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From our first breath at birth, the
rate and depth of our respiration is
unconsciouslymatched to our activi-
ties, whether studying, sleeping, talk-
ing, eating, or exercising. We can
voluntarily stop breathing, but within a
few seconds we must breathe again.
Breathing isso characteristic of life that, along
with the pulse, it’sone of the first things we check
for to determine ifan unconscious person is alive.
Breathing isnecessary because all living cells of the body require oxygen
and produce carbon dioxide. The respiratorysystem allows exchange of these
gasesbetween the air and the blood, and the cardiovascular system transports
them between the lungsand the cells of the body. The capacity to carryout nor-
malactivity is reduced without healthy respiratory and cardiovascular systems.
Respirationincludes: (1) ventilation, the movement of air into and out of
the lungs; (2)gas exchange between the air in the lungs and the blood, some-
timescalled external respiration; (3) transport of oxygen and carbon dioxide in
the blood; and (4)gas exchange between the blood and the tissues, sometimes
called internalrespiration. The term respiration is also used in reference to cell
metabolism, which isconsidered in chapter 25.
This chapter explainsthe functions of the respiratory system (814), the
anatomyand histology of the respiratorysystem (814), ventilation (828), measur-
ing lung function(833), physical principlesof gas exchange (835), oxygen and car-
bon dioxide transportin the blood (838), rhythmic ventilation (843), modification
of ventilation (845), and respiratoryadaptations to exercise (849). We conclude
the chapter bylooking at the effects of aging on the respiratorysystem (850).
Respiratory
System
Colorized scanning electron micrograph (SEM) of
the lung, showing alveoli, which are small
chamberswhere gas exchange takesplace
between the airand the blood.
CHAPTER
23
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Functions of the Respiratory
System
Objective
■ Describe the functionsof the respiratory system.
Respiration is necessary because all living cells of the body
require oxygen and produce carbon dioxide.The respiratory sys-
tem assists in gas exchange and performs other functions as well.
1. Gas exchange.The respiratory system allows oxygen from the
air to enter the blood and carbon dioxide to leave the blood
and enter the air.The cardiovascular system transports
oxygen from the lungs to the cells ofthe body and carbon
dioxide from the cells ofthe body to the lungs. Thus, the
respiratory and cardiovascular systems work together to
supply oxygen to all cells and to remove carbon dioxide.
2. Regulation ofblood pH. The respiratory system can alter
blood pH by changing blood carbon dioxide levels.
3. Voice production.Air movement past the vocal folds makes
sound and speech possible.
4. Olfaction.The sensation of smell occurs when airborne
molecules are drawn into the nasal cavity.
5. Protection.The respiratory system provides protection
against some microorganisms by preventing their entry into
the body and by removing them from respiratory surfaces.
1. Explain the functions of the respiratory system.
Anatomy and Histology of the
Respiratory System
Objectives
■ Describe the structure and functionsof the nasal cavity,
pharynx, and larynx.
■ Describe the airpassageways and the parts of the lungs,
and howthe muscles of respiration change thoracic
volume.
■ Describe the pleural membranes, blood supply, and
lymphaticsupply of the lungs.
The respiratory system consists ofthe nasal cavity, the phar-
ynx, the larynx, the trachea, the bronchi,and the lungs (figure
23.1). The term upper respiratory tract refers to the nose, the
pharynx, and associated structures; and the lower respiratory
tract includes the larynx, trachea, bronchi, and lungs. The di-
aphragm and the muscles of the thoracic and abdominal walls are
responsible for respiratory movements.
Nose
Thenasus (na¯⬘su˘s), or nose, consists ofthe external nose and the
nasal cavity.The external nose is the visible structure that forms a
prominent feature ofthe face. The largest part of the external nose
Part4 Regulationsand Maintenance814
is composed ofcartilage plates (see figure 7.10b). The bridge of the
nose consists ofthe nasal bones plus extensions of the frontal and
maxillary bones.
Thenasal cavity extends from the nares to the choanae (figure
23.2).The nares (na¯⬘res;sing., na¯⬘ris), or nostrils, are the external
openings ofthe nasal cavity and the choanae (ko¯⬘an-e¯) are the open-
ings into the pharynx.The anterior par t of the nasal cavity,just in-
side each naris, is the vestibule (ves⬘ti-bool; entry room). The
vestibule is lined with stratified squamous epithelium that is contin-
uous with the stratified squamous epithelium of the skin.The hard
palate(pal⬘a˘t) is a bony plate covered by a mucous membrane that
forms the floor of the nasal cavity.It separates the nasal cavity from
the oral cavity.The nasal septum is a partition dividing the nasal
cavity into right and left parts (see figure 7.9a).The anterior part of
the nasal septum is cartilage, and the posterior part consists of the
vomer bone and the perpendicular plate ofthe ethmoid bone.
Three bony ridges called conchae (kon⬘ke¯;resembling a
conch shell) modify the lateral walls of the nasal cavity.Beneath
each concha is a passageway called a meatus(me¯-a¯⬘tu˘s;a tunnel or
passageway).Within the superior and middle meatus are openings
from the various paranasal sinuses (see figure 7.10), and the
opening of a nasolacrimal (na¯-zo¯-lak⬘ri-ma˘l) ductis within each
inferior meatus (see figure 15.8).
The nasal cavity has several functions:
1. The nasal cavity is a passageway for air that’s open even
when the mouth is full offood.
2. The nasal cavity cleans the air.The vestibule is lined with
hairs that trap some ofthe large par ticles of dust in the air.
The nasal septum and nasal conchae increase the surface
area ofthe nasal cavity and make airflow within the cavity
Nose
Nasal cavity
Pharynx
(throat)
Larynx
Trachea
Bronchi
Lungs
Upper
respiratory
tract
Lower
respiratory
tract
Figure 23.1
The RespiratorySystem
The upper respiratorytract consists of the nasalcavity and pharynx (throat).
The lower respiratorytract consists of the larynx, trachea, bronchi, and lungs.
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Chapter 23 RespiratorySystem 815
more turbulent,thereby increasing the likelihood that air
comes into contact with the mucous membrane lining the
nasal cavity.This mucous membrane consists of
pseudostratified ciliated columnar epithelium with goblet
cells,which secrete a layer of mucus. The mucus traps debris
in the air,and the cilia on the surface of the mucous
membrane sweep the mucus posteriorly to the pharynx,
where it is swallowed and eliminated by the digestive system.
3. The nasal cavity humidifies and warms the air.Moisture
from the mucous epithelium and from excess tears that
drain into the nasal cavity through the nasolacrimal duct is
added to the air as it passes through the nasal cavity.Warm
blood flowing through the mucous membrane warms the
air within the nasal cavity before it passes into the pharynx,
thus preventing damage from cold air to the rest ofthe
respiratory passages.
Superior
nasal concha
Sphenoidal
sinus
Soft palate
Hard palate
Middle
nasal concha
Inferior
nasal concha
Inferior meatus
Superior
meatus
Middle
meatus
Superior concha
Middle concha
Inferior concha
Vestibule
Naris
Hard palate
Tongue
Palatine tonsil
Lingual tonsil
Epiglottis
Vestibular fold
Vocal fold
Thyroid cartilage
Cricoid cartilage
Nasal cavity
Oral cavity
Larynx
Trachea
Frontal sinus
Cribriform plate
Sphenoidal sinus
Superior meatus
Middle meatus
Inferior meatus
Choana
Pharyngeal tonsil
Opening of auditory tube
Soft palate
Uvula
Nasopharynx
Oropharynx
Laryngopharynx
Nasal cavity
Paranasal
sinuses
Pharynx
Esophagus
Figure 23.2
NasalCavity and Pharynx
(a) Sagittalsection through the nasal cavityand phar ynxviewed from the medial side. (b) Photograph of sagittal section of the head.
(b)
(a)
Seeley−Stephens−Tate:
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PREDICT
Explain whathappens to your throat when you sleep with your mouth
open, especiallywhen your nasal passagesare plugged as a result of
having a cold. Explain whatmay happen to your lungswhen you run a
long wayin very cold weather while breathing rapidly through your
mouth.
4. The olfactory epithelium,the sensory organ for smell, is
located in the most superior part of the nasal cavity (see
figure 15.2).
5. The nasal cavity and paranasal sinuses are resonating
chambers for speech.
Pharynx
The pharynx (far⬘ingks; throat) is the common opening of both
the digestive and respiratory systems.It receives air from the nasal
cavity and air,food, and drink from the oral cavity. Inferiorly, the
pharynx is connected to the respiratory system at the larynx and to
the digestive system at the esophagus.The pharynx is divided into
three regions: the nasopharynx, the oropharynx, and the laryn-
gopharynx (see figure 23.2).
The nasopharynx (na¯⬘zo¯-far⬘ingks) is the superior part of
the pharynx and extends from the choanae to the soft palate,
which is an incomplete muscle and connective tissue partition sep-
arating the nasopharynx from the oropharynx.The uvula (u¯⬘vu¯-la˘;
a grape) is the posterior extension ofthe soft palate. The soft palate
prevents swallowed materials from entering the nasopharynx and
nasal cavity.The nasopharynx is lined with a mucous membrane
containing pseudostratified ciliated columnar epithelium with
goblet cells. Debris-laden mucus from the nasal cavity is moved
through the nasopharynx and swallowed.Two auditory tubes from
the middle ears open into the nasopharynx (see figures 15.22 and
23.2a).Air passes through them to equalize air pressure between
the atmosphere and the middle ears.The posterior surface of the
nasopharynx contains the pharyngeal tonsil, or adenoid (ad⬘e˘-
noyd), which aids in defending the body against infection (see
chapter 22).An enlarged pharyngeal tonsil can interfere with nor-
mal breathing and the passage ofair through the auditory tubes.
The oropharynx (o¯r⬘o¯-far⬘ingks) extends from the uvula to
the epiglottis.The oral cavity opens into the oropharynx through the
fauces (faw⬘se¯z).Thus, air, food, and drink all pass through the
oropharynx.Moist stratified squamous epithelium lines the orophar-
ynx and protects it against abrasion. Two sets oftonsils called the
palatine tonsils and the lingual tonsils are located near the fauces.
The laryngopharynx (la˘-ring⬘go¯ -far-ingks) extends from
the tip ofthe epiglottis to the esophagus and passes posterior to the
larynx. The laryngopharynx is lined with moist stratified squa-
mous epithelium.
Larynx
Thelarynx (lar⬘ingks) consists of an outer casing of nine car tilages
that are connected to one another by muscles and ligaments (figure
23.3).Six of the nine cartilages are paired, and three are unpaired.
Part4 Regulationsand Maintenance816
The largest of the cartilages is the unpaired thyroid (shield; refers
to the shape ofthe cartilage) car tilage,or Adam’s apple.
The most inferior cartilage of the larynx is the unpaired
cricoid(krı¯⬘koyd;ring-shaped) cartilage,which forms the base of
the larynx on which the other cartilages rest.
The third unpaired cartilage is the epiglottis (ep-i-glot⬘is;
on the glottis).It’s attached to the thyroid cartilage and projects as
a free flap toward the tongue.The epiglottis differs from the other
cartilages in that it consists of elastic rather than hyaline cartilage.
During swallowing,the epiglottis covers the opening of the larynx
and prevents materials from entering it.
