcers was dis-

missed for such a long time isthat is was

assumed that the extreme acid environ-

mentkilled all bacteria. Apparently not only

can H. pylori survive in such an environ-

ment, butit may even thrive there.

People experiencing severe anxietyfor

a long time are the mostprone to develop

duodenal ulcers. They often have a high

rate of gastric secretion (as much as 15

timesthe normal amount) between meals.

Thissecretion results in highly acidic chyme

entering the duodenum. The duodenum is

usually protected by sodium bicarbonate

(secreted mainly by the pancreas), which

neutralizesthe chyme. When large amounts

of acid enter the duodenum, however, the

sodium bicarbonate isnot adequate to neu-

tralize it. The acid tendsto reduce the mu-

cousprotection of the duodenum, perhaps

leaving thatpart of the digestive tract open

to the action ofH. pylori, which may further

destroythe mucous lining.

In one study, itwas determined that ul-

cer patients prefer their hot drinks extra

hot, 62⬚Ccompared with 56⬚C for a control

group without ulcers. The high tempera-

tures of the drinks maycause thinning of

the mucous lining of the stomach, thus

making these people more susceptible to

ulcers, again perhaps by increasing their

sensitivityto H. pylori invasion.

In some patientswith gastric ulcers, of-

ten normal or even low levels of gastrichy-

drochloricacid secretion exist. The stomach

hasa reduced resistance to itsown acid, how-

ever. Such inhibited resistance can result

from excessive ingestion ofalcoholor aspirin.

Reﬂux of duodenal contentsinto the

pyloruscan also cause gastric ulcers. In this

case, bile, which ispresent in the reﬂux,

has a detergenteffect that reduces gastric

mucosalresistance to acid and bacteria.

An ulcer may become perforated

(ahole in the stomach or duodenum), caus-

ing peritonitis. The perforation mustbe cor-

rected surgically. Selective vagotomy,

cutting branchesof the vagus (X) nerve go-

ing to the stomach, is sometimes per-

formed atthe time of surgery to reduce acid

production in the stomach.

Chapter 24 Digestive System 879

Inhibition ofgastric secretions is also under nervous

control.Distention of the duodenal wall, the presence of

irritating substances in the duodenum,reduced pH, and

hypertonic or hypotonic solutions in the duodenum

activate the enterogastric reﬂex.The enterogastric reﬂex

consists ofa local reﬂex and a reﬂex integrated within the

medulla oblongata.It reduces gastric secretions.

Movementsof the Stomach

Stomach Filling

As food enters the stomach,the rugae ﬂatten, and the stomach vol-

ume increases.Despite the increase in volume, the pressure within

the stomach doesn’t increase until the volume nears maximum ca-

pacity because smooth muscle can stretch without an increase in

tension (see chapter 9) and because of a reﬂex integrated within

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Anatomy and Physiology,

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IV. Regulations and

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24. Digestive System

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the medulla oblongata.This reﬂex inhibits muscle tone in the body

ofthe stomach.

Mixing of Stomach Contents

Ingested food is thoroughly mixed with the secretions ofthe stom-

ach glands to form chyme.This mixing is accomplished by gentle

mixing waves, which are peristaltic-like contractions that occur

about every 20 seconds and proceed from the body toward the py-

loric sphincter to mix the ingested material with the secretions of

the stomach. Peristaltic wavesoccur less frequently, are signiﬁ-

cantly more powerful than mixing waves,and force the chyme near

the periphery of the stomach toward the pyloric sphincter. The

more solid material near the center ofthe stomach is pushed supe-

riorly toward the cardiac region for further digestion (ﬁgure

Part4 Regulationsand Maintenance880

24.14). Roughly 80% of the contractions are mixing waves, and

20% are peristaltic waves.

Stomach Emptying

The amount of time food remains in the stomach depends on a

number offactors, including the type and volume of food. Liquids

exit the stomach within 1

–2

⁄

hours after ingestion.After a typ-

ical meal,the stomach is usually empty within 3–4 hours. The py-

loric sphincter usually remains partially closed because of mild

tonic contraction.Each peristaltic contraction is sufﬁciently strong

to force a small amount ofchyme through the pyloric opening and

into the duodenum. The peristaltic contractions responsible for

movement of chyme through the partially closed pyloric opening

are called the pyloric pump.

Mixing waves initiated in the body

of the stomach progress toward

the pyloric region (

pink arrows

directed inward

The more fluid part of the chyme

is pushed toward the pyloric

region (

blue arrows

), whereas the

more solid center of the chyme

squeezes past the peristaltic

constriction back toward the body

of the stomach (

orange arrow

Additional mixing waves (

purple

arrows

) move in the same

direction and in the same way as

the earlier waves (1) that reach

the pyloric region.

Again, the more fluid part of the

chyme is pushed toward the

pyloric region (

blue arrows

whereas the more solid center of

the chyme squeezes past the

peristaltic constriction back

toward the body of the stomach

(

orange arrow

Some of the most fluid chyme is

squeezed through the pyloric

opening into the duodenum

(

small blue arrows

), whereas

most of the chyme is forced back

toward the body of the stomach for

further mixing (

orange arrows

Esophagus

Body of

stomach

Chyme

First wave

Second wave

Pyloric

sphincter

First

wave

Pyloric

region

Duodenum

More fluid

chyme

More solid

chyme

ProcessFigure 24.14

Movementsin the Stomach

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Chapter 24 Digestive System 881

Hunger Contractions

Hunger contractionsare peristaltic contractionsthat approach tetanic

contractionsfor periods of about 2–3 minutes. Low blood glucose levels

cause the contractionsto increase and become sufﬁciently strong to

create uncomfortable sensationscalled “hunger pangs.” Hunger pangs

usuallybegin 12–24 hours after the previous meal or in less time for

some people. Ifnothing is ingested, they reach their maximum intensity

within 3–4 daysand then become progressively weaker.

Regulation of Stomach Emptying

If the stomach empties too fast, the efﬁciency of digestion and ab-

sorption is reduced,and acidic gastric contents dumped into the duo-

denum may damage its lining.If the rate of emptying is too slow, the

highly acidic contents of the stomach may damage the stomach wall

and reduce the rate at which nutrients are digested and absorbed.

Stomach emptying is regulated to prevent these two extremes.Some

of the hormonal and neural mechanisms that stimulate stomach se-

cretions also are involved with increasing stomach motility.For exam-

ple,during the gast ric phase of stomach secretion,distention of the

stomach stimulates local reﬂexes,CNS reﬂexes,and the release of gas-

trin,all of which increase stomach motility and cause relaxation of the

pyloric sphincter.The result is an increase in stomach emptying.Con-

versely,some of the hormonal and neural mechanisms that decrease

gastric secretions also inhibit gastric motility,increase constriction of

the pyloric sphincter,and decrease the rate of stomach emptying.

Vomiting

Vomitingcan result from irritation (e.g., overdistention or overexcitation)

anywhere along the GI tract. Action potentialstravel through the vagus

nerve and spinalvisceral afferent nerves to the vomiting center in the

medulla oblongata. Once the vomiting center isstimulated and the reﬂex

isinitiated, the following events occur: (1) a deep breath is taken; (2) the

hyoid bone and larynxare elevated, opening the upper esophageal

sphincter; (3) the opening ofthe larynx is closed; (4) the soft palate is

elevated, closing the posterior nares; (5) the diaphragm and abdominal

musclesare forcefully contracted, strongly compressing the stomach and

increasing the intragastricpressure; (6) the lower esophageal sphincter

isrelaxed; and (7) the gastric contents are forced outof the stomach,

through the esophagusand oral cavity, to the outside.

21. Describe the parts of the stomach. List the layers of the

stomach wall. Howis the stomach different from the

esophagus?

22. What are gastric pits and gastric glands? Name the different

cell typesin the stomach and the secretions they produce.

23. Describe the three phases of regulation of stomach

secretion, and discussthe cause and result of each phase.

24. How are gastric secretions inhibited? Why is this inhibition

necessary?

25. Why does pressure in the stomach not greatly increase as

the stomach ﬁlls?

26. What are two kinds of stomach movements? How are stomach

movementsregulated by hormones and nervous control?

Small Intestine

Objectives

■ Describe the anatomy of the small intestine.

■ List the secretionsof the small intestine, and explain how

secretion and movementare regulated.

The small intestine consists of three parts: the duodenum,

the jejunum,and the ileum (ﬁgure 24.15). The entire small intes-

tine is about 6 m long (range: 4.6–9 m).The duodenum is about

25 cm long (the term duodenum means 12, suggesting that it is

12inches long). The jejunum, constituting about two-ﬁfths of the

total length of the small intestine, is about 2.5 m long; and the

ileum, constituting three-ﬁfths of the small intestine, is about

3.5m long. Two major accessory glands,the liver and the pancreas,

are associated with the duodenum.

The small intestine is the site at which the greatest amount of

digestion and absorption occur.Each day,about 9 L of water enters

the digestive system.It comes from water that is ingested and from

ﬂuid secretions produced by glands along the length ofthe diges-

tive tract.Most of the water, 8–8.5 L, moves by osmosis, with the

absorbed solutes,out of the small intestine. A small part, 0.5–1 L,

enters the colon.

Anatomyof the Small Intestine

Duodenum

The duodenum nearly completes a 180-degree arc as it curves

within the abdominal cavity (ﬁgure 24.16), and the head of the

pancreas lies within this arc. The duodenum begins with a short

superior part, which is where it exits the pylorus of the stomach,

and ends in a sharp bend,which is where it joins the jejunum.

Stomach

Jejunum

Ileum

Appendix

Duodenum

Ascending

colon

Mesentery

Ileocecal

junction

Cecum

Figure 24.15

The SmallIntestine

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Two small mounds are within the duodenum about two-

thirds of the way down the descending part: the major duodenal

papillaand the lesser duodenal papilla. At the major papilla, the

common bile duct and pancreatic duct join to form the he-

patopancreatic ampulla (Vater’s ampulla),which empties into

the duodenum.A smooth muscle sphincter,the hepatopancreat ic

ampullar sphincter (sphincter of Oddi) regulates the opening of

the ampulla.An accessory pancreatic duct, present in most people,

opens at the tip ofthe lesser duodenal papilla.

The surface ofthe duodenum has several modiﬁcations that

increase its surface area about 600-fold to allow for more efﬁcient

digestion and absorption of food. The mucosa and submucosa

form a series of folds called the circular folds, or plicae (plı¯⬘se¯;

folds) circulares (ﬁgure 24.17a), which run perpendicular to the

long axis of the digestive tract. Tiny ﬁngerlike projections ofthe

mucosa form numerous villi(vil⬘ı¯;shaggy hair),which are 0.5–1.5

mm in length (ﬁgure 24.17b). Each villus is covered by simple

columnar epithelium and contains a blood capillary network and a

lymphatic capillary called a lacteal(lak⬘te¯-a˘l) (ﬁgure 24.17c). Most

ofthe cells that make up the surface of the villi have numerous cy-

Part4 Regulationsand Maintenance882

toplasmic extensions (about 1 ␮m long) called microvilli,which

further increase the surface area (ﬁgure 24.17d).The combined mi-

crovilli on the entire epithelial surface form the brush border.

These various modiﬁcations greatly increase the surface area ofthe

small intestine and,as a result, greatly enhance absorption.

The mucosa of the duodenum is simple columnar epithe-

lium with four major cell types: (1) absorptive cellsare cells with

microvilli,which produce digestive enzymes and absorb digested

food; (2) goblet cells, which produce a protective mucus; (3)

granular cells (Paneth’s cells),which may help protect the intes-

tinal epithelium from bacteria; and (4) endocrine cells, which

produce regulatory hormones. The epithelial cells are produced

within tubular invaginations of the mucosa, called intestinal

glands(cry pts of Lieberkühn), at the base of the villi. The absorp-

tive and goblet cells migrate from the intestinal glands to cover the

surface ofthe villi and e ventually are shed from its tip.The granu-

lar and endocrine cells remain in the bottom of the glands. The

submucosa of the duodenum contains coiled tubular mucous

glands called duodenal glands (Brunner’s glands), which open

into the base ofthe intestinal glands.

Common bile duct

Pancreatic duct

Body of pancreas

Tail of pancreas

Jejunum

Pancreatic

islet

Beta cells

(secrete insulin)

Lobule

Acini cells

(secrete enzymes)

Accessory

pancreatic

duct

Duodenum

Minor

duodenal

papilla

Hepato-

pancreatic

ampulla

Major

duodenal

papilla

Circular

folds

Head of

pancreas

Alpha cells

(secrete glucagon)

Interlobular duct

Intercalated duct

Intralobular duct

pancreatic

duct

bloodstream

Interlobular duct

Vein

Figure 24.16

Anatomyand Histology of the Duodenum and Pancreas

(a) The head ofthe pancreas lies within the duodenal curvature, with the pancreaticduct emptying into the duodenum. (b) Histology of the pancreas showing both

the acini and the pancreaticduct system.

(a)

(b)

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Jejunum and Ileum

Thejejunum and ileum are similar in structure to the duodenum

(see ﬁgure 24.15),except that a gradual decrease occurs in the di-

ameter of the small intestine, the thickness of the intestinal wall,

the number of circular folds,and the number of villi as one pro-

gresses through the small intestine.The duodenum and je junum

are the major sites ofnutrient absor ption,although some absor p-

tion occurs in the ileum.Lymph nodules called Peyer’s patchesare

numerous in the mucosa and submucosa ofthe ileum.

The junction between the ileum and the large intestine is the

ileocecal junction. It has a ring of smooth muscle, the ileocecal

sphincter,and a one-way ileocecal valve (see ﬁgure 24.24).

Secretionsof the Small Intestine

The mucosa ofthe small intestine produces secretions that prima-

rily contain mucus,electrolytes, and water.Intestinal secretions lu-

bricate and protect the intestinal wall from the acidic chyme and

the action of digestive enzymes. They also keep the chyme in the

Villus

Circular folds

Epithelium

Submucosa

Circular muscle

Longitudinal muscle

Serosa

Villi

Blood capillary

network

Lacteal

Epithelium

Top of

circular fold

Duodenal

gland

Microvilli

Epithelial cell

Lacteal

(lymph)

Capillary

(blood)

Intestinal

gland

Microvilli of

epithelial cell

surface

Epithelial cell

20,000x

Figure 24.17

Anatomyand Histology of the Duodenum

(a) Wallof the duodenum, showing the circular folds. (b) The villi on a circular fold. (c) A single villus, showing the lactealand capillary network. (d) Transmission

electron micrograph ofmicrovilli on the surface of a villus.