The paired arytenoid (ar-i-te¯⬘noyd;ladle-shaped) carti-
lagesar ticulate with the posterior,superior border of the cricoid
cartilage,and the paired corniculate (ko¯r-nik⬘u¯-la¯t; horn-shaped)
cartilages are attached to the superior tips of the arytenoid carti-
lages.The paired cuneiform (ku¯⬘ne¯-i-fo¯rm; wedge-shaped) car ti-
lages are contained in a mucous membrane anterior to the
corniculate cartilages (see figure 23.3b).
Two pairs ofligaments extend from the anterior surface of the
arytenoid cartilages to the posterior surface of the thyroid cartilage.
The superior ligaments are covered by a mucous membrane called
thevestibular folds, or false vocal cords (see figures 23.3c and 23.4a
andb). When the vestibular folds come together, they prevent food
and liquids from entering the larynx during swallowing and prevent
air from leaving the lungs,as when a person holds his or her breath.
The inferior ligaments are covered by a mucous membrane
called the vocal folds, or true vocal cords (see figure 23.4). The
vocal folds and the opening between them are called the glottis
(glot⬘is). The vestibular folds and the vocal folds are lined with
stratified squamous epithelium. The remainder of the larynx is
lined with pseudostratified ciliated columnar epithelium. An in-
flammation of the mucosal epithelium of the vocal folds is called
laryngitis (lar-in-jı¯⬘tis).
The larynx performs three important functions.
1. The thyroid and cricoid cartilages maintain an open
passageway for air movement.
2. The epiglottis and vestibular folds prevent swallowed
material from moving into the larynx.
3. The vocal folds are the primary source of sound
production.Air moving past the vocal folds causes them
to vibrate and produce sound.The greater the amplitude
ofthe v ibration,the louder is the sound. The force of air
moving past the vocal folds determines the amplitude of
vibration and the loudness of the sound.The frequency of
vibrations determines pitch,with higher frequency
vibrations producing higher pitched sounds and lower
frequency fibrations producing lower pitched sounds.
Variations in the length ofthe vibrating segments of the
vocal folds affect the frequency ofthe vibrations. Higher-
pitched tones are produced when only the anterior parts
ofthe folds v ibrate,and progressively lower tones result
when longer sections ofthe folds v ibrate.Because males
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Chapter 23 RespiratorySystem 817
usually have longer vocal folds than females,they usually
have lower-pitched voices.The sound produced by the
vibrating vocal folds is modified by the tongue,lips, teeth,
and other structures to form words.A person whose
larynx has been removed because ofcarcinoma of the
larynx can produce sound by swallowing air and causing
the esophagus to vibrate.
Movement ofthe arytenoid and other cartilages is
controlled by skeletal muscles,thereby changing the
position and length ofthe vocal folds. When only
breathing,lateral rotation of the arytenoid car tilages
abducts the vocal folds,which allows greater movement of
air (figure 23.4c).Medial rotation of the arytenoid
cartilages adducts the vocal folds,places them in position
for producing sounds,and changes the tension on them.
(figure 23.4d).Anterior/posterior movement of the
arytenoid cartilages also changes the length and tension of
the vocal folds (figure 23.4e).
2. Define upper and lower respiratory tract.
3. How are the structures of the nasal cavity responsible for its
functions?
4. Name the three parts of the pharynx. With what structures
doeseach part communicate?
5. Name and describe the three unpaired cartilages of the
larynx. Whatare their functions?
6. Distinguish between the vestibular and vocal folds. How are
soundsof different loudness and pitch produced by the
vocal folds?
7. How does the position of the arytenoid cartilages change
when justbreathing versus making low-pitched and high-
pitched sounds?
Trachea
The trachea (tra¯⬘ke¯-a˘), or windpipe, is a membranous tube that
consists ofdense regular connective tissue and smooth muscle re-
inforced with 15–20 C-shaped pieces of cartilage. The cartilages
Cuneiform
cartilage
Thyrohyoid
membrane
Corniculate
cartilage
Arytenoid
cartilage
Cricoid
cartilage
Epiglottis
Hyoid
bone
Thyrohyoid
membrane
Fat
Vestibular
fold (false
vocal cord)
Thyroid
cartilage
Cricothyroid
ligament
Vocal fold
(true vocal
cord)
Hyoid
bone
Thyrohyoid
membrane
Thyroid
cartilage
Quadrangular
membrane
Cricoid
cartilage
Tracheal
cartilage
Membranous
part of trachea
Trachea
Cricothyroid
ligament
Superior
thyroid
notch
(a) Anterior view (b) Posterior view (c) Sagittal view
Epiglottis
Larynx
Figure 23.3
Anatomyof the Larynx
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support the anterior and lateral sides of the trachea (figure 23.5a).
They protect the trachea and maintain an open passageway for air.
The posterior wall ofthe trachea is devoid of cartilage and contains
an elastic ligamentous membrane and bundles of smooth muscle
called the trachealis (tra¯⬘ke¯-a¯-lis) muscle. Contraction of the
smooth muscle can narrow the diameter of the trachea. During
coughing,this action causes air to move more rapidly through the
trachea, which helps to expel mucus and foreign objects. The
esophagus lies immediately posterior to the cartilage-free posterior
wall ofthe trachea.
PREDICT
Explain whathappens to the shape of the trachea when a person
swallowsa large mouthful of food. Why is thischange of shape
advantageous?
Part4 Regulationsand Maintenance818
The mucous membrane lining the trachea consists ofpseu-
dostratified ciliated columnar epithelium with numerous goblet
cells (figure 23.5b).The cilia propel mucus and foreign particles
embedded in it toward the larynx, where the mucus enters the
pharynx and is swallowed.Constant irritation to the trachea, such
as occurs in smokers,can cause the tracheal epithelium to become
moist stratified squamous epithelium that lacks cilia and goblet
cells. Consequently,the normal function of the tracheal epithe-
lium is lost.
(c) Vocal folds positioned (d) Vocal folds positioned (e) Changing the tension
for breathing for speaking of the vocal folds
Epiglottis
Vestibular folds
(false vocal cords)
Cuneiform
cartilage
Arytenoid
cartilage
Vocal fold
Cricoid cartilage
Thyroid cartilage
Corniculate
cartilage
Trachea
Vocal folds
(true vocal cords)
Larynx
Posterior
Anterior
Tongue
(b) View through a laryngoscope
(a)
Figure 23.4
Vocal Fo lds
Arrowshows the direction of viewing the vocal folds. (a) The relationship ofthe vocal folds to the vestibular folds and the laryngeal cartilages. (b) Laryngoscopic
view ofthe vocal folds. (c) Lateral rotation of the arytenoid cartilages positionsthe vocal folds for breathing. (d ) Medial rotation ofthe ar ytenoid cartilagespositions
the vocalfolds for speaking. (e) Anterior/posterior movement of the arytenoid cartilages changesthe length and tension of the vocal folds.
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Chapter 23 RespiratorySystem 819
Establishing Airflow
In casesof extreme emergency when the upper air passagewayis
blocked bya foreign object to the extent that the victim cannotbreathe,
quickreaction is required to save the person’s life. The Heimlich
maneuveris designed to force such an object out of the air passage by
the sudden application ofpressure to the abdomen. The person who
performsthe maneuver stands behind the victim with arms under the
victim’sarms and hands over the victim’s abdomen between the navel
and the rib cage. With one hand formed into a fistand the other hand
over it, both handsare suddenly pulled toward the abdomen with an
accompanying upward motion. Thismaneuver, ifdone properly, forces
air up the trachea and dislodgesmost foreign objects.
In rare cases, when the obstruction cannotbe removed using the
Heimlich maneuver, itmay be necessaryto form an ar tificialopening in
the victim’sair passageway, followed with insertion ofa tube to facilitate
the passage ofair. The preferred point of entry in emergencycases is
through the membrane between the cricoid and thyroid cartilages, a
procedure referred to asa cricothyrotomy (krı¯⬘ko¯-thı¯-rot⬘o¯-me¯). A
tracheotomy(tra¯-ke¯-ot⬘o¯-me¯) makesan opening in the trachea, usually
between the second and third cartilage rings. Itis not advisable to enter
the air passagewaythrough the trachea in emergencycases because
arteries, nerves, and the thyroid gland overlie the anterior surface ofthe
trachea.
The trachea has an inside diameter of12 mm and a length of
10–12 cm,descending from the larynx to the level of the fifth tho-
racic vertebra (figure 23.6). The trachea divides to form two
smaller tubes called primary bronchi (brong⬘kı¯;sing., bronchus,
brong⬘ku˘s; windpipe). The most inferior tracheal cartilage forms a
ridge called the carina (ka˘-rı¯⬘na˘), which separates the openings
into the primary bronchi. The carina is an important radiologic
landmark.In addition, the mucous membrane of the carina is very
sensitive to mechanical stimulation,and foreign objects reaching
the carina stimulate a powerful cough reflex.Once a foreign object
passes the carina,coughing usually stops.
TracheobronchialTree
The trachea divides to form primary bronchi, which,in tur n,di-
vide to form smaller and smaller bronchi,until, eventually, many
microscopically small tubes and sacs are formed.Beginning w ith
the trachea,all the respiratory passageways are called the tracheo-
bronchial (tra¯⬘ke¯-o¯-brong⬘ke¯-a˘l)tree (see figure 23.6). Based on
function,the tracheobronchial tree can be subdivided into the con-
ducting zone and the respiratory zone.
Conducting Zone
The conducting zone extends from the trachea to small tubes
called terminal bronchioles (see figure 23.6). Approximately 16
generations of branching occur from the trachea to the terminal
bronchioles.The conducting zone functions as a passageway for air
movement and contains epithelial tissue that helps to remove de-
bris from the air and move it out ofthe tracheobronchial tree.
Goblet cell
Cilia
Trachealis
muscle
Mucous
membrane
Cartilage
Lumen of trachea
Anterior
Esophagus
Trachea
Lumen
Anterior
Transverse plane
through trachea
and esophagus
Esophagus
LM 250x
SEM 2000x
Figure 23.5
Trachea
(a) Photomicrograph ofa transverse section of the trachea. The esophagusis
nextto the trachealis muscle, which connectsthe ends of the cartilage.
(b)Scanning electron micrograph of the surface ofthe mucous membrane lining
the trachea. Gobletcellswith short microvilli are interspersed between ciliated
cells.
(b)
(a)
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Visceral pleura
Parietal pleura
Pleural cavity
Secondary bronchus
Tertiary bronchus
Bronchiole
To terminal
bronchiole
Diaphragm
Primary bronchus
Larynx
Trachea
Carina
Primary bronchus
Secondary bronchus
Tertiary bronchus
Bronchiole
To terminal
bronchiole
Air passageways
decrease in size
but increase
in number
Trachea
Primary
bronchi
Secondary
bronchi
Tertiary
bronchi
Figure 23.6
TracheobronchialTree
(a) The conducting zone ofthe tracheobronchialtree begins at the
trachea and endsat the terminal bronchioles. (b) A bronchogram isa
radiograph ofthe tracheobronchial tree. A contrastmedium, which
makesthe passagewaysvisible, is injected through a catheter after a
topicalanesthetic is applied to the mucous membranes of the nose,
pharynx, larynx, and trachea.
(a)
(b)
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Chapter 23 RespiratorySystem 821
The trachea divides into the left and right primary bronchi,
which extend to the lungs (see figure 23.6). The right primary
bronchus is shorter,has a wider diameter,and is more vertical than
the left primary bronchus.
PREDICT
Into which lung would a foreign objectthat’s small enough to pass
into a primarybronchus most likely become lodged and blockair
movement?
The primary bronchi divide into secondary (lobar) bronchi
within each lung.Two secondary bronchi exist in the left lung, and
three exist in the right lung. The secondary bronchi,in turn, give
rise to tertiary (segmental) bronchi. The bronchi continue to
branch, finally giving rise to bronchioles (brong⬘ke¯-o¯lz),which
are less than 1 mm in diameter.The bronchioles also subdivide sev-
eral times to become even smaller terminal bronchioles.
As the air passageways of the lungs become smaller, the
structure of their walls changes. Like the trachea, the primary
bronchi are supported by C-shaped cartilage connected by smooth
muscle.In the secondar y bronchi,the C-shaped cartilages are re-
placed with cartilage plates,and smooth muscle forms a layer be-
tween the cartilage and the mucous membrane. As the bronchi
become smaller,the car tilage becomes more sparse and smooth
muscle becomes more abundant. The terminal bronchioles have
no cartilage,and the smooth muscle layer is prominent. Relaxation
and contraction of the smooth muscle within the bronchi and
bronchioles can change the diameter of the air passageways and
thereby change the volume ofair moving through them. For exam-
ple,during exercise, the diameter can increase, which reduces the
resistance to airflow and thereby increases the volume of air
moved. During an asthma attack, however,contraction of the
smooth muscle in the terminal bronchioles,which have no carti-
lage in their walls,can result in decreased diameter, increased re-
sistance to airflow,and greatly reduced airflow. In severe cases,air
movement can be so restricted that death results.
The bronchi are lined with a pseudostratified ciliated colum-
nar epithelium.The larger bronchioles are lined with ciliated sim-
ple columnar epithelium, which changes to ciliated simple
cuboidal epithelium in the terminal bronchioles.The epithelium in
the conducting part of the air passageways functions as a mu-
cus–cilia escalator,w hich traps debris in the air and removes it
from the respiratory system.
RespiratoryZone
The respiratory zone extends from the terminal bronchioles to
small air-filled chambers called alveoli(al-ve¯⬘o¯-lı¯;hollow cavity),
which are the sites ofgas exchange between the air and blood. Ap-
proximately seven generations ofbranching are present in the res-
piratory zone. The terminal bronchioles divide to form
respiratory bronchioles(figure 23.7), which have a limited ability
for gas exchange because ofa few attached alveoli. As the respira-
tory bronchioles divide to form smaller respiratory bronchioles,
the number ofattached alveoli increases. The respiratory bronchi-
oles give rise to alveolar (al-ve¯⬘o¯-la˘r) ducts, which are like long
branching hallways with many open doorways.The doorways open
into alveoli,which become so numerous that the alveolar duct wall
is little more than a succession ofalveoli. The alveolar ducts end as
two or three alveolar sacs,which are chambers connected to two
or more alveoli.
The tissue surrounding the alveoli contains elastic fibers that
allow the alveoli to expand during inspiration and recoil during ex-
piration.The lungs are very elastic, and when inflated, they are ca-
pable ofexpelling the air and returning to their original, uninflated
state.Even when not inflated, however, the lungs retain some air,
which gives them a spongy quality.
The walls of respiratory bronchioles consists of collagenous
and elastic connective tissue with bundles of smooth muscle.The
epithelium in the respiratory bronchioles is a simple cuboidal ep-
ithelium. The alveolar ducts and alveoli consist of simple squa-
mous epithelium.Although the epithelium of the respiratory zone
is not ciliated,debris from the air can be removed by macrophages
that move over the surfaces ofthe cells. The macrophages don’t ac-
cumulate in the respiratory zone because they either move into
nearby lymphatic vessels or enter terminal bronchioles,thereby be-
coming entrapped in mucus that is swept to the pharynx.
Approximately 300 million alveoli are in the two lungs.The
average diameter ofthe alveoli is approximately 250 m, and their
walls are extremely thin.Two types of cells form the alveolar wall
(figure 23.8a).Type I pneumocytes are thin, squamous epithelial
cells that form 90% of the alveolar surface.Most gas exchange be-
tween alveolar air and the blood takes place through these cells.
Type II pneumocytes are round or cube-shaped secretory cells
that produce surfactant,which makes it easier for the alveoli to ex-
pand during inspiration (see “Lung Recoil”on p.829).
The respiratory membrane of the lungs is where gas ex-
change between the air and blood takes place.It is mainly formed
by the alveolar walls and surrounding pulmonary capillaries (fig-
ure 23.8b),but there’s some contribution by the respiratory bron-
chioles and alveolar ducts.The respiratory membrane is very thin
to facilitate the diffusion ofgases. It consists of
1. a thin layer offluid lining the alveolus;
2. the alveolar epithelium composed ofsimple squamous
epithelium;
3. the basement membrane ofthe alveolar epithelium;
4. a thin interstitial space;
5. the basement membrane ofthe capillary endothelium;
6. the capillary endothelium composed ofsimple squamous
epithelium.
Lungs
Thelungs are the principal organs of respiration, and on a volume
basis they are among the largest organs of the body.Each lung is
conical in shape, with its base resting on the diaphragm and
its apex extending superiorly to a point approximately 2.5 cm
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Part4 Regulationsand Maintenance822
Terminal bronchiole
Respiratory bronchioles
Alveolar ducts
Alveolar sac
Alveoli
Connective
tissue
Visceral pleura
Pleural cavity
Parietal pleura
Smooth muscle
Bronchial vein, artery, and nerve
Branch of pulmonary artery
Deep lymphatic vessel
Alveolus
Superficial lymphatic vessel
Lymph nodes
Pulmonary capillaries
Branch of pulmonary vein
Elastic fibers
Figure 23.7
Bronchiolesand Alveoli
(a) Alveoli, the sitesof gas exchange between air and blood, are connected to
respiratorybronchioles and alveolar ducts and are surrounded by capillaries.
(b) Photomicrograph oflung tissue.
Terminal
bronchus
Respiratory
bronchiole
Alveolar
duct
Alveolar
sacs
Alveoli
LM 30x
(a)
(b)
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Chapter 23 RespiratorySystem 823
superior to the clavicle. The right lung is larger than the left and
weighs an average of 620 g, whereas the left lung weighs an aver-
ageof 560 g.
The hilum (hı¯⬘lu˘m) is a region on the medial surface of the
lung where structures,such as the primar y bronchus,blood vessels,
nerves,and lymphatic vessels, enter or exit the lung. All the structures
passing through the hilum are referred to as the root ofthe lung.
The right lung has three lobes, and the left lung has two
(figure 23.9).The lobes are separated by deep,prominent fissures
on the surface ofthe lung, and each lobe is supplied by a second-
ary bronchus.The lobes are subdivided into bronchopulmonary
segments,which are supplied by the tertiary bronchi. Nine bron-
chopulmonary segments are present in the left lung, and 10 are
present in the right lung. The bronchopulmonary segments are
Alveolar
epithelium
(wall)
Red blood cell
Capillary endothelium
(wall)
Air space
within
alveolus
Macrophage
Type II pneumocyte
(surfactant-
secreting cell)
Type I pneumocyte
Alveolus
Capillary
Respiratory
membrane
Diffusion of O
2
Diffusion of CO
2
Alveolar fluid
(with surfactant)
Alveolar epithelium
Basement membrane of
alveolar epithelium
Interstitial space
Basement membrane of
capillary endothelium
Capillary endothelium
Red blood cell
Figure 23.8
Alveolusand the Respiratory Membrane
(a) Section ofan alveolus showing the air-filled interior and thin walls composed ofsimple squamous epithelium. The alveolus is surrounded by elastic connective
tissue and blood capillaries. (b) Diffusion ofoxygen and carbon dioxide acrossthe six thin layers of the respiratory membrane.
(a)
(b)
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separated from each other by connective tissue partitions,which
are not visible as surface fissures. Individual diseased bron-
chopulmonary segments can be surgically removed, leaving the
rest ofthe lung relatively intact, because major blood vessels and
bronchi don’t cross the connective tissue partitions.The bron-
Part4 Regulationsand Maintenance824
chopulmonary segments are subdivided into lobules by incom-
plete connective tissue walls. The lobules are supplied by the
bronchioles.
8. What are the parts of the conducting and respiratory zones
of the tracheobronchial tree?
Apical
Anterior
Middle
lobe
Superior
lobe
Superior
lobe
Oblique
fissure
Inferior
lobe
Primary
bronchus
Secondary
bronchi
Tertiary
bronchi
Inferior
lobe
Middle
lobe
Medial view of right lung Medial view of left lung
Oblique
fissure
Horizontal
fissure
Anterior
Medial
Posterior
Superior
lobe
Superior
Medial view of
right lung
Posterior
basal
Lat.
basal
Ant.
basal
Medial
basal
Lateral
Inferior
lobe
Inferior
lobe
Superior
lobe
Middle
lobe
Trachea
Primary bronchi
(
green
) to lungs
Inferior
lobe
Superior
lobe
Secondary
bronchi (
red
)
to lobes
Tertiary bronchi
(
all other colors
)
to bronchopulmonary
segments
Inferior
lobe
Medial
basal
Apical–
posterior
(combined)
Anterior
Superior
Medial view of
left lung
Post.
basal
Lateral
basal
Ant.
basal
Superior
lobe
Anterior
Inferior
lingular
Figure 23.9
Lobesand Bronchopulmonary Segments of the Lungs
(a) The trachea (blue), primarybronchi (green), secondary bronchi (red), and tertiary bronchi (all other colors) are in the center of the figure, surrounded by two
viewsof each lung, showing the bronchopulmonary segments. In general, each bronchopulmonarysegment is supplied by a tertiary bronchus (color-coded to match
the bronchopulmonarysegment it supplies). (b) Photograph of the lungsshowing the bronchi supplying the lung lobes.
(b)
(a)
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Chapter 23 RespiratorySystem 825
9. Describe the arrangement of cartilage, smooth muscle, and
epithelium in the tracheobronchial tree. Explain why
breathing becomesmore difficult during an asthma attack.
10. How is debris removed from the conducting and respiratory
zones?
11. Name the two types of cells in the alveolar wall, and state
theirfunctions.
12. List the parts of the respiratory membrane.
13. Distinguish among a lung, a lung lobe, a
bronchopulmonarysegment, and a lobule. How are they
related to the tracheobronchial tree?