(a)

(b)

(c)

(d)

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small intestine in a liquid form to facilitate the digestive process

(see table 24.2).The intestinal mucosa produces most of the secre-

tions that enter the small intestine,but the secretions of the liver

and the pancreas also enter the small intestine and play essential

roles in the process ofdigestion. Most of the digestive enzymes en-

tering the small intestine are secreted by the pancreas.The intes-

tinal mucosa also produces enzymes,but these remain associated

with the intestinal epithelial surface.

The duodenal glands, intestinal glands, and goblet cells se-

crete large amounts ofmucus. This mucus provides the wall of the

intestine with protection against the irritating effects of acidic

chyme and against the digestive enzymes that enter the duodenum

from the pancreas.Secretin and cholecystokinin are released from

the intestinal mucosa and stimulate hepatic and pancreatic secre-

tions (see ﬁgures 24.21 and 24.23).

The vagus nerve, secretin,and chemical or tactile irritation

of the duodenal mucosa stimulate secretion from the duodenal

glands.Goblet cells produce mucus in response to the tactile and

chemical stimulation ofthe mucosa.

DuodenalUlcer

Sympatheticnerve stimulation inhibits duodenal gland secretion, thus

reducing the coating ofmucus on the duodenal wall, which protectsit

againstacid and gastric enzymes. If a person is highly stressed, elevated

sympatheticactivity may therefore inhibit duodenal gland secretion and

increase the person’ssusceptibility to duodenal ulcers.

Enzymes of the intestinal mucosa are bound to the mem-

branes of the absorptive cell microvilli. These surface-bound en-

zymes include disaccharidases, which break disaccharides down

to monosaccharides; peptidases, which hydrolyze the peptide

bonds between small amino acid chains; and nucleases, which

break down nucleic acids (see table 24.2).Although these enzymes

are not secreted into the intestine, they inﬂuence the digestive

process signiﬁcantly,and the large surface area of the intestinal ep-

ithelium brings these enzymes into contact with the intestinal con-

tents. Small molecules, which are breakdown products of

digestion,are absorbed through the microvilli and enter the circu-

latory or lymphatic systems.

Movementin the Small Intestine

Mixing and propulsion ofchyme are the primary mechanical events

that occur in the small intestine.These functions are the result of seg-

mental or peristalic contractions, which are accomplished by the

smooth muscle in the wall of the small intestine and which are only

propagated for short distances.Segmental contract ions (see ﬁgure

24.3) mix the intestinal contents,and peristaltic contractions propel

the intestinal contents along the digestive tract.A few peristaltic con-

tractions may proceed the entire length ofthe intestine. Frequently,

intestinal peristaltic contractions are continuations ofperistaltic con-

tractions that begin in the stomach.These contractions both mix and

propel substances through the small intestine as the wave ofcontrac-

Part4 Regulationsand Maintenance884

tion proceeds.The contractions move at a rate of about 1 cm/min.

The movements are slightly faster at the proximal end ofthe small in-

testine and slightly slower at the distal end.It usually takes 3–5 hours

for chyme to move from the pyloric region to the ileocecal junction.

Local mechanical and chemical stimuli are especially impor-

tant in regulating the motility ofthe small intestine. Smooth mus-

cle contraction increases in response to distention ofthe intestinal

wall. Solutions that are either hypertonic or hypotonic,solutions

with a low pH,and certain products of digestion like amino acids

and peptides also stimulate contractions ofthe small intestine. Lo-

cal reﬂexes,which are integrated within the enteric plexus of the

small intestine,mediate the response of the small intestine to these

mechanical and chemical stimuli. Stimulation through parasym-

pathetic nerve ﬁbers may also increase the motility ofthe small in-

testine, but the parasympathetic inﬂuences in the small intestine

are not as important as those in the stomach.

The ileocecal sphincter at the juncture between the ileum

and the large intestine remains mildly contracted most ofthe time,

but peristaltic waves reaching it from the small intestine cause it to

relax and allow movement ofchyme from the small intestine into

the cecum. Cecal distention,however, initiates a local reﬂex that

causes more intense constriction ofthe ileocecal sphincter. Closure

ofthe sphincter facilitates digestion and absorption in the small in-

testine by slowing the rate ofchyme movement from the small in-

testine into the large intestine and prevents material from

returning to the ileum from the cecum.

27. Name and describe the three parts of the small intestine.

Whatare the major and lesser duodenal papilla?

28. What are the circular folds, villi, and microvilli in the small

intestine? Whatare their functions?

29. Name the four types of cells found in the duodenal mucosa,

and state theirfunctions.

30. What are the functions of the intestinal glands and

duodenal glands? State the factorsthat stimulate secretion

from the duodenal glandsand from goblet cells.

31. List the enzymes of the small intestine wall and give their

functions.

32. What are two kinds of movement of the small intestine?

Howare they regulated?

33. What is the function of the ileocecal sphincter?

Liver

Objective

■ Describe the structure and function of the liver.

Anatomyof the Liver

Theliver is the largest internal organ of the body, weighing about

1.36 kg (3 pounds),and it is in the right-upper quadrant of the ab-

domen,tucked against the inferior surface of the diaphragm (see

ﬁgures 24.1 and 24.18).The liver consists of two major lobes, left

andright, and two minor lobes, caudate and quadrate.

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Inferior vena cava

Caudate lobe

Right lobe

Left lobe

Lesser

omentum

Hepatic duct

Hepatic portal vein

Hepatic artery

Quadrate lobe

Gallbladder

Porta

Coronary ligament

Bare area

Inferior vena cava

Gallbladder

Round ligament

Right lobe

Falciform

ligament

Left lobe

Falciform ligament

Bare area

Esophagus

Inferior

vena cava

Hepatic veins

Left lobe

Right lobe

Hepatic cords

Bile canaliculi

Hepatocyte

Hepatic

sinusoid

Central vein

Liver lobule

Hepatic duct

Hepatic portal vein

Portal triad

Hepatic artery

Liver

Coronary ligament

Figure 24.18

Anatomyand Histology

ofthe Liver

(a) Anterior view. (b) Inferior view. (c) Superior view.

(d) Liver lobuleswith triads at the corners and central

veinsin the center of the lobules.

(a)

(b)

(c)

(d)

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Aporta (gate) is on the inferior surface of the liver, where the

various vessels,ducts, and nerves enter and exit the liver. The he-

patic(he-pat⬘ik; associated with the liver) portal vein, the hepatic

artery,and a small hepatic nerve plexus enter the liver through the

porta (ﬁgure 24.19).Lymphatic vessels and two hepatic ducts, one

each from the right and left lobes,exit the liver at the porta. The he-

patic ducts transport bile out of the liver.The right and left hepatic

ducts unite to form a single common hepatic duct.The cystic duct

from the gallbladder joins the common hepatic duct to form the

common bile duct, which joins the pancreatic duct at the he-

patopancreatic ampulla (he˘-pat⬘o¯-pan-cre¯-at⬘ik am-pul⬘ la˘), an

enlargement where the hepatic and pancreatic ducts come together.

The hepatopancreatic ampulla empties into the duodenum at the

major duodenal papilla (see ﬁgures 24.16a and 24.20). A smooth

muscle sphincter surrounds the common bile duct where it enters

the hepatopancreatic ampulla.The gallbladder is a small sac on the

Part4 Regulationsand Maintenance886

inferior surface of the liver that stores bile.Bile can ﬂow from the

gallbladder through thecystic duct into the common bile duct, or it

can ﬂow back up the cystic duct into the gallbladder.

Histology ofthe Liver

A connective tissue capsule and visceral peritoneum cover the liver,

except for the bare area, which is a small area on the diaphrag-

matic surface surrounded by the coronary ligament (see ﬁgure

24.18c).At the porta, the connective tissue capsule sends a branch-

ing network of septa (walls) into the substance of the liver to pro-

vide its main support. Vessels, nerves, and ducts follow the

connective tissue branches throughout the liver.

The connective tissue septa divide the liver into hexagon-

shapedlobules with a por tal triad at each corner.The tr iads are so

named because three vessels—the hepatic portal vein,hepatic ar-

tery,and hepatic duct—are commonly located in them (see ﬁgure

24.18d).Hepatic nerves and lymphatic vessels, often too small to be

easily seen in light micrographs, are also located in these areas.A

central vein is in the center of each lobule. Central veins unite

tofo rm hepatic veins, which exit the liver on its posterior and su-

perior surfaces and empty into the inferior vena cava (see ﬁgure

24.19).

Hepatic cordsradiate out from the central vein of each lob-

ule like the spokes ofa wheel. The hepatic cords are composed of

hepatocytes, the functional cells of the liver. The spaces between

the hepatic cords are blood channels called hepatic sinusoids.The

sinusoids are lined with a very thin,irregular squamous endothe-

lium consisting oftwo cell populations: (1) extremely thin, sparse

endothelial cellsand (2) hepatic phagocytic cells (Kupffer cells).

A cleftlike lumen, the bile canaliculus (kan-a˘-lik⬘u¯-lu˘s; little

canal),lies between the cells within each cord (see ﬁgure 24.18d).

Hepatocytes have six major functions (described in more de-

tail starting on the next page):(1) bile production, (2) storage, (3)

interconversion ofnutr ients,(4) detoxiﬁcation, (5) phagocytosis,

and (6) synthesis of blood components. Nutrient-rich, oxygen-

poor blood from the viscera enters the hepatic sinusoids from

branches of the hepatic portal vein and mixes with oxygen-rich,

nutrient-depleted blood from the hepatic arteries.From the blood,

the hepatocytes can take up the oxygen and nutrients,which are

stored,detoxiﬁed, used for energy, or used to synthesize new mol-

ecules.Molecules produced by or modiﬁed in the hepatocytes are

released into the hepatic sinusoids or into the bile canaliculi.

Mixed blood in the hepatic sinusoids ﬂows to the central

vein,where it exits the lobule and then exits the liver through the

hepatic veins. Bile, produced by the hepatocytes and consisting

primarily of metabolic by-products,ﬂows through the bile canali-

culi toward the hepatic triad and exits the liver through the hepatic

ducts.Blood, therefore, ﬂows from the triad toward the center of

each lobule,whereas bile ﬂows away from the center of the lobule

toward the triad.

In the fetus,special blood vessels bypass the liver sinusoids.

The remnants offetal blood vessels can be seen in the adult as the

round ligament (ligamentum teres) and the ligamentum venosum

(see chapter 29).

Inferior vena cava Aorta

Hepatic veins

Hepatic portal

vein

Hepatic

artery

Hepatic

ducts

Porta

of liver

Small

intestine

Heart

Nutrient-rich,

oxygen-poor

blood

Bile

Oxygen-rich

blood

Oxygen-rich

blood

Liver

Figure 24.19

Blood and Bile Flow Through the Liver

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Chapter 24 Digestive System 887

Liver Rupture or Enlargement

The liver iseasily ruptured because it islarge, ﬁxed in position, and

fragile, or itcan be lacerated by a broken rib. Liver rupture or laceration

resultsin severe internal bleeding.

The liver maybecome enlarged as a result of heart failure, hepatic

cancer, cirrhosis, or Hodgkin’sdisease (a lymphaticcancer).

Functionsof the Liver

The liver performs important digestive and excretory functions,

stores and processes nutrients, synthesizes new molecules, and

detoxiﬁes harmful chemicals.

Bile Production

The liver produces and secretes about 600–1000 mL ofbile each

day (see table 24.2).Bile contains no digestive enzymes, but it plays

a role in digestion because it neutralizes and dilutes stomach acid

and emulsiﬁes fats.The pH of chyme as it leaves the stomach is too

low for the normal function of pancreatic enzymes. Bile helps to

neutralize the acidic chyme and to bring the pH up to a level at

which pancreatic enzymes can function. Bile salts emulsify fats

(described in more detail on p.896). Bile also contains excretory

products like bile pigments.Bilirubin is a bile pigment that results

from the breakdown ofhemoglobin. Bile also contains cholesterol,

fats,fat-soluble hormones, and lecithin.

Secretin stimulates bile secretion,primarily by increasing the

water and bicarbonate ion content of bile (ﬁgure 24.21). Bile salts

also increase bile secretion through a positive-feedback system.Most

bile salts are reabsorbed in the ileum and carried in the blood back to

the liver,where they contribute to further bile secretion. The loss of

bile salts in the feces is reduced by this recycling process.Bile secre-

tion into the duodenum continues until the duodenum empties.

Storage

Hepatocytes can remove sugar from the blood and store it in the

form of glycogen.They can also store fat, vitamins (A, B

,D,E,

and K), copper,and iron. This storage function is usually short

term, and the amount of stored material in the hepatocytes and,

thus,the cell size ﬂuctuate during a given day.

Hepatocytes help control blood sugar levels within very nar-

row limits.If a large amount of sugar enters the general circulation

after a meal,it will increase the osmolalit y of the blood and pro-

duce hyperglycemia.This is prevented because the blood from the

intestine passes through the hepatic portal vein to the liver,where

glucose and other substances are removed from the blood by hepa-

tocytes, stored, and secreted back into the circulation when

needed.

NutrientInterconversion

Interconversion ofnutrients is another important function of the

liver.Ingested nutrients are not always in the proportion needed by

the tissues.If this is the case, the liver can convert some nutrients

into others.If, for example, a person is on a diet that is excessively

Gallbladder

Cystic duct

Hepatic portal vein

Duodenum

(cutaway view)

Hepatopancreatic

ampulla

Major duodenal

papilla

Common bile duct

Minor duodenal

papilla

1.The hepatic ducts from the liver lobes

combine to form the common hepatic

duct.

2.The common hepatic duct combines

with the cystic duct from the gallbladder

to form the common bile duct.

3.The common bile duct and the

pancreatic duct combine to form the

hepatopancreatic ampulla.

4.The hepatopancreatic ampulla empties

into the duodenum at the major

duodenal papilla.

5. Pancreatic secretions also enter the

duodenum through the

hepatopancreatic ampulla. The

accessory pancreatic duct also empties

into the duodenum.

Liver

Hepatic ducts

Common

hepatic duct

Spleen

Pancreatic

duct

Pancreas

Accessory pancreatic

duct

Figure 24.20

The Liver, Gallbladder, Pancreas, and DuctSystem

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high in protein,an oversupply of amino acids and an undersupply

oflipids and carbohydrates may be delivered to the liver.The hepa-

tocytes break down the amino acids and cycle many of them

through metabolic pathways so they can be used to produce

adenosine triphosphate,lipids, and glucose (see chapter 25).