ThoracicWall and Muscles of Respiration
The thoracic wall consists of the thoracic vertebrae, ribs, costal
cartilages,the sternum, and associated muscles (see chapters 7 and
10).The thoracic cavity is the space enclosed by the thoracic wall
and the diaphragm (dı¯⬘a˘-fram, meaning partition), which sepa-
rates the thoracic cavity from the abdominal cavity. The di-
aphragm and other skeletal muscles associated with the thoracic
wall are responsible for respiration (figure 23.10).The muscles of
inspiration include the diaphragm, external intercostals, pec-
toralis minor,and scalenes. Contraction of the diaphrag m is re-
sponsible for approximately two-thirds ofthe increase in thoracic
volume during inspiration. The external intercostals,pectoralis
minor and scalene muscles also increase thoracic volume by elevat-
ing the ribs.The muscles of expiration consist of muscles that de-
press the ribs and sternum,such as the abdominal muscles and the
internal intercostals. Although the internal intercostals are most
active during expiration,and the external intercostals are most ac-
tive during inspiration,the primary function of these muscles is to
stiffen the thoracic wall by contracting at the same time.By so do-
ing,they prevent inward collapse of the thoracic cage during inspi-
ration.
The diaphragm is dome-shaped, and the base of the dome
attaches to the inner circumference of the inferior thoracic cage
(see figure 10.15).The top of the dome is a flat sheet of connective
tissue called the central tendon.In normal, quiet inspiration, con-
traction ofthe diaphrag m results in inferior movement of the cen-
tral tendon with very little change in the overall shape ofthe dome.
Inferior movement ofthe central tendon can occur because of re-
laxation ofthe abdominal muscles, which allows the abdominal or-
gans to move out of the way of the diaphragm. As the depth of
inspiration increases, inferior movement of the central tendon is
prevented by the abdominal organs.Continued contraction of the
diaphragm causes it to flatten as the lower ribs are elevated. In ad-
dition,other muscles of inspiration can elevate the ribs. As the ribs
are elevated, the costal cartilages allow lateral rib movement and
lateral expansion of the thoracic cavity (figure 23.11). The ribs
slope inferiorly from the vertebrae to the sternum,and elevation of
the ribs also increases the anterior–posterior dimension of the
thoracic cavity.
Expiration during quiet breathing occurs when the di-
aphragm and external intercostals relax and the elastic properties of
the thorax and lungs cause a passive decrease in thoracic volume.In
End of
inspiration
Labored breathing:
Additional muscles
contract, causing
additional expansion
of the thorax.
Abdominal
muscles
relax.
The diaphragm contracts,
increasing the superior–inferior
dimension of the thoracic cavity.
Quiet breathing:
The external
intercostal
muscles contract,
elevating the
ribs and moving
the sternum.
Sternocleidomastoid
Scalenes
Pectoralis
minor
External
intercostals
Muscles
of
inspiration
Diaphragm
relaxed
Clavicle
(cut)
Internal
intercostals
Abdominal
muscles
Muscles
of
expiration
End of
expiration
Diaphragm
Figure 23.10
Effectof the Muscles of Respiration on Thoracic Volume
(a) Musclesof respiration at the end of expiration. (b) Muscles of respiration at the end of inspiration.
(a) (b)
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addition,contraction of the abdominal muscles helps to push ab-
dominal organs and the diaphragm in a superior direction.
The Role ofAbdominal Muscles in Breathing
The importance ofthe abdominal muscles in breathing can be observed
in a person with a spinalcord injury that causes flaccid paralysisof the
abdominalmuscles. In the upright position, the abdominal organs and
diaphragm are notpushed superiorly and passive recoil of the thorax
and lungsis inadequate for normal expiration. An elastic binder around
the abdomen can help such patients. When lying down, the weightof the
abdominalorgans can assist in expiration.
Several differences can be recognized between normal,quiet
breathing and labored breathing.During labored breathing, all of
the inspiratory muscles are active,and they contract more forcefully
than during quiet breathing,causing a greater increase in thoracic
volume (see figure 23.10b).During labored breathing, forceful con-
traction ofthe internal intercostals and the abdominal muscles pro-
duces a more rapid and greater decrease in thoracic volume than
would be produced by the passive recoil ofthe thorax and lungs.
Pleura
The lungs are contained within the thoracic cavity,but each lung is
surrounded by a separate pleural (ploor⬘a˘l;relating to the ribs)
cavityformed by the pleural serous membranes (figure 23.12). The
mediastinum (me¯⬘de¯-as-tı¯⬘nu˘m),a midline partition formed by
the heart,t rachea,esophagus, and associated structures, separates
the pleural cavities.The parietal pleura covers the inner thoracic
wall,the superior surface of the diaphragm, and the mediastinum.
At the hilum,the par ietal pleura is continuous with the visceral
pleura,which covers the surface of the lung.
Part4 Regulationsand Maintenance826
The pleural cavity is filled with pleural fluid, which is pro-
duced by the pleural membranes. The pleural fluid does two
things: (1) it acts as a lubricant,allowing the parietal and v isceral
pleural membranes to slide past each other as the lungs and the
thorax change shape during respiration,and (2) it helps hold the
parietal and visceral pleural membranes together.When thoracic
volume changes during respiration,lung volume changes because
the parietal pleura is attached to the diaphragm and inner thoracic
wall, and the visceral pleura is attached to the lungs.The pleural
fluid is analogous to a thin film of water between two sheets of
glass (the visceral and parietal pleurae); the glass sheets can easily
slide over each other,but it’s difficult to separate them.
Blood Supply
Blood that has passed through the lungs and picked up oxygen is
calledoxygenated blood, and blood that has passed through the tis-
sues and released some of its oxygen is called deoxygenated blood.
Two blood flow routes to the lungs exist.The major route brings de-
oxygenated blood to the lungs,where it is oxygenated (see chapter 21
and figure 23.12b). The deoxygenated blood flows through pul-
monary arteries to pulmonary capillaries, becomes oxygenated,and
returns to the heart through pulmonary veins.The other route brings
oxygenated blood to the tissues ofthe bronchi down to the respira-
tory bronchioles.The oxygenated blood flows from the thoracic aorta
through bronchial arteries to capillaries, where oxygen is released.
Deoxygenated blood from the proximal part ofthe bronchi returns to
the heart through the bronchial veins and the azygos venous system
(see chapter 21).More distally,the venous drainage from the bronchi
enters the pulmonary veins. Thus,the oxygenated blood returning
from the alveoli in the pulmonary veins is mixed with a small amount
ofdeoxygenated blood returning from the bronchi.
Vertebra
Lateral
increase
in volume
Sternum
Sternum
Anterior
increase
in volume
Figure 23.11
Effectof Rib and Sternum Movement on Thoracic Volume
(a) Elevation ofthe rib in the “bucket-handle” movement laterallyincreases thoracic volume. (b) As the rib is elevated, rotation of the rib in the “pump-handle”
movementincreases thoracic volume anteriorly.
(a)
(b)
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Chapter 23 RespiratorySystem 827
LymphaticSupply
The lungs have two lymphatic supplies.The superficial lymphatic
vesselsare deep to the visceral pleura and function to drain lymph
from the superficial lung tissue and the visceral pleura.The deep
lymphatic vesselsfollow the bronchi and function to drain lymph
from the bronchi and associated connective tissues.No lymphatic
vessels are located in the walls of the alveoli.Both the superficial
and deep lymphatic vessels exit the lung at the hilum.
Phagocytic cells pick up carbon particles and other debris
from inspired air and move them to the lymphatic vessels.In older
people,the surface of the lungs can appear gray to black because of
the accumulation ofthese particles, especially if the person smokes
or has lived most ofhis or her life in a city with air pollution. Can-
cer cells from the lungs can spread to other parts of the body
through the lymphatic vessels.
14. List the muscles of respiration and describe their role in
quietinspiration and expiration. How does this change
during labored breathing?
15. Name the pleurae of the lungs. What is their function?
16. What are the two major routes of blood flow to and from the
lungs? Whatis the function of each route?
17. Describe the lymphatic supply of the lungs.
Parietal pleura
Pleural cavity
Visceral pleura
Lung
Vertebra
Esophagus (in posterior
mediastinum)
Right lung
Right primary
bronchus
Right pulmonary
artery
Right pulmonary
vein
Pulmonary trunk
Heart
Anterior mediastinum
Sternum
Left lung
Pleural cavity
Visceral pleura
Parietal pleura
Fibrous pericardium
Parietal pericardium
Pericardial cavity
Visceral pericardium
Root of
lung
at hilum
Figure 23.12
PleuralCavities and Membranes
(a) Each lung issurrounded by a pleural cavity. The parietalpleura lines the wall of each pleural cavity, and the visceralpleura covers the surface of the lungs. The
space between the parietaland visceral pleurae issmall and filled with pleural fluid. (b) Transverse section of the thorax, at the level indicated in part (a), showing
the relationship ofthe pleural cavities to the thoracic organs.
(a)
(b)
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Clinical Focus Cough and Sneeze Reflexes
The function of both the cough reflex and
the sneeze reflexis to dislodge foreign mat-
ter or irritating material from respiratory
passages. The bronchi and trachea contain
sensory receptorsthat are sensitive to for-
eign particlesand irritating substances. The
cough reflex isinitiated when the sensor y
receptorsdetect such substances and initi-
ate action potentialsthat pass along the va-
gusnerves to the medulla oblongata, where
the cough reflexis triggered.
The movements resulting in a cough
occur as follows: approximately2.5 L of
air is inspired; the vestibular and vocal
folds close tightlyto trap the inspired air
in the lungs; the abdominalmuscles con-
tract to force the abdominal contentsup
against the diaphragm; and the muscles
ofexpiration contract forcefully. As a con-
sequence, the pressure in the lungs in-
creasesto 100 mm Hg or more. Then the
vestibular and vocalfolds open suddenly,
the soft palate is elevated, and the air
rushes from the lungs and out the oral
cavity at a high velocity, carrying foreign
particleswith it.
The sneeze reflex is similar to the
cough reflex, butit differs in several ways.
The source of irritation that initiates the
sneeze reflex is in the nasal passages
instead ofin the trachea and bronchi, and
the action potentialsare conducted along
the trigeminal nerves to the medulla ob-
longata, where the reflexis triggered. Dur-
ing the sneeze reflex the soft palate is
depressed so thatair is directed primarily
through the nasal passages, although a
considerable amountpasses through the
oral cavity. The rapidly flowing air dis-
lodges particulate matter from the nasal
passagesand can propel it a considerable
distance from the nose.
Ventilation
Objectives
■ Describe the factorsthat affect the flow of air through a
tube and the factorsthat determine the pressure of a gas in
a container.
■ Explain the movementof air into and out of the lungs.
■ Describe the factorsthat cause the alveoli to collapse and
expand.
Pressure Differencesand Airflow
Ventilationis the process of moving air into and out of the lungs.
The flow ofair into the lungs requires a pressure gradient from the
outside of the body to the alveoli,and airflow from the lungs re-
quires a pressure gradient in the opposite direction.The physics of
airflow in tubes,such as the ones that make up the respiratory pas-
sages,is the same as the flow of blood in blood vessels (see chapter
21).Thus, the following relationships hold:
F⫽
P
1
⫺P
2
R
where F is airflow (milliliters per minute) in a tube, P
1
is pres-
sure at point one,P
2
is pressure at point two,and R is resistance
to airflow.
Air moves through tubes because of a pressure difference.
When P
1
is greater than P
2
,gas flows from P
1
to P
2
at a rate that’s
proportional to the pressure difference.For example, during in-
spiration, air pressure outside the body is greater than air pres-
sure in the alveoli,and air flows through the trachea and bronchi
to the alveoli.
Part4 Regulationsand Maintenance828
DisordersThat Decrease the Radius of Air
Passageways
The flow ofair decreases when the resistance to airflow is increased by
conditionsthat reduce the radius of the respiratory passageways.
According to Poiseuille’slaw (see chapter 21), the resistance to airflow is
proportionalto the radius (r) of a tube raised to the fourth power (r
4
).
Thus, a smallchange in radius results in a large change in resistance,
which greatlydecreases airflow. For example, asthma resultsin the
release ofinflammatory chemicals such asleukotrienes that cause
severe constriction ofthe bronchioles. Emphysema produces increased
airwayresistance because the bronchioles are obstructed asa result of
inflammation and because damaged bronchiolescollapse during
expiration, thustrapping air within the alveoli. Cancer can also occlude
respiratorypassages as the tumor replaceslung tissue. Increasing the
pressure difference between alveoli and the atmosphere can help to
maintain airflow despite increased resistance. Within limits, thiscan be
accomplished byincreased contraction of the musclesof respiration.
Pressure and Volume
The pressure in a container,such as the thoracic cavity or an alveo-
lus,is described according to the general gas law.
P ⫽
nRT
V
where Pis pressure, n is the number of gram moles of gas (a meas-
ure ofthe number of molecules present), R is the gas constant, T is
absolute temperature,and V is volume.
The value ofR is a constant, and the values of n and T (body
temperature) are considered constants in humans.Thus, the gen-
eral gas law reveals that air pressure is inversely proportional to
volume. As volume increases,pressure decreases; and as volume
decreases,pressure increases (table 23.1).
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Chapter 23 RespiratorySystem 829
18. Define the term ventilation.
19. How do pressure differences and resistance affect airflow
through a tube?
20. What happens to the pressure within a container when the
volume of the containerincreases?
Airflow into and out of Alveoli
Respiratory physiologists use three conventions to help simplify
the numbers used to express pressures.First, barometric air pres-
sure (P
B
), which is atmospheric air pressure outside the body,is
assigned a value ofzero. Thus, whether at sea level with a pressure
of760 mm Hg or at 10,000 feet above sea level on a mountaintop
with a pressure of523 mm Hg, P
B
is always zero.Second,the small
pressures in respiratory physiology are usually expressed in cen-
timeters of water (cm H
2
O).A pressure of 1 cm H
2
O is equal to
0.74 mm Hg.Third, other pressures are measured in reference to
barometric air pressure.For example, alveolar pressure (P
alv
) is
the pressure inside an alveolus.An alveolar pressure of 1 cm H
2
O
is 1 cm H
2
O greater pressure than barometric air pressure,and an
alveolar pressure of ⫺1 cm H
2
O is 1 cm H
2
O less pressure than
barometric air pressure.
Movement of air into and out of the lungs results from
changes in thoracic volume,which cause changes in alveolar vol-
ume.The changes in alveolar volume produce changes in alveolar
pressure.The pressure difference between barometric air pressure
and alveolar pressure (P
B
⫺P
alv
) results in air movement.The de-
tails ofthis process during quiet breathing are described as follows:
1. End ofexpiration (figure 23.13 1).At the end of expiration,
barometric air pressure and alveolar pressure are equal.
Therefore,no movement of air into or out of the lungs
takes place.
2. During inspiration(figure 23.13 2).As inspiration begins,
contraction ofinspirator y muscles increases thoracic
volume,which results in expansion of the lungs and an
increase in alveolar volume (see following section on
“Changing Alveolar Volume”).The increased alveolar
volume causes a decrease in alveolar pressure below
barometric air pressure to approximately ⫺1cm H
2
O.Ai r
flows into the lungs because barometric air pressure is
greater than alveolar pressure.
3. End ofinspiration (figure 23.13 3).At the end of inspiration,
the thorax stops expanding,the alveoli stop expanding, and
alveolar pressure becomes equal to barometric air pressure
because ofairflow into the lungs. No movement of air
occurs after alveolar pressure becomes equal to barometric
pressure,but the volume of the lungs is larger than at the
end ofexpiration.
4. During expiration(figure 23.13 4). During expiration,the
volume ofthe thorax decreases as the diaphragm relaxes,
and the thorax and lungs recoil.The decreased thoracic
volume results in a decrease in alveolar volume and an
increase in alveolar pressure over barometric air pressure to
approximately 1cm H
2
O.Air flows out of the lungs because
alveolar pressure is greater than barometric air pressure.As
expiration ends,the decrease in thoracic volume stops and
the alveoli stop changing size.The process repeats
beginning at step 1.
Changing Alveolar Volume
It’s important to understand how alveolar volume is changed be-
cause these changes cause the pressure differences resulting in ven-
tilation.In addition, many respiratory disorders affect how alveolar
volume changes.Lung recoil and changes in pleural pressure cause
changes in alveolar volume.
Lung Recoil
Lung recoil causes the alveoli to collapse, and it results from
(1)elastic recoil caused by the elastic fibers in the alveolar walls and
(2) surface tension ofthe film of fluid that lines the alveoli. Surface
tension occurs at the boundary between water and air because the
polar water molecules are attracted to one another more than they
Table 23.1
Description Importance
Gas Law
General Gas Law
The pressure of a gas is Air flows from areas higher to lower
inversely proportional pressure. When alveolar volume
to its volume (at a increases, causing pleural
constant temperature, pressure to decrease below
this is referred atmosphereic pressure, air moves
to as Boyle's law). into the lungs. When alveolar
volume decreases, causing
pleural pressure to increase above
atmospheric pressure, air moves
out of the lungs.
Dalton’s Law
The partial pressure of a gas Gases move from areas of higher to
in a mixture of gases is areas of lower partial pressures.
the percentage of the gas The greater the differernce in
in the mixture times the partial pressure between two
total pressure of the points, the greater the rate of gas
mixture of gases. movement. Maintaining partial
pressure differences ensures
gas movements.
Henry’s Law
The concentration of a Only a small amount of the gases in
gas dissolved in a liquid air dissolves in the fluid lining
is equal to the partial the alveoli. Carbon dioxide,
pressure of the gas over however, is 24 times more
the liquid times the soluble than oxygen; therefore,
solubility coefficient of carbon dioxide passes out
the gas. through the respiratory
membrane more readily than
oxygen enters.
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Part4 Regulationsand Maintenance830
P
B
= 0
No air
movement
P
alv
= 0
Diaphragm
End of expiration
P
B
= P
alv
P
B
= 0
Air moves in
P
alv
= –1
(alveolar
volume
increases)
Diaphragm
contracts
During inspiration
P
B
> P
alv
Thorax
expands
1. Barometric air pressure (P
B
) is equal to
alveolar pressure (P
alv
) and there is no air
movement.
2. Increased thoracic volume results in
increased alveolar volume and decreased
alveolar pressure. Barometric air pressure
is greater than alveolar pressure, and air
moves into the lungs.
Air moves out
P
B
= 0 P
B
= 0
P
alv
= 1
P
alv
= 0
(alveolar
volume
decreases)
During expiration
> P
B
P
alv
Diaphragm
relaxes
Thorax
recoils
4. Decreased thoracic volume results in
decreased alveolar volume and increased
alveolar pressure. Alveolar pressure is
greater than barometric air pressure, and
air moves out of the lungs.
3. End of inspiration.
No air
movement
ProcessFigure 23.13
Alveolar Pressure ChangesDuring Inspiration and Expiration
The combined space ofall the alveoli is represented by a large “bubble.” The alveoli are actuallymicroscopic in size and cannot be seen in the illustration.
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Chapter 23 RespiratorySystem 831
are attracted to the air molecules.Consequently, the water mole-
cules are drawn together,tending to form a droplet. Because the
water molecules of the alveolar fluid are also attracted to the sur-
face ofthe alveoli, formation of a droplet causes the alveoli to col-
lapse,thus producing fluid-filled alveoli with smaller volumes than
air-filled alveoli.
Surfactant (ser-fak⬘ta˘nt) is a mixture of lipoprotein mole-
cules produced by the type II pneumocytes of the alveolar epithe-
lium.The surfactant molecules form a monomolecular layer over
the surface ofthe fluid within the alveoli to reduce the surface ten-
sion.With surfactant, the force produced by surface tension is ap-
proximately 4 cm H
2
O;without surfactant, the force can be as high
as 40 cm H
2
O.Thus, surfactant greatly reduces the tendency of the
lungs to collapse.
RespiratoryDistress Syndrome
In premature infants, respiratorydistresssyndrome, or hyaline (hı¯⬘a˘-
lin)membrane disease, is common, especially for infantswith a
gestation age ofless than 7 months. This occursbecause surfactant is
notproduced in adequate quantities until approximately 7 months of
development. Thereafter, the amountproduced increases asthe fetus
matures. Cortisolcan be given to pregnant women who are likely to
deliver prematurely, because itcrosses the placenta into the fetusand
stimulatessurfactant synthesis.
Ifinsufficient surfactant is produced bya newborn, the lungs tend
to collapse. Thus, a greatdeal of energy mustbe exer ted bythe muscles
ofrespiration to keep the lungs inflated, and even then inadequate
ventilation occurs. Withoutspecialized treatment, most babies with this
disease die soon after birth asa result of inadequate ventilation of the
lungsand fatigue of the respiratory muscles. Positive end-expiratory
pressure deliversoxygen-rich, pressurized air to the lungsthrough a tube
passed through the respiratorypassages. The pressure helpsto keep the
alveoli inflated. In addition, human surfactantadministered with the
pressurized air can reduce surface tension in the alveoli.
Pleural Pressure
Pleural pressure (P
pl
) is the pressure in the pleural cavity. When
pleural pressure is less than alveolar pressure,the alveoli tend to ex-
pand. This principle can be understood by considering a balloon.
The balloon expands when the pressure outside the balloon is less
than the pressure inside. This pressure difference is normally
achieved by increasing the pressure inside the balloon when a person
forcefully blows into it.This pressure difference,however, can also be
achieved by decreasing the pressure outside the balloon.For exam-
ple,if the balloon is placed in a chamber from which air is removed,
the pressure around the balloon becomes lower than atmospheric
pressure,and the balloon expands. The lower the pressure outside
the balloon,the greater the tendency for the higher pressure inside
the balloon to cause it to expand. In a similar fashion,decreasing
pleural pressure can result in expansion ofthe alveoli.
Normally the alveoli are expanded because of a negative
pleural pressure that is lower than alveolar pressure.At the end of a
normal expiration, pleural pressure is ⫺5 cm H
2
O,and alveolar
pressure is 0 cm H
2
O.Pleural pressure is lower than alveolar pres-
sure because ofa “suction effect”caused by lung recoil. As the lungs
recoil, the visceral and parietal pleurae tend to be pulled apart.
Normally the lungs don’t pull away from the thoracic wall because
pleural fluid holds the visceral and parietal pleurae together.
Nonetheless,this pull decreases pressure in the pleural cavity,an ef-
fect that can be appreciated by putting water on the palms of the
hands and putting them together.A sensation of negative pressure
is felt as the hands are gently pulled apart.
When pleural pressure is lower than alveolar pressure,the
alveoli tend to expand.This expansion is opposed by the tendency
ofthe lungs to recoil. If the pleural pressure is sufficiently low, lung
recoil is overcome and the alveoli expand.If the pleural pressure is
not low enough to overcome lung recoil,then the alveoli collapse.