Hepatocytes also transform substances that cannot be used by

most cells into more readily usable substances.For example,ingested

fats are combined with choline and phosphorus in the liver to pro-

duce phospholipids, which are essential components of plasma

membranes.Vitamin D is hydroxylated in the liver.The hydroxylated

form ofvitamin D is the major circulating form of vitamin D, which

is transported through the circulation to the kidney,where it’s again

hydroxylated.The double-hydroxylated vitamin D is the active form

ofthe vitamin, which functions in calcium maintenance.

Part4 Regulationsand Maintenance888

Detoxiﬁcation

Many ingested substances are harmful to the cells ofthe body. In

addition,the body itself produces many by-products of metabo-

lism that,if accumulated,are toxic. The liver forms a major line

ofdefense against many of these harmful substances. It detoxiﬁes

many substances by altering their structure to make them less

toxicor make their elimination easier. Ammonia, for example, a

by-product ofamino acid metabolism, is toxic and is not readily

removed from the circulation by the kidneys. Hepatocytes re-

move ammonia from the circulation and convert it to urea,which

is less toxic than ammonia and is secreted into the circulation and

then eliminated by the kidneys in the urine.Other substances are

removed from the circulation and excreted by the hepatocytes

into the bile.

Brain

Vagus nerves

Bile

Liver

Gallbladder

Secretin

Stomach

Pancreas

Duodenum

Circulation

1. Secretin, produced by the

duodenum (

purple arrows

) and

carried through the circulation to the

liver, stimulates bile secretion by the

liver (

green arrows inside the liver

2. Cholecystokinin, produced by the

duodenum (

pink arrows

) and

carried through the circulation to the

gallbladder, stimulates the

gallbladder to contract, thereby

releasing bile into the duodenum

(

green arrow outside the liver

3. Vagal nerve stimulation (

red arrow

)

causes the gallbladder to contract,

thereby releasing bile into the

duodenum.

ProcessFigure 24.21

Controlof Bile Secretion and Release

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Phagocytosis

Hepatic phagocytic cells (Kupffer cells),which lie along the sinu-

soid walls of the liver,phagocytize “worn-out” and dying red and

white blood cells,some bacteria, and other debris that enters the

liver through the circulation.

Synthesis

The liver can also produce its own unique new compounds.It pro-

duces many blood proteins,such as albumins, ﬁbrinogen, globu-

lins, heparin, and clotting factors, which are released into the

circulation (see chapter 19).

Hepatitis, Cirrhosis, and Liver Damage

Strictlydeﬁned, hepatitis is an inﬂammation of the liver and does not

necessarilyresult from an infection. Hepatitis can be caused byalcohol

consumption or infection. Infectioushepatitis is caused byviral

infections. HepatitisA, also called infectious hepatitis, is responsible for

about30% of hepatitis cases in the U.S. Hepatitis B, also called serum

hepatitis, isa more chronic infection responsible for halfthe hepatitis

casesin the U.S. Hepatitis C, also called non-A and non-B hepatitis,

causes20% of the U.S. hepatitis cases. It’s caused byone or more virus

typesthat cannot be identiﬁed in blood tests. It’s spread by blood

transfusionsor sexual intercourse. If hepatitis isnot treated, liver cells

die and are replaced byscar tissue, resulting in loss of liver function.

Death caused byliver failure can occur.

Cirrhosis(sir-ro¯⬘sis) ofthe liver involvesthe death of hepatocytes

and their replacementby ﬁbrous connective tissue. The liver becomes

pale in color (the term cirrhosismeans a tawny or orange condition)

because ofthe presence of excess white connective tissue. Italso

becomesﬁrmer, and the surface becomes nodular. The lossof

hepatocyteseliminates the function of the liver, often resulting in

jaundice, and the buildup ofconnective tissue can impede blood ﬂow

through the liver. Cirrhosisfrequently develops in alcoholicsand may

develop asa result of biliary obstruction, hepatitis, or nutritional

deﬁciencies.

Under mostconditions, mature hepatocytes can proliferate and

replace lostparts of the liver. If the liver is severely damaged, however,

the hepatocytesmay not have enough regenerative power to replace the

lostparts. In this case, a liver transplant may be necessary. Recent

evidence suggeststhat the liver also maintains an undifferentiated stem

cellpopulation, called “oval” cells, which givesrise to two cell lines, one

forming bile ductepithelium and the other producing hepatocytes. It is

hoped thatthese stem cells can be used to reconstitute a severely

damaged liver. Itmay even be possible at some time in the future to

remove stem cellsfrom a person with hemophilia, genetically engineer

the cellsto produce the missing clotting factors, and then reintroduce

the altered stem cellsinto the person’s liver.

34. Describe the lobes of the liver. What is the porta?

35. Diagram the duct system from the liver, gallbladder, and

pancreasthat empties into the major duodenal papilla.

36. What are the hepatic cords and the sinusoids?

37. Describe the ﬂow of blood to and through the liver.

Describe the ﬂowof bile away from the liver.

38. Explain and give examples of the major functions of the liver.

39. What stimulates bile secretion from the liver?

Gallbladder

Objective

■ Describe the structure and function of the gallbladder.

Thegallbladder is a saclike structure on the inferior surface

ofthe liver that is about 8 cm long and 4 cm wide (see ﬁgure 24.20).

Three tunics form the gallbladder wall:(1) an inner mucosa folded

into rugae that allow the gallbladder to expand;(2) a muscularis,

which is a layer of smooth muscle that allows the gallbladder to

contract;and (3) an outer covering of serosa. The cystic duct con-

nects the gallbladder to the common bile duct.

Bile is continually secreted by the liver and ﬂows to the gall-

bladder,where 40–70 mL of bile can be stored. While the bile is in

the gallbladder,water and electrolytes are absorbed, and bile salts

and pigments become as much as 5–10 times more concentrated

than they were when secreted by the liver.Shortly after a meal, the

gallbladder contracts in response to stimulation by cholecystokinin

and, to a lesser degree,in response to vagal stimulation, thereby

dumping large amounts of concentrated bile into the small intes-

tine (see ﬁgure 24.21).

Gallstones

Cholesterol, secreted bythe liver, may precipitate in the gallbladder to

produce gallstones(ﬁgure A). Occasionally, a gallstone can passout

ofthe gallbladder and enter the cystic duct, blocking release of bile.

Such a condition interfereswith normal digestion, and the gallstone

often mustbe removed surgically. If the gallstone moves far enough

down the duct, itcan also block the pancreatic duct, resulting in

pancreatitis.

Drastic dieting with rapid weight loss may lead to gallstone

production.In one study, 25% of obese people participating in an

8-week,quick-weight-loss program developed gallstones. Six per-

cent required surgical removal ofthe stones. No gallstones devel-

oped in nondieting obese controls.

40. Describe the three tunics of the gallbladder wall.

41. What is the function of the gallbladder? What stimulates the

release of bile from the gallbladder?

Figure A

Gallstones

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Pancreas

Objective

■ Explain the structure and function of the pancreas.

Anatomyof the Pancreas

Thepancreas is a complex organ composed of both endocrine and

exocrine tissues that perform several functions.The pancreas con-

sists ofa head, located within the curvature of the duodenum (see

ﬁgure 24.16a),a body, and a tail, which extends to the spleen.

The endocrine part of the pancreas consists of pancreatic

islets (islets of Langerhans; see ﬁgure 24.16b). The islet cells pro-

duce insulin and glucagon,which are very important in controlling

blood levels ofnutrients, such as glucose and amino acids, and so-

matostatin, which regulates insulin and glucagon secretion and

may inhibit growth hormone secretion (see chapter 18).

The exocrine part of the pancreas is a compound acinar

gland (see discussion of glands in chapter 4). The acini (as⬘i-nı¯;

grapes; see ﬁgure 24.16b) produce digestive enzymes.Clusters of

acini form lobules that are separated by thin septa. Lobules are

connected by small intercalated ducts to intralobular ducts,

which leave the lobules to join interlobular ductsbetween the lob-

ules. The interlobular ducts attach to the main pancreatic duct,

which joins the common bile duct at the hepatopancreatic ampulla

(see ﬁgures 24.16a and 24.20). The ducts are lined with simple

cuboidal epithelium,and the epithelial cells of the acini are pyramid-

shaped.A smooth muscle sphincter surrounds the pancreatic duct

where it enters the hepatopancreatic ampulla.

PancreaticSecretions

The exocrine secretions ofthe pancreas are called pancreatic juice

and have two major components:an aqueous component and an

enzymatic component.Pancreatic juice is produced in the pancreas

and is then delivered through the pancreatic ducts to the small in-

testine,where it functions in digestion. The aqueous component

is produced principally by columnar epithelial cells that line the

smaller ducts of the pancreas. It contains Na

⫹

and K

⫹

ions in

about the same concentration found in extracellular ﬂuid.Bicar-

bonate ions are a major part of the aqueous component,and they

neutralize the acidic chyme that enters the small intestine from the

stomach.The increased pH caused by pancreatic secretions in the

duodenum stops pepsin digestion but provides the proper envi-

ronment for the function ofpancreatic enzymes. Bicarbonate ions

are actively secreted by the duct epithelium,and water follows pas-

sively to make the pancreatic juice isotonic.The cellular mecha-

nism that is responsible for the secretion of bicarbonate ions is

diagrammed in ﬁgure 24.22.

The enzymes ofthe pancreatic juice are produced by the aci-

nar cells ofthe pancreas and are important for the digestion of all

major classes of food.Without the enzymes produced by the pan-

creas, lipids, proteins,and carbohydrates are not adequately di-

gested (see tables 24.1 and 24.2).

The proteolytic pancreatic enzymes,which digest proteins, are

secreted in inactive forms,whereas many of the other enzymes are

secreted in active form.The major proteolytic enzymes are trypsin,

chymotrypsin, and car boxypeptidase. The y are secreted in their

inactive forms as trypsinogen, chymotrypsinogen, and procar-

Part4 Regulationsand Maintenance890

boxypeptidase and are activated by the removal ofcertain peptides

from the larger precursor proteins.If these were produced in their

active forms,they would digest the tissues producing them. The pro-

teolytic enzyme enterokinase (en⬘te¯r-o¯-kı¯⬘na¯ s; intestinal enzyme),

which is an enzyme attached to the brush border ofthe small intes-

tine,activates trypsinogen. Trypsin then activates more trypsinogen,

as well as chymotrypsinogen and procarboxypeptidase.

Pancreatic juice also contains pancreaticamylase, which con-

tinues the polysaccharide digestion that was initiated in the oral cavity.

In addition,pancreatic juice contains a group of lipid-digesting en-

zymes called pancreatic lipases,which break down lipids into free

fatty acids,glycerides, cholesterol, and other components.

Enzymes that reduce DNA and ribonucleic acid to their

component nucleotides,deoxyribonucleases and rib onucleases,

respectively,are also present in pancreatic juice.

Pancreatitisand Pancreatic Cancer

Pancreatitisis an inﬂammation of the pancreas and occursquite

commonly. Pancreatitisinvolves the release ofpancreatic enzymes

within the pancreasitself, which digest pancreatic tissue. Itmay result

from alcoholism, use ofcertain drugs, pancreatic duct blockage, cystic

ﬁbrosis, viralinfection, or pancreatic cancer. Symptomscan range from

mild abdominalpain to systemic shock and coma.

Cancer ofthe pancreas can obstruct the pancreaticand the

common hepaticducts, resulting in jaundice. Pancreatic cancer maynot

be detected untilthe mass has become fairlylarge and may become so

large asto block off the pyloricregion of the stomach.

Regulation ofPancreatic Secretion

Both hormonal and neural mechanisms (ﬁgure 24.23) control the

exocrine secretions of the pancreas.Secretin stimulates the secre-

tion ofa watery solution that contains a large amount of bicar bon-

ate ions from the pancreas. The primary stimulus for secretin

release is the presence ofacidic chyme in the duodenum.

PREDICT

Explain whysecretin production in response to acidic chyme and its

stimulation ofbicarbonate ion secretion constitute a negative-

feedbackmechanism.

Cholecystokinin stimulates the release ofbile from the gall-

bladder and the secretion of pancreatic juice rich in digestive en-

zymes.The major stimulus for the release of cholecystokinin is the

presence offatty acids and other lipids in the duodenum.

Parasympathetic stimulation through the vagus (X) nerves

also stimulates the secretion ofpancreatic juices rich in pancreatic

enzymes,and sympathetic impulses inhibit secretion. The effect of

vagal stimulation on pancreatic juice secretion is greatest during

the cephalic and gastric phases ofstomach secretion.

42. Describe the parts of the pancreas responsible for

endocrine and exocrine secretions. Diagram the duct

system of the pancreas.

43. Name the two kinds of exocrine secretions produced by the

pancreas. Whatstimulates their production and what is

theirfunction?

44. What are the enzymes present in pancreatic juice? Explain

the function of each.

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Chapter 24 Digestive System 891

Large Intestine

Objective

■ Describe the anatomy and functionsof the large intestine.

The large intestine is the portion of the digestive tract ex-

tending from the ileocecal junction to the anus.It consists of the ce-

cum, colon, rectum,and anal canal. Normally 18–24 hours are

required for material to pass through the large intestine,in contrast

to the 3–5 hours required for movement of chyme through the

small intestine.Thus, the movements of the colon are more sluggish

than those ofthe small intestine. While in the colon, chyme is con-

verted to feces.Absorption of water and salts, the secretion of mu-

cus,and extensive action of microorganisms are involved in the for-

mation offeces, which the colon stores until the feces are eliminated

by the process ofdefecation. About 1500 mL of chyme enters the ce-

cum each day,but more than 90% of the volume is reabsorbed so

that only 80–150 mL offeces is normally eliminated by defecation.

Anatomyof the Large Intestine

Cecum

Thececum (se¯⬘ku˘m; blind) is the proximal end of the large intes-

tine.It’s where the large and small intestines meet at the ileocecal

junction.The cecum extends inferiorly about 6 cm past the ileoce-

cal junction in the form of a blind sac (ﬁgure 24.24).Attached to

1. Water (H

O) and carbon dioxide (CO

) combine under the

influence of carbon anhydrase (CA) to form carbonic acid.

2. Carbonic acid (H

) dissociates to form hydrogen ions (H

)

and bicarbonate ions (HCO

3. The H

are exchanged for sodium ions (Na

) and are removed in

the bloodstream.

4. The HCO

are actively transported into the intercalated ducts.

and water follow the HCO

ions into the ducts.

ADP

Intercalated

duct cell

(produces

aqueous

component

of pancreatic

juice)

Blood vessel

HCO

–

HCO

–

HCO

–

O + CO

To intercalated

duct lumen

Blood vessel

Acinar cell (produces

enzymatic component

of pancreatic juice)

Intercalated

duct

To interlobular

duct

–

ProcessFigure 24.22

Bicarbonate Ion Production in the

Pancreas

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Part4 Regulationsand Maintenance892

Brain

Stomach

Pancreas

Duodenum

Cholecystokinin

Secretin

Cholecystokinin

Secretin

Pancreatic

juices

Vagus nerves

Circulation

1. Secretin (

purple arrows

)

released from the duodenum,

stimulates the pancreas to

release a watery secretion, rich

in bicarbonate ions.

2. Cholecystokinin (

pink arrows

)

released from the duodenum,

causes the pancreas to release

a secretion rich in digestive

enzymes.

3. Parasympathetic stimulation

from the vagus nerve (

red

arrow

) causes the pancreas to

release a secretion rich in

digestive enzymes.

ProcessFigure 24.23

Controlof Pancreatic Secretion

Left colic flexure

(splenic flexure)

Descending colon

Haustra

Epiploic

appendages

Teniae coli

Sigmoid colon

Internal anal sphincter

External anal sphincter

Rectum

Anal canal

Ascending

colon

Ileocecal

valve

Ileum

Cecum

Vermiform

appendix

Right colic flexure

(hepatic flexure)

Transverse colon

Figure 24.24

Large Intestine

(a) Large intestine (i.e., cecum, colon, and rectum) and analcanal. The teniae coli and epiploicappendages are along the length of the colon. (b) A radiograph of

the large intestine following a barium enema.

(a) (b)

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Chapter 24 Digestive System 893

the cecum is a small blind tube about 9 cm long called the vermi-

form(ver⬘mi-fo¯rm;worm-shaped) appendix.The walls of the ap-

pendix contain many lymphatic nodules.

Appendicitis

Appendicitisis an inﬂammation of the vermiform appendix and usually

occursbecause of obstruction of the appendix. Secretionsfrom the

appendixcannot pass the obstruction and accumulate, causing

enlargementand pain. Bacteria in the area can cause infection of the

appendix. Symptomsinclude sudden abdominal pain, particularly in the

rightlower portion of the abdomen, along with a slight fever, loss of

appetite, constipation or diarrhea, nausea, and vomiting. In the right-

lower quadrantof the abdomen, about midway along a line between the

umbilicusand the right anterior superior iliac spine, is an area on the

body’ssurface called McBurney’s point. This area becomesvery tender

in patientswith acute appendicitis because of pain referred from the

inﬂamed appendixto the body’s surface. Each year, 500,000 people in

the United Statessuffer an appendicitis. An appendectomy is removal

ofthe appendix. If the appendix bursts, the infection can spread

throughoutthe peritoneal cavity, causing peritonitis, with life-

threatening results.

Colon

The colon (ko¯⬘lon) is about 1.5–1.8 m long and consists offour

parts:the ascending colon, transverse colon, descending colon,and

sigmoid colon (see ﬁgure 24.24).The ascending colon extends su-

periorly from the cecum and ends at the right colic ﬂexure (hepatic

ﬂexure) near the right inferior margin of the liver.The transverse

colon extends from the right colic ﬂexure to the left colic ﬂexure

(splenic ﬂexure),and the descending colon extends from the left

colic ﬂexure to the superior opening ofthe true pelvis, where it be-

comes the sigmoid colon.The sigmoid colon forms an S-shaped

tube that extends into the pelvis and ends at the rectum.

The circular muscle layer ofthe colon is complete, but the

longitudinal muscle layer is incomplete. The longitudinal layer

doesn’t completely envelop the intestinal wall but forms three

bands,called the teniae coli (te¯⬘ne¯-e¯ ko¯⬘lı¯;a band or tape along

the colon),that run the length of the colon (see ﬁgures 24.24 and

24.25).Contractions of the teniae coli cause pouches called haus-

tra(haw⬘stra˘;to draw up) to form along the length of the colon,

giving it a puckered appearance.Small, fat-ﬁlled connective tis-

sue pouches called epiploic (ep⬘i-plo¯⬘ik;related to the omen-

tum) appendages are attached to the outer surface of the colon

along its length.

The mucosal lining of the large intestine consists of simple

columnar epithelium.This epithelium is not formed into folds or

villi like that ofthe small intestine but has numerous straight tubu-

lar glands called crypts(see ﬁgure 24.25). The crypts are somewhat

similar to the intestinal glands ofthe small intestine, with three cell

types that include absorptive,goblet, and granular cells. The major

difference is that in the large intestine goblet cells predominate and

the other two cell types are greatly reduced in number.

Rectum

Therectum is a straight, muscular tube that begins at the termina-

tion of the sigmoid colon and ends at the anal canal (see ﬁgure

24.24).The mucosal lining of the rectum is simple columnar ep-

ithelium,and the muscular tunic is relatively thick compared to the

rest ofthe digestive tract.

Anal Canal

The last 2–3 cm of the digestive tract is the anal canal (see ﬁgure

24.24).It begins at the inferior end of the rectum and ends at the

anus (external GI tract opening). The smooth muscle layer of the

anal canal is even thicker than that ofthe rectum and forms the in-

ternal anal sphincterat the superior end of the anal canal. Skeletal

muscle forms the external anal sphincterat the inferior end of the

canal.The epithelium of the superior par t of the anal canal is sim-

ple columnar and that ofthe inferior part is str atiﬁed squamous.

Hemorrhoids

Hemorrhoidsare the enlargement, or inﬂammation, of the hemorrhoidal

veins, which supplythe anal canal. The condition isalso called varicose

hemorrhoidalveins. Hemorrhoids cause pain, itching, and bleeding

around the anus. Treatmentsinclude increasing the bulk(indigestible

ﬁber) in the diet, taking sitzbaths, and using hydrocortizone

suppositories. Surgerymay be necessary if the condition isextreme and

doesn’trespond to other treatments.

Secretionsof the Large Intestine

The mucosa of the colon has numerous goblet cells that are scat-

tered along its length and numerous crypts that are lined almost

entirely with goblet cells. Little enzymatic activity is associated

with secretions of the colon when mucus is the major secretory

product (see tables 24.1 and 24.2).Mucus lubricates the wall of the

colon and helps the fecal matter stick together.Tactile stimuli and

irritation of the wall of the colon trigger local enteric reﬂexes that

increase mucous secretion. Parasympathetic stimulation also in-

creases the secretory rate ofthe goblet cells.

Diarrhea

When the large intestine isirritated and inﬂamed, such as in patients

with bacterialenteritis (inﬂamed intestine resulting from bacterial

infection ofthe bowel), the intestinal mucosa secretes large amounts of

mucusand electrolytes, and water moves by osmosis into the colon. An

abnormallyfrequent discharge of ﬂuid fecesis called diarrhea. Although

such discharge increasesﬂuid and electrolyte loss, it also movesthe

infected fecesout of the intestine more rapidly and speeds recovery from

the disease.

A molecular pump exchanges bicarbonate ions for chloride

ions in epithelial cells ofthe colon in response to acid produced by

colic bacteria.Another pump exchanges sodium ions for hydrogen

ions.Water crosses the wall of the colon through osmosis with the

sodium chloride gradient.

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Anatomy and Physiology,

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IV. Regulations and

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24. Digestive System

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The feces that leave the digestive tract consist ofwater, solid

substances (e.g.,undigested food), microorganisms, and sloughed-

offepithelial cells.

Numerous microorganisms inhabit the colon.They repro-

duce rapidly and ultimately constitute about 30% ofthe dry weight

of the feces. Some bacteria in the intestine synthesize vitamin K,

which is passively absorbed in the colon,and break down a small

amount ofcellulose to glucose.

Bacterial actions in the colon produce gases called ﬂatus

(ﬂa¯⬘tu˘s;blowing). The amount of ﬂatus depends partly on the bac-

terial population present in the colon and partly on the type offood

Part4 Regulationsand Maintenance894

consumed.For example, beans, which contain certain complex car-

bohydrates,are well known for their ﬂatus-producing effect.

Movementin the Large Intestine

Segmental mixing movements occur in the colon much less often

than in the small intestine.Peristaltic waves are largely responsible for

moving chyme along the ascending colon.At widely spaced intervals

(normally three or four times each day),large parts of the transverse

and descending colon undergo several strong peristaltic contractions,

calledmass movements. Each mass movement contraction extends

over a much longer part ofthe digestive tract (≥ 20 cm) than does a

Teniae coli

Haustra

Epiploic

appendages

Epithelial

cell

Lamina

propria

Goblet cells

in crypt

Surface

goblet cells

Crypt

Submucosa

Circular

muscle

Longitudinal

muscle

Serosa

Lymphatic

nodule

Crypts

Opening

of crypts

Epithelium

Figure 24.25

Histologyof the Large Intestine

(a) Section ofthe transverse colon cut open to show the inner surface. (b) Enlargementof the inner surface, showing openings of the crypts. (c) Higher

magniﬁcation ofa single crypt.

(a)

(b)

(c)

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Chapter 24 Digestive System 895

peristaltic contraction and propels the colon contents a considerable

distance toward the anus (ﬁgure 24.26).Mass movements are very

common after meals because the presence offood in the stomach or

duodenum initiates them.Mass movements are most common about

15 minutes after breakfast.The y usually persist for 10–30 minutes

and then stop for perhaps half a day.Local reﬂexes in the enteric

plexus,which are called gastrocolic reﬂexes if initiated by the stom-

ach or duodenocolic reﬂexesif initiated by the duodenum, integrate

mass movements.

Distention ofthe rectal wall by feces acts as a stimulus that initi-

ates the defecation reﬂex. Local reﬂexes cause weak contractions of

the rectum and relaxation ofthe internal anal sphincter. Parasympa-

thetic reﬂexes cause strong contractions of the rectum and are nor-

mally responsible for most of the defecation reﬂex.Action potentials

produced in response to the distention travel along afferent nerve

ﬁbers to the sacral region ofthe spinal cord, where efferent action po-

tentials are initiated that reinforce peristaltic contractions in the lower

colon and rectum.The defecation reﬂex reduces action potentials to

the internal anal sphincter, causing it to relax.The external anal

sphincter,which is composed of skeletal muscle and is under con-

scious cerebral control,prevents the movement of feces out of the rec-

tum and through the anal opening.If this sphincter is relaxed volun-

tarily,feces are expelled. The defecation reﬂex persists for only a few

minutes and quickly declines.Generally, the reﬂex is reinitiated after a

period that may be as long as several hours.Mass movements in the

colon are usually the reason for the reinitiation ofthe defecation reﬂex.

Defecation is usually accompanied by voluntary movements

that support the expulsion of feces. These voluntary movements

include a large inspiration of air followed by closure of the larynx

and forceful contraction of the abdominal muscles. As a conse-

quence, the pressure in the abdominal cavity increases,thereby

helping force the contents ofthe colon through the anal canal and

out ofthe anus.

45. Describe the parts of the large intestine. What are teniae

coli, haustra, and crypts?

46. Explain the difference in structure between the internal anal

sphincterand the external anal sphincter.

47. Name the substances secreted and absorbed by the large

intestine. Whatis the role of microorganisms in the colon?

48. What kind of movements occur in the colon? Describe the

defecation reﬂex.

Stomach

Colon

Rectum

Stimulation

of local

defecation

reflexes

Stimulates

mass

movement

Stimulation of

parasympathetic

controlled

defecation

reflexes

Feces

Mass movements

Presence of food

in the stomach

Presence of chyme

in the duodenum

1. The presence of food in the

stomach and chyme in the

duodenum stimulate mass

movement in the colon.

2. Mass movements are integrated

by the enteric plexus.

3. They propel the contents of the

colon toward the rectum.

4. The presence of feces in the

rectum stimulates

parasympathetic and local

reflexes that result in defecation.

ProcessFigure 24.26

Reﬂexesin the Colon and Rectum

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On Being “Regular”

The importance ofregularity of defecation has been greatly

overestimated. Manypeople have the misleading notion that a daily

bowelmovement is critical for good health. As with manyother body

functions, whatis “normal” differs from person to person. Whereas many

people defecate one or more timesper day, some normal, healthy adults

defecate on the average onlyevery other day. A defecation rate of only

twice per week, however, isusually described as constipation. Habitually

postponing defecation when the defecation reﬂexoccurs can lead to

constipation and mayeventually result in desensitization ofthe rectum

so thatthe defecation reﬂex is greatly diminished.

Digestion, Absorption,

and Transport

Objectives

■ Describe the processof digestion, absorption, and

transportfor carbohydrates, lipids, and proteins.

■ Describe the movement of waterand ions through the

intestinal wall.

Digestion is the breakdown of food to molecules that are

small enough to be absorbed into the circulation.Mechanical di-

gestionbreaks large food particles down into smaller ones. Chem-

ical digestioninvolves the breaking of covalent chemical bonds in

organic molecules by digestive enzymes.Carbohydrates are broken

down into monosaccharides,proteins are broken down into amino

acids,and fats are broken down into fatty acids and glycerol. Ab-

sorption and transport are the means by which molecules are

moved out ofthe digestive tract and into the circulation for distri-

bution throughout the body.Not all molecules (e.g., vitamins,

minerals,and water) are broken down before being absorbed. Di-

gestion begins in the oral cavity and continues in the stomach,but

most digestion occurs in the proximal end of the small intestine,

especially in the duodenum.

Absorption of certain molecules can occur all along the di-

gestive tract. A few chemicals,such as nitroglycerin, can be ab-

sorbed through the thin mucosa of the oral cavity below the

tongue.Some small molecules (e.g., alcohol and aspirin) can dif-

fuse through the stomach epithelium into the circulation.Most ab-

sorption, however, occurs in the duodenum and jejunum,

although some absorption occurs in the ileum.

Once the digestive products have been absorbed,they are

transported to other parts of the body by two different routes.Wa-

ter,ions, and water-soluble digestion products,such as glucose and

amino acids,enter the hepatic portal system and are transported to

the liver.The products of lipid metabolism are coated with pro-

teins and transported into lymphatic capillaries called lacteals (see

ﬁgure 24.17c).The lacteals are connected by lymphatic vessels to

the thoracic duct (see chapter 21),which empties into the left sub-

clavian vein.The protein-coated lipid products then travel in the

circulation to adipose tissue or to the liver.