Pneumothorax
Apneumothorax is the introduction of air into the pleural cavity through
an opening in the thoracicwall or lung. Pneumothorax can resultfrom
penetrating trauma bya knife, bullet, broken rib, or other object;
nonpenetrating trauma such asa blow to the chest; medicalprocedures
such asinserting a catheter to withdraw pleural fluid; disease, such as
infectionsor emphysema; or can be of unknown cause.
Pleuralpressure becomes equal to barometric air pressure when
the pleuralcavity is connected to the outside through an opening in the
thoracicwall or the surface of the lung. The alveoli, therefore, don’ttend
to expand, lung recoilis unopposed, and the lung collapsesand pulls
awayfrom the thoracic wall. Pneumothorax can occur in one lung while
the lung on the opposite side remainsinflated because the two pleural
cavitiesare separated by the mediastinum.
The mostcommon symptoms of pneumothorax are chest pain
and shortnessof breath. Treatment ofpneumothorax depends upon its
cause and severity. In patientswith mild symptoms, the pneumothorax
mayresolve on its own. In other cases, a chest tube thataspirates the
pleuralcavity and restores a negative pressure can cause re-expansion
ofthe lung. Surgery may also be necessary to close the opening into the
pleuralcavity.
In a tension pneumothorax,the pressure within the thoracic
cavityis always higher than barometric air pressure. A tissue flap or air
passagewayforms a flutter valve that allowsair to enter the pleural
cavityduring inspiration but not exit during expiration. The resultis an
increase in air and pressure within the pleuralcavity that can compress
blood vesselsreturning blood to the heart, causing decreased venous
return, low blood pressure, and inadequate deliveryof oxygen to tissues.
Insertion ofa large bore needle into the pleural cavity allows air to
escape and releasesthe pressure.
Pressure ChangesDuring Inspiration and Expiration
At the end ofa normal expiration, pleural pressure is ⫺5 cm H
2
O,
and alveolar pressure is equal to barometric pressure (0 cm H
2
O)
(figure 23.14).During nor mal,quiet inspiration, pleural pressure
decreases to ⫺8 cm H
2
O.Consequently, the alveolar volume in-
creases,alveolar pressure decreases below barometric air pressure,
and air flows into the lungs.As air flows into the lungs, alveolar
pressure increases and becomes equal to barometric pressure at the
end ofinspiration.
The decrease in pleural pressure during inspiration occurs
for two reasons.First, because of the effect of changing volume on
pressure(general gas law), when the volume of the thoracic cavity
increases,pleural pressure decreases. Second, as the thoracic cavity
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expands,the lung s expand because they adhere to the inner tho-
racic wall through the pleurae.As the lungs expand, the tendency
for the lungs to recoil increases,resulting in an increased suction
effect and a lowering of pleural pressure. The tendency for the
lungs to recoil increases as the lungs are stretched,similar to the in-
creased force generated in a stretched rubber band.
During expiration,pleural pressure increases because of de-
creased thoracic volume and decreased lung recoil (see figure
23.14). As pleural pressure increases,alveolar volume decreases,
alveolar pressure increases above barometric air pressure,and air
flows out of the lungs.As air flows out of the lungs, alveolar pres-
sure decreases and becomes equal to barometric pressure at the
end ofexpiration.
21. Define barometric and alveolar pressures.
Part4 Regulationsand Maintenance832
22. Explain how changes in alveolar volume cause air to move
into and outof the lungs.
23. Name two things that cause the lungs to recoil. How does
surfactantreduce lung recoil? What happens if there are
inadequate amountsof surfactant in the alveoli?
24. Define pleural pressure. What happens to alveolar volume
when pleural pressure decreases? Name two thingsthat
cause pleural pressure to decrease.
25. How does an opening in the chest wall cause the lung to
collapse?
PREDICT
How doesthe pleural pressure at the end of expiration in a newborn with
respiratorydistress syndrome compare to thatof a healthy newborn?
How doesthe pleural pressure compare during inspiration? Explain.
1
4
2
5
3
6
–5
Pleural pressure
(cm H
2
O)
Changes during
inspiration
During inspiration, air
flows into the lungs
because alveolar pressure
is lower than barometric
air pressure.
During expiration, air
flows out of the lungs
because alveolar pressure
is greater than barometric
air pressure.
Pleural pressure increases
because thoracic volume
decreases.
Changes during
expiration
Inspiration Expiration
–7
–9
Alveolar pressure
(cm H
2
O)
Change in
lung volume (L)
0
–1
+1
0
012
Time (s)
345
+0.5
Pleural pressure decreases
because thoracic volume
increases.
As inspiration begins,
alveolar pressure decreases
below barometric air
pressure (0 on the graph)
because the decreased
pleural pressure causes
alveolar volume to increase.
By the end of inspiration,
alveolar and barometric air
pressure are equal.
As expiration begins,
alveolar pressure increases
above barometric air
pressure (0 on the graph)
because the increased
pleural pressure causes
alveolar volume to decrease.
By the end of expiration,
alveolar and barometric air
pressure are equal.
1.
2.
3.
4.
5.
6.
ProcessFigure 23.14
Dynamicsof a Normal Breathing Cycle
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Chapter 23 RespiratorySystem 833
Measuring Lung Function
Objectives
■ Define the term compliance, and explain itssignificance.
■ Listthe pulmonary volumes and capacities, and define each
of them.
■ Explain the significance of forced expiratoryvolume in one
second, minute ventilation, and alveolarventilation.
A variety of measurements can be used to assess lung func-
tion. Each of these tests compares a subject’s measurements to a
normal range. These measurements can be used to diagnose dis-
eases,track the progress of diseases, or track recovery from diseases.
Compliance ofthe Lungs and the Thorax
Complianceis a measure of the ease with which the lungs and the
thorax expand.The compliance of the lungs and thor ax is the vol-
ume by which they increase for each unit ofpressure change in alve-
olar pressure. It is usually expressed in liters (volume of air) per
centimeter ofwater (pressure), and for the normal person the com-
pliance ofthe lungs and thorax is 0.13 L/cm H
2
O.That is, for every 1
cm H
2
O change in alveolar pressure,the volume changes by 0.13 L.
The greater the compliance,the easier it is for a change in
pressure to cause expansion ofthe lungs and thorax. For example,
one possible result ofemphysema is the destruction of elastic lung
tissue. This reduces the elastic recoil force of the lungs,thereby
making expansion of the lungs easier and resulting in a higher-
than-normal compliance.A lower-than-normal compliance means
that it’s harder to expand the lungs and thorax.Conditions that de-
crease compliance include deposition ofinelastic fibers in lung tis-
sue (pulmonary fibrosis), collapse of the alveoli (respiratory
distress syndrome and pulmonary edema),increased resistance to
airflow caused by airway obstruction (asthma,bronchitis, and lung
cancer),and deformities of the thoracic wall that reduce the ability
ofthe thoracic volume to increase (kyphosis and scoliosis).
Effectsof Decreased Compliance
Pulmonarydiseases can markedly affectthe total amount of energy
required for ventilation, aswell as the percentage ofthe total amount of
energyexpended by the body. Diseases that decrease compliance can
increase the energyrequired for breathing up to 30% of the total energy
expended bythe body.
PulmonaryVolumes and Capacities
Spirometry(spı¯-rom⬘e˘-tre¯) is the process of measuring volumes of
air that move into and out ofthe respiratory system, and a spirom-
eter (spı¯-rom⬘e˘-ter) is a device used to measure these pulmonary
volumes (figure 23.15a).The four pulmonary volumes and repre-
sentative values (figure 23.15b) for a young adult male follow:
1. Tidal volumeis the volume of air inspired or expired
during a normal inspiration or expiration (approximately
500 mL).
2. Inspiratory reserve volumeis the amount of air that can
be inspired forcefully after inspiration ofthe normal tidal
volume (approximately 3000 mL).
3. Expiratory reserve volumeis the amount of air that can be
forcefully expired after expiration ofthe normal tidal
volume (approximately 1100 mL).
4. Residual volumeis the volume of air still remaining in the
respiratory passages and lungs after the most forceful
expiration (approximately 1200 mL).
Pulmonary capacitiesare the sum of two or more pulmonary
volumes (see figure 23.15b).Some pulmonary capacities follow:
1. Inspiratory capacityis the tidal volume plus the
inspiratory reserve volume,which is the amount of air that
a person can inspire maximally after a normal expiration
(approximately 3500 mL).
2. Functional residual capacityis the expiratory reserve
volume plus the residual volume,which is the amount of air
remaining in the lungs at the end ofa normal expiration
(approximately 2300 mL).
3. Vital capacityis the sum of the inspiratory reserve volume,
the tidal volume,and the expiratory reserve volume, which
is the maximum volume ofair that a person can expel from
the respiratory tract after a maximum inspiration
(approximately 4600 mL).
4. Total lung capacityis the sum ofthe inspiratory and
expiratory reserve volumes plus the tidal volume and the
residual volume (approximately 5800 mL).
Factors like sex, age,body size, and physical conditioning
cause variations in respiratory volumes and capacities from one in-
dividual to another.For example, the vital capacity of adult females
is usually 20%–25% less than that ofadult males. The vital capacity
reaches its maximum amount in the young adult and gradually de-
creases in the elderly.Tall people usually have a greater vital capac-
ity than short people,and thin people have a greater vital capacity
than obese people. Well-trained athletes can have a vital capacity
30%–40% above that ofuntrained people. In patients whose respi-
ratory muscles are paralyzed by spinal cord injury or diseases like
poliomyelitis or muscular dystrophy,vital capacity can be reduced
to values not consistent with survival (less than 500–1000 mL).Fac-
tors that reduce compliance also reduce vital capacity.
The forced expiratory vital capacity is a simple and clini-
cally important pulmonary test.The individual inspires maximally
and then exhales maximally into a spirometer as rapidly as possi-
ble.The volume of air expired at the end of the test is the person’s
vital capacity.The spirometer also records the volume of air that
enters it per second.The forced expiratory volume in one second
(FEV
1
)is the amount of air expired during the first second ofthe
test.In some conditions, the vital capacity may not be dramatically
affected, but how rapidly air is expired can be greatly decreased.
Airway obstruction,caused by asthma, collapse of bronchi in em-
physema,or a tumor, and disorders that reduce the ability of the
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lungs or chest wall to deflate,such as pulmonary fibrosis, silicosis,
kyphosis,and scoliosis, can cause a decreased FEV
1
.
Minute Ventilation and Alveolar Ventilation
Minute ventilationis the total amount of air moved into and out
of the respiratory system each minute,and it is equal to tidal vol-
ume times the respiratory rate. Respiratory rate, or respiratory
frequency, is the number of breaths taken per minute. Because
resting tidal volume is approximately 500 mL and respiratory rate
is approximately 12 breaths per minute,minute ventilation aver-
ages approximately 6 L/min.
Part4 Regulationsand Maintenance834
Although minute ventilation measures the amount of air
moving into and out ofthe lungs per minute, it’s not a measure of
the amount of air available for gas exchange because gas exchange
takes place mainly in the alveoli and to a lesser extent in the alveo-
lar ducts and the respiratory bronchioles.The part of the respira-
tory system where gas exchange does not take place is called the
dead space.A distinction can be made between anatomic and phys-
iologic dead space. Anatomic dead space, which measures 150
mL, is formed by the nasal cavity, pharynx, larynx, trachea,
bronchi,bronchioles, and terminal bronchioles. Physiologic dead
space is anatomic dead space plus the volume of any alveoli in
0
1000
2000
3000
Volume (mL)
Maximum
inspiration
Maximum
expiration
Time
Volumes
Inspiratory reserve volume
(3000 mL)
Tidal
volume
(500
mL)
Expiratory
reserve
volume
(1100 mL)
Residual
volume
(1200 mL)
Functional residual capacity
(2300 mL)
Inspiratory capacity
(3500 mL)
Vital capacity (4600 mL)
Total lung capacity (5800 mL)
Capacities
4000
5000
6000
Figure 23.15
Spirometer, Lung Volumes, and Lung
Capacities
(a) A spirometer used to measure lung volumesand capacities. (b) Lung
volumesand capacities. The tidal volume in the figure isthe tidal volume
during resting conditions.
(a)
(b)
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Chapter 23 RespiratorySystem 835
which gas exchange is less than normal. In a healthy person,
anatomic and physiologic dead spaces are nearly the same,mean-
ing that few nonfunctional alveoli exist.
Emphysema and Dead Space
In patientswith emphysema, alveolar walls degenerate, and small
alveoli combine to form larger alveoli. The resultis fewer alveoli, but
alveoli with an increased volume and decreased surface area. Although
the enlarged alveoli are stillventilated, surface area is inadequate for
complete gasexchange, and the physiologicdead space increases.
During inspiration, much of the inspired air fills the dead
space first before reaching the alveoli and,thus, is unavailable for gas
exchange.The volume of air available for gas exchange per minute is
calledalveolar ventilation (
•
V
A
),and it is calculated as follows:
•
V
A
⫽f (V
T
⫺V
D
)
where
•
V
A
is alveolar ventilation (milliliters per minute),f is respiratory
rate (frequency;breaths per minute), V
T
is tidal volume (milliliters per
respiration),and V
D
is dead space (milliliters per respiration).
26. Define the term compliance. What is the effect on lung
expansion when compliance increasesor decreases?
27. Define the terms tidal volume, inspiratory reserve volume,
expiratoryreserve volume, and residual volume.
28. Define the terms inspiratory capacity, functional residual
capacity, vital capacity, and total lung capacity.
29. What is forced expiratory volume in one second, and why is
itclinically important?
30. Define the terms minute ventilation and alveolar
ventilation.
31. What is dead space? What is the difference between
anatomicand physiologic dead space?
PREDICT
Whatis the alveolar ventilation of a resting person with a tidal volume
of500 mL, a dead space of 150 mL, and a respiratory rate of 12
breathsper minute? If the person exercises and tidal volume
increasesto 4000 mL, dead space increases to 300 mL asa result of
dilation ofthe respiratory passageways, and respiratory rate increases
to 24 breathsper minute, what is the alveolar ventilation? How is the
change in alveolar ventilation beneficialfor doing exercise?
Physical Principlesof
Gas Exchange
Objectives
■ Define the termspartial pressure of a gas and water vapor
pressure.
■ Describe the factorsaffecting the movement of gas into and
through a liquid.
■ Explain the factorsthat affect gas movement through the
respiratorymembrane.
■ Describe the effectthat ventilation and pulmonary capillary
blood flowhave on gas exchange.
Ventilation supplies atmospheric air to the alveoli.The next
step in the process ofrespiration is the diffusion of gases between
alveoli and blood in the pulmonary capillaries. The molecules of
gas move randomly,and if a gas is in a higher concentration at one
point than at another,random motion ensures that the net move-
ment ofgas is from the higher concentration toward the lower con-
centration until a homogeneous mixture ofgases is achieved. One
measurement ofthe concentration of gases is par tial pressure.
Partial Pressure
At sea level,atmospheric pressure is approximately 760 mm Hg,
which means that the mixture of gases that constitute atmo-
spheric air exerts a total pressure of760 mm Hg. The major com-
ponents of dry air are nitrogen (approximately 79%) and oxygen
(approximately 21%).According to Dalton’s law,in a mixture of
gases,the part of the total pressure resulting from each type of gas
is determined by the percentage of the total volume represented
by each gas type (see table 23.1).The pressure exerted by each type
of gas in a mixture is referred to as the partial pressure of that
gas. Because nitrogen makes up 78.62% of the volume of atmo-
spheric air,the partial pressure resulting from nitrogen is 0.7862
times 760 mm Hg,or 597.5 mm Hg. Because oxygen is 20.84% of
the volume ofatmospheric air, the partial pressure resulting from
oxygen is 0.2084 times 760 mm Hg,or 158.4 mm Hg .It’s tradi-
tional to designate the partial pressure of individual gases in a
mixture with a capital P followed by the symbol for the gas.Thus,
the partial pressure ofnitrogen is denoted P
N
2
,oxygen is P
O
2
,and
carbon dioxide is P
CO
2
.
When air comes into contact with water,some of the water
turns into a gas and evaporates into the air.Water molecules in
gaseous form also exert a partial pressure. This partial pressure
(P
H
2
O
) is sometimes referred to as water vapor pressure.The com-
position ofdr y,humidified, alveolar,and expired air is presented in
table 23.2.The composition of alveolar air and of expired air is not
identical to the composition of dry atmospheric air for three rea-
sons.First, air entering the respiratory system during inspiration is
humidified;second, oxygen diffuses from the alveoli into the blood,
and carbon dioxide diffuses from the pulmonary capillaries into the
alveoli;and third, the air within the alveoli is only partially replaced
with atmospheric air during each inspiration.
Diffusion ofGases Through Liquids
When a gas comes into contact with a liquid such as water,it tends
to dissolve in the liquid.At equilibrium, the concentration of a gas
in the liquid is determined by its partial pressure in the gas and by
its solubility in the liquid.This relationship is described by Henry’s
law(see table 23.1).
Concentration of
⫽
Partial pressure
⫻
Solubility
dissolved gas of gas coefficient
The solubility coefficient is a measure of how easily the gas
dissolves in the liquid.In water,the solubility coefficient for oxygen
is 0.024,and for carbon dioxide it is 0.57. Thus, carbon dioxide is
approximately 24 times more soluble in water than is oxygen.
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Gases don’t actually produce a partial pressure in a liquid as
they do when in the gaseous state.Using the general gas law equa-
tion and the concentration ofa gas in a liquid, however, the partial
pressure ofthe gas if it were in a gaseous state can be calculated. Be-
cause the calculated partial pressure of a gas in a liquid is a meas-
ure of concentration,it can be used to determine the direction of
diffusion ofthe gas through the liquid: the gas moves from areas of
higher to areas oflower partial pressure.
PREDICT
Asa SCUBA diver descends, the pressure of the water on the body
preventsnormal expansion of the lungs. To compensate, the diver
breathespressurized air, which hasa greater pressure than air at sea
level. Whateffect does the increased pressure have on the amountof
gasdissolved in the diver’s body fluids? A SCUBA diver who suddenly
ascendsto the surface from a great depth can develop decompression
sickness(the bends) in which bubbles of nitrogen gasform. The
expanding bubblesdamage tissues or blockblood flow through small
blood vessels. Explain the developmentof the bubbles.
Diffusion ofGases Through the
RespiratoryMembrane
The factors that influence the rate ofgas diffusion across the respi-
ratory membrane include (1) the thickness of the membrane; (2)
the diffusion coefficient of the gas in the substance of the mem-
brane,which is approximately the same as the diffusion coefficient
for the gas through water; (3) the surface area of the membrane;
and (4) the difference of the partial pressures of the gas between
the two sides ofthe membrane.
RespiratoryMembrane Thickness
Increasing the thickness ofthe respiratory membrane decreases the
rate of diffusion.The thickness of the respiratory membrane nor-
mally averages 0.6 m, but diseases can cause an increase in the
thickness. If the thickness of the respiratory membrane increases
two or three times, the rate of gas exchange markedly decreases.
Pulmonary edema caused by failure of the left side of the heart is
the most common cause ofan increase in the thickness of the res-
piratory membrane. Left side heart failure increases venous pres-
Part4 Regulationsand Maintenance836
sure in the pulmonary capillaries and results in the accumulation
offluid in the alveoli. Conditions such as tuberculosis, pneumonia,
or advanced silicosis that result in inflammation ofthe lung tissues
can also cause fluid accumulation within the alveoli.
Diffusion Coefficient
The diffusion coefficient is a measure of how easily a gas dif-
fuses through a liquid or tissue,taking into account the solubil-
ity of the gas in the liquid and the size of the gas molecule
(molecular weight). If the diffusion coefficient of oxygen is as-
signed a value of 1,then the relative diffusion coefficient of car-
bon dioxide is 20,which means carbon dioxide diffuses through
the respiratory membrane about 20 times more readily than
oxygen does.
When the respiratory membrane becomes progressively
damaged as a result of disease,its capacity for allowing the move-
ment of oxygen into the blood is often impaired enough to cause
death from oxygen deprivation before the diffusion of carbon
dioxide is dramatically reduced.If life is being maintained by ex-
tensive oxygen therapy,which increases the concentration of oxy-
gen in the lung alveoli, the reduced capacity for the diffusion of
carbon dioxide across the respiratory membrane can result in sub-
stantial increases in carbon dioxide in the blood.
Surface Area
In a healthy adult,the total surface area of the respiratory mem-
brane is approximately 70 m
2
(approximately the floor area ofa
25- by 30-foot room). Several respiratory diseases, including
emphysema and lung cancer,cause a decrease in the surface area
of the respiratory membrane. Even small decreases in this sur-
face area adversely affect the respiratory exchange ofgases dur-
ing strenuous exercise. When the total surface area of the
respiratory membrane is decreased to one-third or one-fourth
of normal, the exchange of gases is significantly restricted even
under resting conditions.
A decreased surface area for gas exchange can also result
from the surgical removal oflung tissue, the destruction of lung
tissue by cancer, the degeneration of the alveolar walls by
Table 23.2
Dry Air Humidified Air Alveolar Air Expired Air
Gases mm Hg % mm Hg % mm Hg % mm Hg %
Nitrogen 597.5 78.62 563.4 74.09 569.0 74.9 566.0 74.5
Oxygen 158.4 20.84 149.3 19.67 104.0 13.6 120.0 15.7
Carbon dioxide 0.3 0.04 0.3 0.04 40.0 5.3 27.0 3.6
Water vapor 0.0 0.0 47.0 6.20 47.0 6.2 47.0 6.2
Partial Pressures of Gases at Sea Level
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Chapter 23 RespiratorySystem 837
emphysema,or the replacement of lung tissue by connective tis-
sue caused by tuberculosis.More acute conditions that cause the
alveoli to fill with fluid also reduce the surface area for gas ex-
change.Examples include pneumonia and pulmonary edema re-
sulting from failure ofthe left ventricle.