Part4 Regulationsand Maintenance896

Carbohydrates

Ingested carbohydratesconsist primarily of polysaccharides, such

as starches and glycogen; disaccharides, such as sucrose (table

sugar) and lactose (milk sugar);and monosaccharides, such as glu-

cose and fructose (found in many fruits). During the digestion

process,polysaccharides are broken down into smaller chains and

ﬁnally into disaccharides and monosaccharides.Disaccharides are

broken down into monosaccharides.Car bohydrate digestion be-

gins in the oral cavity with the partial digestion of starches by sali-

vary amylase (am⬘il-a¯s). A minor amount of digestion occurs in

the stomach through the action of gastric amylase and gelatinase.

Carbohydrate digestion is continued in the intestine by pancreatic

amylase(table 24.4). A series of disaccharidases that are bound to

the microvilli ofthe intestinal epithelium digest disaccharides into

monosaccharides.

Lactose Intolerance

Lactase deﬁciencyresults in lactose intolerance, which is an inabilityto

digestmilk products. This disorder is primarily hereditary, affecting

5%–15% ofEuropeans and 80%–90% of Africans and Asians.

Symptomsinclude cramps, bloating, and diarrhea.

Monosaccharides such as glucose and galactose are taken up

into intestinal epithelial cells by cotransport,powered by a sodium

ion gradient (ﬁgure 24.27).Monosaccharides such as fructose are

taken up by facilitated diffusion.The monosaccharides are trans-

ferred by facilitated diffusion to the capillaries ofthe intestinal villi

and are carried by the hepatic portal system to the liver,where the

nonglucose sugars are converted to glucose.Glucose enters the cells

through facilitated diffusion. The rate of glucose transport into

most types of cells is greatly inﬂuenced by insulin and may in-

crease 10-fold in its presence.

Type I DiabetesMellitus

In patientswith type I diabetes mellitus, insulin is lacking, and

insufﬁcientglucose is transported into the cells ofthe body. As a result,

the cellsdo not have enough energy for normal function, blood glucose

levelsbecome signiﬁcantly elevated, and abnormal amounts of glucose

are released into the urine. Thiscondition is discussed more fullyin

chapter 18.

Lipids

Lipidsare molecules that are insoluble or only slightly soluble in wa-

ter.They include tr iglycerides,phospholipids, cholesterol, steroids,

and fat-soluble vitamins. Triglycerides(t rı¯-glis⬘er-ı¯dz), also called

triacylglycerol (trı¯-as⬘il-glis⬘er-ol),consist of three fatty acids and

one glycerol molecule covalently bound together.The ﬁrst step in

lipid digestion is emulsiﬁcation (e¯-mu˘l⬘si-ﬁ-ka¯⬘shu˘ n), which is the

transformation of large lipid droplets into much smaller droplets.

The enzymes that digest lipids are water-soluble and can digest the

lipids only by acting at the surface ofthe droplets. The emulsiﬁcation

process increases the surface area ofthe lipid exposed to the digestive

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24. Digestive System

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Chapter 24 Digestive System 897

enzymes by decreasing the droplet size. Emulsiﬁcation is accom-

plished by bile saltssecreted by the liver and stored in the gallbladder.

Lipase (lip⬘a¯s) digests lipid molecules (see table 24.4). The

vast majority oflipase is secreted by the pancreas. A minor amount

oflingual lipase is secreted in the oral cavity, is swallowed with the

food,and digests a small amount (<10%) of lipid in the stomach.

The stomach also produces very small amounts of gastric lipase.

The primary products of lipase digestion are free fatty acids and

glycerol.Cholesterol and phospholipids also constitute part of the

lipid digestion products.

Table 24.4

Carbohydrates Proteins Lipids

Digestion of the Three Major Food Types

Mouth (Salivary Glands)

Stomach

Duodenum (Pancreas)

Lining of Small Intestine

Salivary

amylase

Pancreatic

amylase

Disaccharides

Trypsin

Chymotrypsin

Carboxypeptidase

Gastric

amylase and

gelatinase

Pepsin Lingual lipase

Gastric lipaseDipeptides

Polypeptides

Polysaccharides

Disaccharides

Lactase

Sucrase

Maltase

Isomaltase

Monosaccharides Amino acids

Dipeptides

Glycerol

Fatty acids

Tripeptides

Lipase

Esterase

Aminopeptidase

Peptidase

1. Monosaccharides are absorbed by secondary active

transport into intestinal epithelial cells.

2. Monosaccharides move out of intestinal epithelial cells by

facilitated diffusion.

3.They enter the capillaries of the intestinal villi and are

carried through the hepatic portal vein to the liver.

Intestinal

epithelial cell

Capillary Lacteal

Monosaccharides

Secondary

active transport

To liver

Facilitated

diffusion

ProcessFigure 24.27

Monosaccharide Transport

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Once lipids are digested in the intestine,bile salts aggregate

around the small droplets to form micelles(mi-selz⬘,mı¯-selz⬘;a

small morsel;ﬁgure 24.28). The hydrophobic ends of the bile salts

are directed toward the free fatty acids,cholesterol, and glycerides

at the center of the micelle;and the hydrophilic ends are directed

outward toward the water environment.When a micelle comes

into contact with the epithelial cells ofthe small intestine, the con-

tents ofthe micelle pass by means of simple diffusion through the

plasma membrane ofthe epithelial cells.

CysticFibrosis

Cysticﬁbrosisis a hereditary disorder that occurs in 1 of every 2000

birthsand affects 33,000 people in the United States; it’s the most

common lethalgenetic disorder among Caucasians. The mostcritical

effectsof the disease, accounting for 90% of the deaths, are on the

respiratorysystem. Several other problems occur, however, in affected

people. Because the disease isa disorder in chloride ion transport

channelproteins, which affects chloride transportand, as a result,

movementof water, all exocrine glands are affected. The buildup ofthick

mucusin the pancreatic and hepatic ducts causes blockage ofthe ducts

so thatbile salts and pancreatic digestive enzymes are prevented from

reaching the duodenum. Asa result, fats and fat-soluble vitamins, which

require bile saltsto form micelles and which cannot be adequately

digested withoutpancreatic enzymes, are not well digested and

absorbed. The patientsuffers from vitamin A, D, E, and K deﬁciencies,

which resultin conditions like night blindness, skin disorders, rickets,

and excessive bleeding. Therapyincludesadministering the missing

vitaminsto the patient and reducing dietary fat intake.

Lipid Transport

Within the smooth endoplasmic reticulum ofthe intestinal epithe-

lial cells,free fatty acids are combined with glycerol molecules to

form triglycerides.Proteins synthesized in the epithelial cells attach

Part4 Regulationsand Maintenance898

to droplets oftriglycerides, phospholipids, and cholesterol to form

chylomicrons(kı¯-lo¯-mi⬘kronz; small particles in the chyle, or fat-

ﬁlled lymph).The chylomicrons leave the epithelial cells and enter

the lacteals ofthe ly mphatic system within the villi.Chylomicrons

enter the lymphatic capillaries rather than the blood capillaries be-

cause the lymphatic capillaries lack a basement membrane and are

more permeable to large particles like chylomicrons (about 0.3

mm in diameter). Chylomicrons are about 90% triglyceride,5%

cholesterol,4% phospholipid, and 1% protein (ﬁgure 24.29). They

are carried through the lymphatic system to the bloodstream and

then by the blood to adipose tissue. Before entering the adipose

cells,triglyceride is broken back down into fatty acids and glycerol,

which enter the fat cells and are once more converted to triglyc-

eride. Triglycerides are stored in adipose tissue until an energy

source is needed elsewhere in the body.In the liver, the chylomi-

cron lipids are stored,converted into other molecules, or used as

energy.The chylomicron remnant, minus the triglyceride, is con-

veyed through the circulation to the liver,where it is broken up.

Because lipids are either insoluble or only slightly soluble in

water,they are transported through the blood in combination with

proteins,which are water-soluble.Lipids combined with proteins are

called lipoproteins. Chylomicrons are one type of lipoprotein.

Other lipoproteins are referred to as high- or low-density lipopro-

teins.Density describes the compactness of a substance and is the ra-

tio of mass to volume.Lipids are less dense than water and tend to

ﬂoat in water.Proteins, which are denser than water,tend to sink in

water.A lipoprotein with a high lipid content has a very low density,

whereas a lipoprotein with a high protein content has a relatively

high density.Chylomicrons, which are made up of 99% lipid and

only 1% protein, have an extremely low density.The other major

transport lipoproteins are very low-density lipoprotein (VLDL),

which is 92% lipid and 8% protein,low-density lipoprotein (LDL),

which is 75% lipid and 25% protein,and high-density lipoprotein

(HDL),which is 55% lipid and 45% protein (see ﬁgure 24.29).

1. Bile salts surround fatty acids and glycerol to form

micelles.

2. Micelles attach to the plasma membranes of intestinal

epithelial cells, and the fatty acids and glycerol pass

by simple diffusion into the intestinal epithelial cells.

3. Within the intestinal epithelial cell, the fatty acids and

glycerol are converted to triglyceride; proteins coat the

triglyceride to form chylomicrons, which move out of

the intestinal epithelial cells by exocytosis.

4. The chylomicrons enter the lacteals of the intestinal

villi and are carried through the lymphatic system to

the general circulation.

Simple

diffusion

Triglyceride

Intestinal

epithelial cell

Protein coat

LactealCapillary

Fatty acids

and glycerol

Micelles contact

epithelial plasma

membrane

Exocytosis

Lymphatic

system

Chylomicron

Bile

salt

Micelle

ProcessFigure 24.28

Lipid Transport

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Chapter 24 Digestive System 899

About 15% of the cholesterol in the body is ingested in the

food we eat,and the remaining 85% is manufactured in the cells of

the body,mostly in the liver and intestinal mucosa.Most of the lipid

taken into or manufactured in the liver leaves the liver in the form

ofVLDL. Most of the triglycerides are removed from the VLDL to

be stored in adipose tissue and,as a result, VLDL becomes LDL.

The cholesterol in LDL is critical for the production of

steroid hormones in the adrenal cortex and the production ofbile

acids in the liver.It’s also an important component of plasma

membranes.LDL is delivered to cells of various tissues through the

circulation. Cells have LDL receptorsin “pits” on their surfaces,

which bind the LDL.Once LDL is bound to the receptors, the pits

on the cell surface become endocytotic vesicles, and the LDL is

taken into the cell by receptor-mediated endocytosis (ﬁgure 24.30).

Each ﬁbroblast,as an example of a tissue cell, has 20,000–50,000

LDL receptors on the surface.Those receptors are conﬁned to cell

surface pits, however,which occupy only 2% of the cell surface.

Once inside the cell,the endocytotic vesicle combines with a lyso-

some,and the LDL components are separated for use in the cell.

Cells not only take in cholesterol and other lipids from LDLs,

but they also make their own cholesterol.When the combined in-

take and manufacture ofcholesterol exceeds a cell’s needs, a nega-

tive-feedback system functions,which reduces the amount of LDL

receptors and cholesterol manufactured by the cell.Excess lipids

are also packaged into HDLs by the cells. These are transported

back to the liver for recycling or disposal.

Phospholipid (4%)

Triglyceride (90%)

Cholesterol (5%)

Protein (1%)

Phospholipid (18%)

Protein (8%)

Cholesterol (14%)

Triglyceride (60%)

Phospholipid (20%)

Protein (25%)

Cholesterol (45%)

Triglyceride (10%)

Phospholipid (30%)

Protein (45%)

Cholesterol (20%)

Triglyceride (5%)

Chylomicron

Very low-density lipoprotein

(VLDL)

Low-density lipoprotein

(LDL)

High-density lipoprotein

(HDL)

Figure 24.29

Lipoproteins

LDL

LDL receptor

"Pit" on cell surface

Cells have pits on the surface, which contain LDL receptors.

LDL

LDL receptor

LDL binds to the LDL receptors in the pits.

The LDL, bound to LDL receptors, is taken into the cell by endocytosis.

LDL

LDL receptor

Endocytotic vesicle

Figure 24.30

Transportof LDL into Cells

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Cholesteroland Coronary Heart Disease

Cholesterolis a major component of atherosclerotic plaques. The level of

plasma cholesterolis positivelylinked to coronary heart disease (CHD).

Cholesterollevels of over 200 mg/100 mL increase the riskof CHD. Other

riskfactors, which are additive to high cholesterollevels, are hypertension,

diabetesmellitus, cigarette smoking, and low plasma HDLlevels. Low HDL

levelsare linked to obesity, and weight reduction increasesHDL levels.

Aerobicexercise can decrease LDLlevels and increase HDL levels. Ingestion

ofsaturated fatty acids raisesplasma cholesterol levels by stimulating LDL

production and inhibiting LDLreceptor production, which would enhance

HDLproduction and cholesterol clearance. Ingestion ofunsaturated fatty

acidslowers plasma cholesterol levels. Replacing fatsby carbohydrates in

the dietcan also reduce blood cholesterol levels. The American Heart

Association recommendsthat no more than 30% of an adult’stotal caloric

intake should be from fatsand that only10% be from saturated fats. Our

totalcholesterol intake should be no more than 300 mg/day. People

should eatno more than 7 ounces of meatper day, and that should be

chicken, ﬁsh, or lean meat. We should eatonlytwo eggs per week and drink

milkwith 1% or less butter fat. Young children, however, require more fats

in their dietto stimulate normal brain development, and whole milk is

recommended in their diets. Some evidence also existsthatseverely

reducing plasma cholesterollevels, below about 180 mg/100 mLmay be

harmfulin adults. Cholesterol is required for normal membrane structure in

cells. Abnormallylow cholesterollevels, may lead to weakened blood

vesselwalls and an increased riskfor cerebral hemorrhage.

A smallnumber of people have a genetic disorder in the production

or function ofLDL receptors, resulting in poor LDL clearing, and, asa

result, have whatis called familialhypercholesterolemia. These people

commonlydevelop premature atherosclerosis and are prone to die atan

earlyage of a heart attack. In some of these disorders, the LDLreceptor is

notproduced. In other cases, the receptor isproduced, but it has a lower-

than-normalafﬁnity for LDL. In yet other cases, the receptor bindsto LDL

butthe receptor–LDL complex isnot taken into the cell by endocytosis.

Part4 Regulationsand Maintenance900

Proteins

Proteinsare taken into the body from a number of dietary sources.

Pepsinsecreted by the stomach (see table 24.3) catalyzes the cleav-

age of covalent bonds in proteins to produce smaller polypeptide

chains.Gastric pepsin digests as much as 10%–20% of the total in-

gested protein.Once the proteins and polypeptide chains leave the

stomach,proteolytic enzymes produced in the pancreas continue

the digestive process and produce small peptide chains.These are

broken down into dipeptides,tripeptides, and amino acids by pep-

tidasesbound to the microvilli of the small intestine. Each pepti-

dase is speciﬁc for a certain peptide chain length or for a certain

peptide bond.

Dipeptides and tripeptides enter intestinal epithelial cells

through a group of related carrier molecules, by a cotransport

mechanism, powered by a sodium ion concentration gradient

similar to that described for glucose. Separate molecules trans-

port basic, acidic, and neutral amino acids into the epithelial

cells.Acidic and most neutral amino acids are cotransported with

a sodium ion gradient, whereas basic amino acids enter the ep-

ithelial cells by facilitated diffusion. The total amount of each

amino acid that enters the intestinal epithelial cells as dipeptides

or tripeptides is considerably more than the amount that enters

as single amino acids. Once inside the cells, dipeptidases and

tripeptidases split the dipeptides and tripeptides into their com-

ponent amino acids. Individual amino acids then leave the ep-

ithelial cells and enter the hepatic portal system,which transports

them to the liver (ﬁgure 24.31).The amino acids may be modiﬁed

in the liver or released into the bloodstream and distributed

throughout the body.

Amino acids are actively transported into the various cells of

the body.This transport is stimulated by growth hormone and in-

sulin.Most amino acids are used as building blocks to form new pro-

teins (see chapter 2),but some amino acids may be used for energy.

Secondary

active transport

Intestinal

epithelial cell

Capillary Lacteal

To liver

Amino acids

Active

transport

1. Amino acids are absorbed by secondary active transport

into intestinal epithelial cells.

2.Amino acids move out of intestinal epithelial cells by active

transport.

3.They enter the capillaries of the intestinal villi and are

carried through the hepatic portal vein to the liver.

2 3

ProcessFigure 24.31

Amino Acid Transport

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24. Digestive System

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Chapter 24 Digestive System 901

Water

About 9 L ofwater enters the digestive tract each day,of which about

92% is absorbed in the small intestine,and another 6%–7% is ab-

sorbed in the large intestine (ﬁgure 24.32).Water can move in either

direction across the wall of the small intestine. Osmotic gradients

across the epithelium determine the direction ofits diffusion. When

the chyme is dilute,water is absorbed by osmosis across the intestinal

wall into the blood.When the chyme is very concentrated and con-

tains very little water,water moves by osmosis into the lumen of the

small intestine.As nutrients are absorbed in the small intestine, its

osmotic pressure decreases;as a consequence, water moves from the

intestine into the surrounding extracellular ﬂuid.Water in the extra-

cellular ﬂuid can then enter the circulation.Because of the osmotic

gradient produced as nutrients are absorbed in the small intestine,

92% of the water that enters the small intestine by way of the oral

cavity,stomach, or intestinal secretions is reabsorbed.

Ions

Active transport mechanisms for sodium ions are present within

the epithelial cells of the small intestine. Potassium,calcium,

magnesium, and phosphate are also actively transported. Chlo-

rideions move passively through the intestinal wall of the duode-

num and the jejunum following the positively charged sodium

ions,but chloride ions are actively transported from the ileum. Al-

though calcium ions are actively transported along the entire

length of the small intestine,v itamin D is required for that trans-

port process.The absor ption of calcium is under hormonal con-

trol, as is its excretion and storage. Parathyroid hormones,

calcitonin,and vitamin D all play a role in regulating blood levels

ofcalcium in the circulatory system (see chapters 6, 18, and 27).

49. Describe the mechanism of absorption and the route of

transportfor water-soluble and lipid-soluble molecules.

50. Describe the enzymatic digestion of carbohydrates, lipids,

and proteins, and listthe breakdown products of each.

51. Explain how fats are emulsiﬁed. Describe the role of

micelles, chylomicrons, VLDLs, LDLs, and HDLsin the

absorption and transportof lipids in the body.

52. Explain how dipeptides and tripeptides enter intestinal

epithelial cells.

53. Describe the movement of water through the intestinal wall.

54. When and where are various ions absorbed?

Effects of Aging on the

Digestive System

Objective

■ Describe the effects of aging on the digestive tract.

As a person ages,gradual changes occur throughout the en-

tire digestive tract. The connective tissue layers ofthe digestive

tract,the submucosa and serosa, tend to thin. The blood supply to

the digestive tract decreases.There is also a decrease in the number

of smooth muscle cells in the muscularis, resulting in decreased

motility in the digestive tract. In addition,goblet cells within the

mucosa secrete less mucus.Glands along the digestive tract,such as

the gastric pits,the liver, and the pancreas, also tend to secrete less

with age.These changes by themselves don’t appreciably decrease

the function ofthe digestive system.

Through the years, however,the digestive tract, like the skin

and lungs,is directly exposed to materials from the outside environ-

ment.Some of those substances can cause mechanical damage to the

digestive tract and others may be toxic to the tissues.Because the con-

nective tissue ofthe digestive tract becomes thin with age and because

the protective mucus covering is reduced,the digestive tract ofelderly

people becomes less and less protected from these outside inﬂuences.

In addition,the mucosa of elderly people tends to heal more slowly

following injury.The liver’s ability to detoxify certain chemicals tends

to decline,the ability of the hepatic phagocytic cells to remove partic-

ulate contaminants decreases,and the liver’s ability to store glycogen

decreases.These problems are increased in people who smoke.

This overall decline in the defenses ofthe digestive tract with

advancing age leaves elderly people more susceptible to infections

and to the effects of toxic agents.Elderly people are more likely to

develop ulcerations and cancers of the digestive tract. Colorectal

cancers,for example, are the second leading cause of cancer deaths

in the United States, with an estimated 135,000 new cases and

57,000 deaths each year.

Ingestion

(2 L)

Salivary gland secretions

(1 L)

Gastric

secretions

(2 L)

Pancreatic

secretions

(1.2 L)

92%

absorbed in the

small intestine

Bile

(0.7 L)

Small intestine

secretions

(2 L)

1% in

feces

(Water in feces ⫽ ingested ⫹ secreted ⫺ absorbed)

Ingestion or

secretion

Absorption

6 – 7%

absorbed in the

large intestine

Figure 24.32

Fluid Volumesin the Digestive Tract

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IV. Regulations and

Maintenance

24. Digestive System

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Clinical Focus IntestinalDisorders

InﬂammatoryBowel Disease

Inﬂammatorybowel disease (IBD) is the

generalname given to either Crohn’s dis-

ease or ulcerative colitis. IBD occursat a

rate in Europe and North America of ap-

proximately4 to 8 new cases per 100,000

people per year, which is much higher

than in Asia and Africa. Malesand females

are affected about equally. IBD is ofun-

known cause, but infectious, autoim-

mune, and hereditary factors have been

implicated. Crohn’s disease involves lo-

calized inflammatory degeneration that

may occur anywhere along the digestive

tractbut most commonly involves the dis-

talileum and proximal colon. The degener-

ation involves the entire thicknessof the

digestive tractwall. The intestinal wall of-

ten becomes thickened, constricting the

lumen, with ulcerationsand ﬁssures in the

damaged areas. The disease causesdiar-

rhea, abdominal pain, fever, and weight

loss. Treatment centers around anti-

inﬂammatorydrugs, but other treatments,

including avoiding foods that increase

symptoms and even surgery, are em-

ployed. Ulcerative colitisis limited to the

mucosa of the large intestine. The in-

volved mucosa exhibitsinﬂammation, in-

cluding edema, vascular congestion,

hemorrhage, and the accumulation of

plasma cells, lymphocytes, neutrophils,

and eosinophils. Patientsmay experience

abdominal pain, fever, malaise, fatigue,

and weight loss, as well asdiarrhea and

hemorrhage. In rare cases, severe diar-

rhea and hemorrhage mayrequire transfu-

sions. Treatment includes the use of

anti-inflammatory drugs and, in some

cases, avoiding foods that increase

symptoms.

Irritable BowelSyndrome

Irritable bowelsyndrome (IBS) is a disor-

der of unknown cause in which intestinal

mobility is abnormal. The disorder ac-

countsfor over half of all referrals to gas-

troenterologists. Male and female

children are affected equally, butadult fe-

males are affected twice as often as

males. IBSpatients experience abdominal

pain mainlyin the left lower quadrant, es-

peciallyafter eating. They also have alter-

nating boutsof constipation and diarrhea.

There isno speciﬁc histopathology in the

digestive tractsof IBS patients. There are

no anatomicabnormalities, no indication

of infection, and no sign of metabolic

causes. Patients with IBS appear to ex-

hibitgreater-than-normal levelsof psycho-

logical stress or depression and show

increased contractions of the esophagus

and smallintestine during times of stress.

There is a high familial incidence. Some

patients might present with a history of

traumaticevents such as physical or sex-

ualabuse. Treatments include psychiatric

counseling and stressmanagement, diets

with increased fiber and limited gas-

producing foods, and loose clothing. In

some patients, drugs that reduce

parasympatheticstimulation of the diges-

tive system maybe useful.

Malabsorption Syndrome

Malabsorption syndrome (sprue) is a

spectrum of disorders of the small intes-

tine that results in abnormal nutrient ab-

sorption. One type of malabsorption

results from an immune response to

gluten, which ispresent in certain types of

grains and involves the destruction of

newly formed epithelial cellsin the intes-

tinal glands. These cellsfail to migrate to

the villi surface, the villi become blunted,

and the surface area decreases. As a re-

sult, the intestinalepithelium is less capa-

ble ofabsorbing nutrients. Another type of

malabsorption (called tropicalmalabsorp-

tion) isapparently caused by bacteria, al-

though no specific bacterium has been

identiﬁed.

Gastroesophageal reﬂux disorder (GERD) increases with ad-

vancing age.It is probably the main reason that elderly people take

antacids,H

antagonists,and proton pump inhibitors. Disorders that

are not necessarily age-induced,such as hiatal hernia and irregular or

inadequate esophageal motility,may be worsened by the effects of ag-

ing,because of a general decreased motility in the digestive tract.

The enamel on the surface of elderly people’s teeth becomes

thinner with age and may expose the underlying dentin.In addition,

the gingiva covering the tooth root recedes, exposing additional

dentin.Exposed dentin may become painful and change the person’s

eating habits.Many elderly people also lose teeth, which can have a

marked effect on eating habits unless artiﬁcial teeth are provided.

The muscles of mastication tend to become weaker and,as a result,

older people tend to chew their food less before swallowing.

Part4 Regulationsand Maintenance902

Another complication of the age-related changes in the di-

gestive system is the way medications and other chemicals are ab-

sorbed from the digestive tract. The decreased mucous covering

and the thinned connective tissue layers allow chemicals to pass

more readily from the digestive tract into the circulatory system.

However,a decline in the blood supply to the digestive tract hin-

ders the absorption ofsuch chemicals. Drugs administered to treat

cancer,which occurs in many elderly people, may irritate the mu-

cosa ofthe digestive tract, resulting in nausea and loss of appetite.

55. What is the general effect of aging on digestive tract

secretions?

56. What are the effects of the overall decline in the defenses of

the digestive tractwith advancing age?

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IV. Regulations and

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24. Digestive System

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Chapter 24 Digestive System 903

Enteritis

Enteritisis any inﬂammation of the intes-

tines that can result in diarrhea, dehydra-

tion, fatigue, and weightloss. It may result

from an infection, chemical irritation, or

from some unknown cause. Regionalen-

teritis,or Crohn’s disease, is a local enteri-

tis of unknown cause characterized by

patchy, deep ulcersdeveloping in the intes-

tinal wall, usually in the distalend of the

ileum. The disease resultsin overprolifera-

tion of connective tissue and invasion of

lymphatic tissue into the involved area,

with a subsequentthickening of the intes-

tinalwall and narrowing of the lumen.

Colitisisan inﬂammation of the colon.

Colon Cancer

Colon canceris the second leading cause of

cancer-related deathsin the United States

and accountsfor 55,000 deaths a year. Sus-

ceptibility to colon cancer can be familial;

however, a correlation existsbetween colon

cancer and dietslow in ﬁber and high in fat.

People who eatbeef, pork, or lamb dailyhave

2.5 timesthe risk of developing colon cancer

compared to people who eat these meats

lessthan once per month. Eating processed

meats increases the risk by an additional

50%–100%. Ingesting calcium in the form of

calcium carbonate antacid tabletsat twice

the recommended dailyallowances may pre-

vent75% of colon cancers. Greatly increased

calcium levelsmay also cause constipation.

A gene for colon cancer maybe present

in as many as 1 in 200 people, making

colon cancer one ofthe most common in-

herited diseases. Nine differentgenes have

been found to be associated with colon

cancer. Mostof those genes are involved in

cellregulation, that is, keeping cell growth

in check, butone gene mutation results in a

high degree of genetic instability. Asa re-

sultof this mutation, the DNA is not copied

accuratelyduring cell division of the colon

cancer cells, causing wholesale errorsand

mutations throughout the genome (allthe

genes). Such genetic instability has been

identiﬁed in 13% ofsporadic (not occurring

in families) colon cancer. Screening for

colon cancer includestesting the stool for

blood content and performing a

colonoscopy, which allowsthe physician to

see into the colon.

Constipation

Constipationis the slow movementof feces

through the large intestine. The fecesoften

become dryand hard because of increased

ﬂuid absorption during the extended time

they are retained in the large intestine. In

the United States, 2.5 million doctor visits

occur each year from people complaining of

constipation, and $400 million dollars is

spenteach year on laxatives.

Constipation often resultsafter a pro-

longed time ofinhibiting normal defecation

reﬂexes. A change in habits, such astravel,

dehydration, depression, disease, meta-

bolic disturbances, certain medications,

pregnancy, or dependencyon laxatives can

allcause constipation. Irritable bowel syn-

drome, also called spasticcolon, which is

ofunknown cause but is stress-related, can

also cause constipation. Constipation can

also occur with diabetes, kidney failure,

colon nerve damage, or spinalcord injuries

or asthe result of an obstructed bowel; of

greatestconcern, the obstruction could be

caused bycolon cancer. Chronic constipa-

tion can resultfrom the slow movement of

feces through the entire colon, in justthe

distalpart (descending colon and rectum),

or in just the rectum. Interestingly, in one

large study of people who claimed to be

suffering from chronic constipation, one-

third were found to have normalmovement

of feces through the large intestine. Defe-

cation frequency was often normal. Many

of those people were suffering from psy-

chologic distress, anxiety, or depression

and just thought theyhad abnormal defe-

cation frequencies.

Anatomyof the Digestive System

(p. 860)

1. The digestive system consists ofa digestive tube and its associated

accessory organs.

2. The digestive system consists ofthe oral cavity, pharynx, esophagus,

stomach,small intestine, large intestine, and anus.

3. Accessory organs such as the salivary glands,liver, gallbladder,and

pancreas are located along the digestive tract.

Functionsof the Digestive System

(p. 860)

The functions of the digestive system are ingestion,mastication, propul-

sion,mixing, secretion, digestion, absorption, and elimination.

Histologyof the Digestive Tract

(p. 862)

The digestive tract is composed offour tunics: mucosa, submucosa, mus-

cularis,and serosa or adventitia.

Mucosa

The mucosa consists of a mucous epithelium, a lamina propria,and a

muscularis mucosae.

Submucosa

The submucosa is a connective tissue layer containing the submucosal

plexus (part ofthe enteric plexus), blood vessels, and small glands.

SUMMARY

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

While on vacation in Mexico, Mr. T wasshopping with hiswife when he

started to experience sharp painsin his abdominal region (ﬁgure B).

He also began to feel hot and sweaty and feltan extreme urge to

defecate. His wife quicklylooked up the word toilet in their handy

Spanish–English pockettraveldictionary, and Mr. T anxiously inquired

ofa local resident where the nearest facility could be found. Once the

immediate need wastaken care of, Mr. and Mrs. T went back to their

hotelroom, where they remained while Mr. T recovered. Over the next

2 days, hisstools were frequent and watery. He also vomited a couple

oftimes. Because they were in a foreign country, Mr. T didn’t consulta

physician. He rested, tookplenty of ﬂuids, and was feeling much bet-

ter, although a little weak, in a couple ofdays.

Background Information

Diarrhea isone of the most common complaints in clinical medicine

and affects more than half of the touristsin developing countries.

Diarrheais deﬁned as any change in bowel habits in which stool fre-

quencyor volume is increased or in which stool ﬂuidity is increased.

Diarrhea isnot itself a disease but is a symptom of a wide variety of

disorders. Normally, about600 mL of ﬂuid enters the colon each day

and all but150 mL is reabsorbed. The loss of more than 200 mL of

stoolper day is considered abnormal.

Mucussecretion by the colon increases dramatically in response

to diarrhea. Thismucus contains large quantities of bicarbonate ions,

which comesfrom the dissociation ofcarbonic acid into bicarbonate ions

(HCO

⫺

) and hydrogen (H

⫹

) ionswithin the blood supply to the colon.

The HCO

⫺

enter the mucussecreted by the colon, whereas the H

⫹

re-

main in the circulation and, asa result, the blood pH decreases. Thus, a

condition called metabolicacidosis can develop (see chapter 27).

Diarrhea in touristsusually results from the ingestion of food or

water contaminated with bacteria or bacterialtoxins. Acute diarrhea

isdeﬁned as lasting less than 2–3 weeks, and diarrhea lasting longer

than that is considered chronic. Acute diarrhea is usually self-

limiting, butsome forms of diarrhea can be fatal if not treated. Diar-

rhea resultsfrom either a decrease in ﬂuid absorption in the gut or an

increase in ﬂuid secretion. Some bacterialtoxins and other chemicals

can also cause an increase in bowelmotor activity. As a result, chyme

ismoved more rapidly through the digestive tract, fewer nutrients and

water are absorbed outof the small intestine, and more water enters

the colon. Symptomscan occur in aslittle as 1–2 hours after bacterial

toxinsare ingested to as long as 24 hours or more for some strains of

bacteria.

Systems Pathology

Diarrhea

Muscularis

1. The muscularis consists ofan inner layer of circular smooth muscle

and an outer layer oflongitudinal smooth muscle.

2. The myenteric plexus is between the two muscle layers.

Serosa or Adventitia

The serosa or adventitia forms the outermost layer ofthe digestive tract.

Regulation ofthe Digestive System

(p. 863)

1. Nervous,hormonal, and local chemical mechanisms regulate digestion.

2. Nervous regulation involves the enteric nervous system and CNS

reﬂexes.

3. The digestive tract produces hormones that regulate digestion.

4. Other chemicals are produced by the digestive tract that exercise

local control ofdigestion.

Part4 Regulationsand Maintenance904

Peritoneum

(p. 864)

1. The peritoneum is a serous membrane that lines the abdominal

cavity and organs.

2. Mesenteries are peritoneum that extends from the body wall to

many ofthe abdominal organs.

3. Retroperitoneal organs are located behind the peritoneum.

OralCavity

(p. 866)

1. The lips and cheeks are involved in facial expression,mastication,

and speech.

2. The roofof the or al cavity is divided into the hard and soft palates.

3. The tongue is involved in speech,taste, mastication,and swallowing.

• The intrinsic tongue muscles change the shape of the tongue, and

the extrinsic tongue muscles move the tongue.

• The anterior two-thirds of the tongue is covered with papillae,the

posterior one-third is devoid ofpapillae.

Figure B

ManyTourists Develop Diarrhea

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

Chapter 24 Digestive System 905

4. Twenty deciduous teeth are replaced by 32 permanent teeth.

• The types of teeth are incisors, canines, premolars, and molars.

• A tooth consists of a crown, a neck, and a root.

• The root is composed of dentin. Within the dentin of the root is

the pulp cavity,which is ﬁlled with pulp, blood vessels,and ner ves.

The crown is dentin covered by enamel.

• Periodontal ligaments hold the teeth in the alveoli.

5. The muscles ofmastication are the masseter, the temporalis, the

medial pterygoid,and the lateral pterygoid.

6. Salivary glands produce serous and mucous secretions.The three

pairs oflarge salivary glands are the parotid, submandibular, and

sublingual.

Pharynx

(p. 870)

The pharynx consists ofthe nasopharynx, oropharynx, and laryngopharynx.

Esophagus

(p. 870)

1. The esophagus connects the pharynx to the stomach.The upper and

lower esophageal sphincters regulate movement.

2. The esophagus consists ofan outer adventitia, a muscular layer

(longitudinal and circular),a submucosal layer (with mucous

glands),and a stratiﬁed squamous epithelium.

Swallowing

(p. 872)

1. During the voluntary phase ofdeg lutition,a bolus of food is moved

by the tongue from the oral cavity to the pharynx.

2. The pharyngeal phase is a reﬂex caused by stimulation ofstretch

receptors in the pharynx.

• The soft palate closes the nasopharynx, and the epiglottis and

vestibular folds close the opening into the larynx.

• Pharyngeal muscles move the bolus to the esophagus.

System Interactions

System Interactions with Digestive System

Integumentary Pallor occurs due to vasoconstriction of blood vessels in the skin, resulting from a decrease in blood fluid levels. Pallor and

sweating increase in response to abdominal pain and anxiety.

Muscular Muscular weakness may result due to electrolyte loss, metabolic acidosis, fever, and general malaise. The involuntary stimulus

to defecate may become so strong as to overcome the voluntary control mechanisms.

Nervous Local reflexes in the colon respond to increased colon fluid volumeby stimulating mass movements and the defecation reflex.

Abdominal pain, much of which is felt asreferred pain, can occur as the result of inflammation and distention of the colon.

Decreased function is due to electrolyte loss. Reduced blood fluid levels stimulate a sensation of thirst in the CNS.

Endocrine A decrease in extracellular fluid volume, due to the loss of fluid in the feces, stimulates the release of hormones (antidiuretic

hormone from the posterior pituitary and aldosterone from the adrenal cortex) that increase water retention and electrolyte

reabsorption in the kidney. In addition, decreased extracellular fluid volume and anxiety result in increased release of

epinephrine and norepinephrine from the adrenal medulla.

Cardiovascular Movement of extracellular fluid into the colon results in a decreased blood volume. The reduced blood volume activates the

baroreceptor reflex, antidiuretic hormone release, the renin-angiotensin-aldosterone mechanism, and the fluid shift

mechanism, which all function to elevate blood volume or increase blood pressure.

Lymphatic and immune White blood cells migrate to the colon in response to infection and inflammation. In the case of bacterial diarrhea, the immune

response is initiated to begin production of antibodies against bacteria and bacterial toxins.

Respiratory As the result of reduced blood pH, the rate of respiration increases to eliminate carbon dioxide, which helps eliminate excess H

⫹

Urinary A decrease in urine volume and an increase in urine concentration results from activation of the baroreceptor reflex, which

decreases blood flow to the kidney; antidiuretic hormone secretion, which increases water reabsorption in the kidney; and

aldosterone secretion, which increases electrolyte and water reabsorption in the kidney. After a period of approximately 24

hours, the kidney is activated to compensate for metabolic acidosis by increasing hydrogen ion secretion and bicarbonate

ion reabsorption.

The Effects of Diarrhea on Other Systems

In casesof short-term acute diarrhea, the infectious agent is sel-

dom identiﬁed. Nearlyany bacterial species is capable of causing diar-

rhea. Some types of bacterial diarrhea include severe vomiting,

whereasothers do not. Some bacterial toxins also induce fever. Some

virusesand amebic parasites can also cause diarrhea. In most cases,

laboratoryanalysis of food or stool is necessary to identify the causal

organism. In casesof mild diarrhea away from home, laboratory evalu-

ation isnot practical, and empiric therapyis usually applied. Fluids and

electrolytes must be replaced, and consumption ofﬂuids with elec-

trolytesis important. The diet should be limited to clear ﬂuids during at

leastthe ﬁrst day or so. Bismuth subsalicylate (Pepto-Bismol) or lop-

eramide (Imodium; exceptin cases of fever) may also be used to help

combatsecretory diarrhea. Milk and milk products should be avoided.

Breads, toast, rice, and baked ﬁsh or chicken can be added to the diet

with improvement. A normaldiet can be resumed after 2–3 days.

PREDICT

Predictthe effects of prolonged diarrhea.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

3. The esophageal phase is a reﬂex initiated by the stimulation of

stretch receptors in the esophagus.A wave of contraction

(peristalsis) moves the food to the stomach.

Stomach

(p. 872)

Anatomyof the Stomach

The openings ofthe stomach are the gastroesophageal (to the esophagus)

and the pyloric (to the duodenum).

Histologyof the Stomach

1. The wall ofthe stomach consists of an external serosa, a muscle layer

(longitudinal,circular, and oblique), a submucosa,and simple

columnar epithelium (surface mucous cells).

2. Rugae are the folds in the stomach when it is empty.

3. Gastric pits are the openings to the gastric glands which contain

mucous neck cells,parietal cells, chief cells, and endocrine cells.

Secretionsof the Stomach

1. Mucus protects the stomach lining.

2. Pepsinogen is converted to pepsin,which digests proteins.

3. Hydrochloric acid promotes pepsin activity and kills

microorganisms.

4. Intrinsic factor is necessary for vitamin B

absorption.

5. The sight,smell, taste, or thought of food initiates the cephalic

phase.Nerve impulses from the medulla stimulate hydrochloric

acid,pepsinogen, gastrin, and histamine secretion.

6. Distention ofthe stomach, which stimulates gastrin secretion and

activates CNS and local reﬂexes that promote secretion,initiates the

gastric phase.

7. Acidic chyme,which enters the duodenum and stimulates neuronal

reﬂexes and the secretion ofhormones that inhibit gastric

secretions,initiates the intestinal phase.

Movementsof the Stomach

1. The stomach stretches and relaxes to increase volume.

2. Mixing waves mix the stomach contents with stomach secretions to

form chyme.

3. Peristaltic waves move the chyme into the duodenum.

4. Gastrin and stretching ofthe stomach stimulate stomach emptying.

5. Chyme entering the duodenum inhibits movement through

neuronal reﬂexes and the release ofhormones.

SmallIntestine

(p. 881)

1. The small intestine is divided into the duodenum,jejunum, and ileum.

2. The wall ofthe small intestine consists of an external serosa, muscles

(longitudinal and circular),submucosa, and simple columnar

epithelium.

3. Circular folds,villi, and microvilli greatly increase the surface area of

the intestinal lining.

4. Absorptive,goblet, and endocrine cells are in intestinal glands.

Duodenal glands produce mucus.

Secretionsof the Small Intestine

1. Mucus protects against digestive enzymes and stomach acids.

2. Digestive enzymes (disaccharidases and peptidases) are bound to

the intestinal wall.

3. Chemical or tactile irritation,vagal stimulation, and secretin

stimulate intestinal secretion.

Movementin the Small Intestine

1. Segmental contractions mix intestinal contents.Peristaltic

contractions move materials distally.

2. Stretch ofsmooth muscles, local reﬂexes, and the parasympathetic

nervous system stimulate contractions.Distention of the cecum

initiates a reﬂex that inhibits peristalsis.

Part4 Regulationsand Maintenance906

Liver

(p. 884)

Anatomyof the Liver

1. The liver has four lobes:right, left, caudate, and quadrate.

2. The liver is divided into lobules.

• The hepatic cords are composed of columns of hepatocytes that

are separated by the bile canaliculi.

• The sinusoids are enlarged spaces ﬁlled with blood and lined with

endothelium and hepatic phagocytic cells.

Histologyof the Liver

1. The portal triads supply the lobules.

• The hepatic arteries and the hepatic portal veins bring blood to the

lobules and empty into the sinusoids.

• The sinusoids empty into central veins, which join to form the

hepatic veins,which leave the liver.

• Bile canaliculi converge to form hepatic ducts,which leave the liver.

2. Bile leaves the liver through the hepatic duct system.

• The hepatic ducts receive bile from the lobules.

• The cystic duct from the gallbladder joins the hepatic duct to form

the common bile duct.

• The common bile duct joins the pancreatic duct at the point at

which it empties into the duodenum.

Functionsof the Liver

1. The liver produces bile,which contains bile salts that emulsify fats.

Secretin increases bile production.

2. The liver stores and processes nutrients,produces new molecules,

and detoxiﬁes molecules.

3. Hepatic phagocytic cells phagocytize red blood cells,bacteria, and

other debris.

4. The liver produces blood components.

Gallbladder

(p. 889)

1. The gallbladder is a small sac on the inferior surface ofthe liver.

2. The gallbladder stores and concentrates bile.

3. Cholecystokinin stimulates gallbladder contraction.

Pancreas

(p. 890)

1. The pancreas is an endocrine and an exocrine gland.Its exocrine

function is the production ofdigestive enzymes.

2. The pancreas is divided into lobules that contain acini.The acini

connect to a duct system that eventually forms the pancreatic duct,

which empties into the duodenum.

4. Secretin stimulates the release ofa watery bicarbonate solution that

neutralizes acidic chyme.

5. Cholecystokinin and the vagus nerve stimulate the release of

digestive enzymes.

Large Intestine

(p. 890)

Anatomyof the Large Intestine

1. The cecum forms a blind sac at the junction ofthe small and large

intestines.The vermiform appendix is a blind tube off the cecum.

2. The ascending colon extends from the cecum superiorly to the right

colic ﬂexure.The transverse colon extends from the right to the left

colic ﬂexure.The descending colon extends inferiorly to join the

sigmoid colon.

3. The sigmoid colon is an S-shaped tube that ends at the rectum.

4. Longitudinal smooth muscles ofthe large intestine wall are

arranged into bands called teniae coli that contract to produce

pouches called haustra.

5. The mucosal lining ofthe large intestine is simple columnar

epithelium with mucus-producing crypts.

6. The rectum is a straight tube that ends at the anus.

7. An internal anal sphincter (smooth muscle) and an external anal

sphincter (skeletal muscle) surround the anal canal.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

Chapter 24 Digestive System 907

Secretionsof the Large Intestine

1. Mucus provides protection to the intestinal lining.

2. Epithelial cells secrete bicarbonate ions.Sodium is absorbed by

active transport,and water is absorbed by osmosis.

3. Microorganisms are responsible for vitamin K production,gas

production,and much of the bulk of feces.

Movementin the Large Intestine

1. Segmental movements mix the colon’s contents.

2. Mass movements are strong peristaltic contractions that occur three

to four times a day.

3. Defecation is the elimination offeces. Reﬂex activity moves feces

through the internal anal sphincter.Voluntary activity regulates

movement through the external anal sphincter.

Digestion, Absorption, and Transport

(p. 896)

1. Digestion is the breakdown oforganic molecules into their

component parts.

2. Absorption and transport are the means by which molecules are moved

out ofthe digestive tract and are distributed throughout the body.

3. Transportation occurs by two different routes.

• Water,ions, and water-soluble products of digestion are

transported to the liver through the hepatic portal system.

• The products of lipid digestion are transported through the

lymphatic system to the circulatory system.

Carbohydrates

1. Carbohydrates consist ofstarches, glycogen, sucrose, lactose,

glucose,and fructose.

2. Polysaccharides are broken down into monosaccharides by a

number ofdifferent enzymes.

3. Monosaccharides are taken up by intestinal epithelial cells by active

transport or by facilitated diffusion.

4. The monosaccharides are carried to the liver where the nonglucose

sugars are converted to glucose.

5. Glucose is transported to the cells that require energy.

6. Glucose enters the cells through facilitated diffusion.

7. Insulin inﬂuences the rate ofglucose transport.

Lipids

1. Lipids include triglycerides,phospholipids, steroids, and fat-soluble

vitamins.

2. Emulsiﬁcation is the transformation oflarge lipid droplets into

smaller droplets and is accomplished by bile salts.

3. Lipase digests lipid molecules to form free fatty acids and glycerol.

4. Micelles form around lipid digestion products and move to

epithelial cells ofthe small intestine, where the products pass into

the cells by simple diffusion.

5. Within the epithelial cells,free fatty acids are combined with

glycerol to form triglyceride.

6. Proteins coat triglycerides,phospholipids, and cholesterol to form

chylomicrons.

7. Chylomicrons enter lacteals within intestinal villi and are carried

through the lymphatic system to the bloodstream.

8. Triglyceride is stored in adipose tissue,converted into other

molecules,or used as energy.

9. Lipoproteins include chylomicrons,VLDL, LDL,and HDL.

10. LDL transports cholesterol to cells,and HDL transports it from cells

to the liver.

11. LDLs are taken into cells by receptor-mediated endocytosis,which is

controlled by a negative-feedback mechanism.

Proteins

1. Pepsin in the stomach breaks proteins into smaller polypeptide

chains.

2. Proteolytic enzymes from the pancreas produce small peptide

chains.

3. Peptidases,bound to the microvilli of the small intestine, break

down peptides.

4. Amino acids are absorbed by cotransport,which requires transport

ofsodium.

5. Amino acids are transported to the liver,where the amino acids can

be modiﬁed or released into the bloodstream.

6. Amino acids are actively transported into cells under the

stimulation ofgrowth hormone and insulin.

7. Amino acids are used as building blocks or for energy.

Water

Water can move in either direction across the wall ofthe small intestine,

depending on the osmotic gradients across the epithelium.

Ions

1. Sodium,potassium, calcium, magnesium, and phosphate are

actively transported.

2. Chloride ions move passively through the wall ofthe duodenum

and jejunum but are actively transported from the ileum.

3. Calcium ions are actively transported,but vitamin D is required for

transport,and the transport is under hormonal control.

Effectsof Aging on the Digestive System

(p. 901)

The mucus layer,the connective tissue,the muscles, and the secretions all

tend to decrease as a person ages.These changes make an older person

more open to infections and toxic agents.

1. Which layer ofthe digestive tract is in direct contact with the food

that is consumed?

a. mucosa

b. muscularis

c. serosa

d. submucosa

2. The enteric plexus is found in the

a. submucosa layer.

b. muscularis layer.

c. serosa layer.

d. both a and b.

e. all ofthe above.

3. The tongue

a. holds food in place during mastication.

b. plays a major role in swallowing.

c. helps to form words during speech.

d. is a major sense organ for taste.

e. all ofthe above.

4. Dentin

a. forms the surface ofthe crown of the teeth.

b. holds the teeth to the periodontal ligaments.

c. is found in the pulp cavity.

d. makes up most of the structure of the teeth.

e. is harder than enamel.

REVIEW AND COMPREHENSION

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

5. The number ofpremolar deciduous teeth is

a. 0.

b. 2.

c. 4.

d. 8.

e. 12.

6. Which ofthese glands does not secrete saliva into the oral cavity?

a. submandibular glands

b. goblet glands

c. sublingual glands

d. parotid glands

7. The portion ofthe digestive tract in which digestion begins is the

a. oral cavity.

b. esophagus.

c. stomach.

d. duodenum.

e. jejunum.

8. During deglutition (swallowing),

a. movement offood results primarily from gravity.

b. the swallowing center in the medulla oblongata is activated.

c. food is pushed into the oropharynx during the pharyngeal phase.

d. the soft palate closes off the opening into the larynx.

9. The stomach

a. has large folds in the submucosa and mucosa called rugae.

b. has two layers of smooth muscle in the muscularis layer.

c. opening from the esophagus is the pyloric opening.

d. has an area closest to the duodenum called the fundus.

e. all ofthe above.

10. Which ofthese stomach cell types is not correctly matched with its

function?

a. surface mucous cells:produce mucus

b. parietal cells: produce hydrochloric acid

c. chiefcells: produce intrinsic factor

d. endocrine cells: produce regulatory hormones

11. HCl

a. is an enzyme.

b. creates the acid condition necessary for pepsin to work.

c. is secreted by the small intestine.

d. activates salivary amylase.

e. all ofthe above.

12. Why doesn’t the stomach digest itself?

a. The stomach wall is not composed ofprotein, so it’s not affected

by proteolytic enzymes.

b. The digestive enzymes of the stomach are not strong enough to

digest the stomach wall.

c. The lining of the stomach wall has a protective layer ofepithelial

cells.

d. The stomach wall is protected by large amounts of mucus.

13. Which ofthese hormones stimulates stomach secretions?

a. cholecystokinin

b. gastric inhibitory peptide

c. gastrin

d. secretin

14. Which ofthese phases of stomach secretion is correctly matched?

a. Cephalic phase:the largest volume of secretion is produced.

b. Gastric phase: gastrin secretion is inhibited by distention of the

stomach.

c. Gastric phase:initiated by chewing, swallowing, or thinking of

food.

d. Intestinal phase: stomach secretions are inhibited.

Part4 Regulationsand Maintenance908

15. Which ofthese structures function to increase the mucosal surface

ofthe small intestine?

a. circular folds

b. villi

c. microvilli

d. length of the small intestine

e. all ofthe above

16. Given these parts ofthe small intestine:

1. duodenum

2. ileum

3. je junum

Choose the arrangement that lists the parts in the order food

encounters them as it passes from the stomach through the small

intestine.

a. 1,2,3

b. 1,3,2

c. 2,1,3

d. 2,3,1

e. 3,1,2

17. Which structures release digestive enzymes in the small intestine?

a. duodenal glands

b. goblet cells

c. endocrine cells

d. absorptive cells

18. The hepatic sinusoids

a. receive blood from the hepatic artery.

b. receive blood from the hepatic portal vein.

c. empty into the central veins.

d. all of the above.

19. Given these ducts:

1. common bile duct

2. common hepatic duct

3. cystic duct

4. hepatic ducts

Choose the arrangement that lists the ducts in the order bile passes

through them when moving from the bile canaliculi ofthe liver to

the small intestine.

a. 3,4,2

b. 3,2,1

c. 3,4,1

d. 4,1,2

e. 4,2,1

20. Which ofthese might occur if a person suffers from a severe case of

hepatitis that impairs liver function?

a. Fat digestion is difﬁcult.

b. By-products of hemoglobin breakdown accumulate in the blood.

c. Plasma proteins decrease in concentration.

d. Toxins in the blood increase.

e. All ofthe above.

21. The gallbladder

a. produces bile.

b. stores bile.

c. contracts and releases bile in response to secretin.

d. contracts and releases bile in response to sympathetic

stimulation.

e. both b and c.

22. The aqueous component ofpancreatic secretions

a. is secreted by the pancreatic islets.

b. contains bicarbonate ions.

c. is released primarily in response to cholecystokinin.

d. passes directly into the blood.

e. all ofthe above.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

Chapter 24 Digestive System 909

23. Given these structures:

1. ascending colon

2. descending colon

3. sig moid colon

4. t ransverse colon

Choose the arrangement that lists the structures in the order that

food encounters them as it passes between the small intestine and

the rectum.

a. 1,2,3,4

b. 1,4,2,3

c. 2,3,1,4

d. 2,4,1,3

e. 3,4,1,2

24. Which ofthese is not a function of the large intestine?

a. absorption offats

b. absorption of certain vitamins

c. absorption of water and salts

d. production of mucus

e. all ofthe above

25. Defecation

a. can be initiated by stretch ofthe rectum.

b. can occur as a result of mass movements.

c. involves local reﬂexes.

d. involves parasympathetic reﬂexes mediated by the spinal cord.

e. all ofthe above.

26. Which ofthese structures produces enzymes that digest

carbohydrates?

a. salivary glands

b. pancreas

c. lining of the small intestine

d. both a and b

e. all ofthe above

27. Bile

a. is an important enzyme for the digestion of fats.

b. is made by the gallbladder.

c. contains breakdown products from hemoglobin.

d. emulsiﬁes fats.

e. both c and d.

28. Micelles are

a. lipids surrounded by bile salts.

b. produced by the pancreas.

c. released into lacteals.

d. stored in the gallbladder.

e. reabsorbed in the colon.

29. Ifthe thoracic duct were tied off, which of these classes of nutrients

wouldnot enter the circulatory system at their normal rate?

a. amino acids

b. glucose

c. lipids

d. fructose

e. nucleotides

30. Which ofthese lipoprotein molecules transports excess lipids from

cells back to the liver?

a. high-density lipoprotein (HDL)

b. low-density lipoprotein (LDL)

c. very low-density lipoprotein (VLDL)

Answers in Appendix F

1. While anesthetized,patients sometimes vomit. Given that the

anesthetic eliminates the swallowing reﬂex,explain why it’s

dangerous for an anesthetized patient to vomit.

2. Achlorhydria is a condition in which the stomach stops producing

hydrochloric acid and other secretions.What effect would

achlorhydria have on the digestive process? On red blood cell count?

3. Victor Worrystudent experienced the pain ofa duodenal ulcer

during ﬁnal examination week.Describe the possible reasons.

Explain what habits could have caused the ulcer,and recommend a

reasonable remedy.

4. Gallstones sometimes obstruct the common bile duct.What are the

consequences ofsuch a blockage?

5. A patient has a spinal cord injury at level L2 ofthe spinal cord. How

will this injury affect the patient’s ability to defecate? What components

ofthe defecation response are still present, and which are lost?

6. The bowel (colon) occasionally can become impacted.Given what

you know about the functions ofthe colon and the factors that

determine the movement ofsubstances across the colon wall,

predict the effect ofthe impaction on the contents of the colon

above the point ofimpaction.

Answers in Appendix G

CRITICAL THINKING

1. A pin placed through the greater omentum passes through four

layers ofsimple squamous epithelium. The greater omentum is

actually a folded mesentery,with each part consisting of two layers

ofserous squamous epithelium.

2. The moist stratiﬁed squamous epithelium ofthe orophar ynx and

the laryngopharynx protects these regions from abrasive food when

it is ﬁrst swallowed.The ciliated pseudostratiﬁed epithelium of the

nasopharynx helps move mucus produced in the nasal cavity and

the nasopharynx into the oropharynx and esophagus.It’s not as

necessary to protect the nasopharynx from abrasion because food

does not normally pass through this cavity.

3. It’s important for the nasopharynx to be closed during swallowing

so that food doesn’t reﬂux into it or the nasal cavity.An explosive

burst oflaughter can relax the soft palate, open the nasopharynx,

and cause the liquid to enter the nasal cavity.

4. Usually ifa person tries to swallow and speak at the same time, the

epiglottis is elevated,the laryngeal muscles closing the opening to

the larynx are mostly relaxed,and food or liquid could enter the

larynx,causing the person to choke.

5. After a heavy meal,blood pH may increase because, as bicarbonate

ions pass from the cells ofthe stomach into the extracellular ﬂuid,

the pH ofthe extracellular ﬂuid increases. As the extracellular ﬂuid

exchanges ions with the blood,the blood pH also increases.

ANSWERS TO PREDICT QUESTIONS

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

IV. Regulations and

Maintenance

24. Digestive System

Companies, 2004

6. Secretin production and its stimulation ofbicarbonate ion secretion

constitute a negative-feedback mechanism because,as the pH of the

chyme in the duodenum decreases as a result ofthe presence of acid,

secretin causes an increase in bicarbonate ion secretion,which

increases the pH and restores the proper pH balance in the

duodenum.

Part4 Regulationsand Maintenance910

7. The major effect ofprolonged diarrhea is on the cardiovascular

system and is much like massive blood loss.Hypovolemia continues

to increase.Blood pressure declines in a positive-feedback cycle and

without intervention can lead to heart failure.

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