Partial Pressure Difference
The partial pressure difference ofa gas across the respiratory mem-
brane is the difference between the partial pressure ofthe gas in the
alveoli and the partial pressure ofthe gas in the blood of the pul-
monary capillaries.When the partial pressure of a gas is greater on
one side of the respiratory membrane than on the other side, net
diffusion occurs from the higher to the lower partial pressure (see
figure 23.8b). Normally,the partial pressure of oxygen (P
O
2
) is
greater in the alveoli than in the blood ofthe pulmonar y capillar-
ies,and the partial pressure of carbon dioxide (P
CO
2
) is greater in
the blood than in the alveolar air.
By increasing alveolar ventilation,the partial pressure differ-
ence for oxygen and carbon dioxide can be raised.The greater vol-
ume ofatmospher ic air exchanged with the residual volume raises
alveolar P
O
2
, lowers alveolar P
CO
2
, and thus promotes gas ex-
change. Conversely,inadequate ventilation causes a lower-than-
normal partial pressure difference for oxygen and carbon dioxide,
resulting in inadequate gas exchange.
Relationship Between Ventilation and
PulmonaryCapillary Blood Flow
Under normal conditions,ventilation of the alveoli and blood flow
through pulmonary capillaries is such that effective gas exchange
occurs between the air and the blood.During exercise, effective gas
exchange is maintained because both ventilation and cardiac out-
put increase.
The normal relationship between ventilation and pul-
monary capillary blood flow can be disrupted in two different
ways.One way occurs when ventilation exceeds the ability of the
blood to pick up oxygen,which can happen because of inadequate
cardiac output after a heart attack.Another way occurs when ven-
tilation is not great enough to provide the oxygen needed to oxy-
genate the blood flowing through the pulmonary capillaries. For
example, constriction of the bronchioles in asthma can decrease
air delivery to the alveoli.
Blood that isn’t completely oxygenated is called shunted
blood. Two sources of shunted blood exist in the lungs. An
anatomic shunt results when deoxygenated blood from the
bronchi and bronchioles mixes with blood in the pulmonary veins
(see section on “Blood Supply”on p. 826). The other source of
shunted blood is blood that passes through pulmonary capillaries
but doesn’t become fully oxygenated.The physiologic shuntis the
combination ofdeoxygenated blood from the anatomic shunt and
the pulmonary capillaries. Normally,1%–2% of cardiac output
passes through the physiologic shunt.
DisordersThat Increase Shunted Blood
Anycondition that decreases gas exchange between the alveoli and the
blood can increase the amountof shunted blood. For example,
obstruction ofthe bronchioles in conditions such asasthma can
decrease ventilation beyond the obstructed areas. The resultisa large
increase in shunted blood because the blood flowing through the
pulmonarycapillaries in the obstructed area remains unoxygenated. In
pneumonia or pulmonaryedema, a buildup of fluid in the alveoli results
in poor gasdiffusion and less oxygenated blood.
When a person is standing,greater blood flow and ventilation
occur in the base ofthe lung than in the top of the lung because of
the effects of gravity.Arterial pressure at the base of the lung is 22
mm Hg greater than at the top of the lung because of hydrostatic
pressure caused by gravity (see chapter 21).This greater pressure in-
creases blood flow and distends blood vessels.The decreased pres-
sure at the top ofthe lung results in less blood flow and vessels that
are less distended,some of which are even collapsed during diastole.
During exercise,cardiac output and ventilation increase.The
increased cardiac output increases pulmonary blood pressure
throughout the lung, which increases blood flow.Blood flow in-
creases most at the top ofthe lung, however, because the increased
pressure expands the less distended vessels and opens the collapsed
vessels. Thus, the effectiveness of gas exchange at the top ofthe
lung increases because ofgreater blood flow.
Although gravity is the major factor affecting regional blood
flow in the lung,under certain circumstances alveolar P
O
2
can have
an effect also.In most tissues, low P
O
2
results in increased blood
flow through the tissues (see chapter 21).In the lung, low P
O
2
has
the opposite effect, causing arterioles to constrict and reducing
blood flow.This response reroutes blood away from areas of low
oxygen toward parts ofthe lung that are better oxygenated. For ex-
ample,if a bronchus becomes partially blocked, ventilation of alve-
oli past the blockage site decreases,which decreases gas exchange
between the air and blood. The effect of this decreased gas ex-
change on overall gas exchange in the lungs is reduced by rerouting
the blood to better-ventilated alveoli.
32. According to Dalton’s law, what is the partial pressure of a
gas? Whatis water vapor pressure?
33. Why is the composition of inspired, alveolar, and expired
airdifferent?
34. According to Henry’s law, how does the partial pressure
and solubilityof a gas affect its concentration in a liquid?
35. Describe four factors that affect the diffusion of gases
acrossthe respiratory membrane. Give examples of
diseasesthat decrease diffusion by altering these factors.
36. Does oxygen or carbon dioxide diffuse most easily through
the respiratorymembrane?
37. What effect do ventilation and pulmonary capillary blood
flowhave on gas exchange? What is the physiologic shunt?
38. What are the effects of gravity and alveolar P
O
2
on blood
flowin the lung?
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23. Respiratory System
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PREDICT
Even people in “good shape” can have trouble breathing athigh
altitudes. Explain how thiscan happen, even when ventilation of the
lungsincreases.
Oxygen and Carbon Dioxide
Transportin the Blood
Objectives
■ Describe the partial pressuresof oxygen and carbon
dioxide in the alveoli, lung capillaries, tissue capillaries,
and tissues.
■ Explain the significance of the oxygen-hemoglobin
dissociation curve, and illustrate howit is affected by
changesin carbon dioxide, pH, temperature, and BPG.
■ Describe howcarbon dioxide is transported in the blood,
and discussthe chloride shift and how respiration can
affectblood pH.
Once oxygen diffuses across the respiratory membrane into
the blood,most of it combines reversibly with hemoglobin, and a
smaller amount dissolves in the plasma. Hemoglobin transports
oxygen from the pulmonary capillaries through the blood vessels
to the tissue capillaries,where some of the oxygen is released. The
oxygen diffuses from the blood to tissue cells,where it is used in
aerobic respiration.
Cells produce carbon dioxide during aerobic metabolism,
and it diffuses from the cells into the tissue capillaries.Once car-
bon dioxide enters the blood, it is transported dissolved in the
plasma,in combination with hemoglobin, and in the form of bi-
carbonate ions.
Oxygen Diffusion Gradients
The P
O
2
within the alveoli averages approximately 104 mm Hg,and
as blood flows into the pulmonary capillaries,it has a P
O
2
ofapprox-
imately 40 mm Hg (figure 23.16). Consequently,oxygen diffuses
from the alveoli into the pulmonary capillary blood because the P
O
2
is greater in the alveoli than in the capillary blood.By the time blood
flows through the first third of the pulmonary capillary beds, an
equilibrium is achieved, and the P
O
2
in the blood is 104 mm Hg,
which is equivalent to the P
O
2
in the alveoli.Even with the greater ve-
locity of blood flow associated with exercise, by the time blood
reaches the venous ends ofthe pulmonary capillaries, the P
O
2
in the
capillaries has achieved the same value as that in the alveoli.
Blood leaving the pulmonary capillaries has a P
O
2
of
104 mm Hg, but blood leaving the lungs in the pulmonary veins
has a P
O
2
ofapproximately 95 mm Hg. The decrease in the P
O
2
oc-
curs because the blood from the pulmonary capillaries mixes with
deoxygenated (shunted) blood from the bronchial veins.
The blood that enters the arterial end ofthe tissue capillaries
has a P
O
2
of approximately 95 mm Hg.The P
O
2
of the interstitial
spaces, in contrast, is close to 40 mm Hg and is probably near
Part4 Regulationsand Maintenance838
20mm Hg in the individual cells. Oxygen diffuses from the tissue
capillaries to the interstitial fluid and from the interstitial fluid into
the cells of the body,where it’s used in aerobic metabolism. Be-
cause oxygen is continuously used by cells,a constant diffusion
gradient exists for oxygen from the tissue capillaries to the cells.
Carbon Dioxide Diffusion Gradients
Carbon dioxide is continually produced as a by-product ofcellular
respiration,and a diffusion gradient is established from tissue cells
to the blood within the tissue capillaries.The intracellular P
CO
2
is
approximately 46 mm Hg,and the interstitial fluid P
CO
2
is approx-
imately 45 mm Hg.At the arterial end of the tissue capillaries, the
P
CO
2
is close to 40 mm Hg.As blood flows through the tissue cap-
illaries,carbon dioxide diffuses from a higher P
CO
2
to a lower P
CO
2
until an equilibrium in P
CO
2
is established.At the venous end of
the capillaries,blood has a P
CO
2
of45 mm Hg (see figure 23.16).
After blood leaves the venous end ofthe capillaries, it’s trans-
ported through the cardiovascular system to the lungs.At the arte-
rial end of the pulmonary capillaries, the P
CO
2
is 45 mm Hg.
Because the P
CO
2
is approximately 40 mm Hg in the alveoli,carbon
dioxide diffuses from the pulmonary capillaries into the alveoli.At
the venous end of the pulmonary capillaries, the P
CO
2
has again
decreased to 40 mm Hg.
39. Describe the partial pressures of oxygen and carbon
dioxide in the alveoli, lung capillaries, tissue capillaries,
and tissues. Howdo these partial pressures account for the
movementof oxygen and carbon dioxide between air and
blood and between blood and tissues?
Hemoglobin and Oxygen Transport
Approximately 98.5% ofthe oxygen transported in the blood from
the lungs to the tissues is transported in combination with hemo-
globin in red blood cells,and the remaining 1.5% is dissolved in the
water part of the plasma.The combination of oxygen with hemo-
globin is reversible.In the pulmonary capillaries, oxygen binds to
hemoglobin, and in the tissue spaces oxygen diffuses away from
hemoglobin and enters the tissues.
Effectof P
O
2
The oxygen-hemoglobin dissociation cur ve describes the per-
centage ofhemoglobin saturated with oxygen at any given P
O
2
.He-
moglobin is saturated when an oxygen molecule is bound to each
ofits four heme g roups (see chapter 19).At any P
O
2
above 80 mm
Hg,approximately 95% of the hemoglobin is saturated w ith oxy-
gen (figure 23.17).Because the P
O
2
in the pulmonary capillaries is
normally 104 mm Hg,the hemoglobin is 98% saturated.
In a resting person,the normal P
O
2
of blood leaving the tis-
sue capillaries of skeletal muscle is 40 mm Hg.At a P
O
2
of 40 mm
Hg, hemoglobin is approximately 75% saturated.Thus, approxi-
mately 23% of the oxygen bound to hemoglobin is released into
the blood and can diffuse into the tissue spaces.During conditions
of vigorous exercise,the blood P
O
2
can decline to levels as low as
Seeley−Stephens−Tate:
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Chapter 23 RespiratorySystem 839
Oxygen diffuses into the
arterial ends of pulmonary
capillaries and carbon
dioxide diffuses into the
alveoli because of
differences in partial
pressures.
As a result of diffusion
at the venous ends of
pulmonary capillaries, the
P
O
2
in the blood is equal
to the PO
2
in the alveoli
and the PCO
2
in the
blood is equal to the
PCO
2
in the alveoli.
The P
O
2
of blood in
the pulmonary veins
is less than in the
pulmonary capillaries
because of mixing with
deoxygenated blood from
veins draining the bronchi
and bronchioles.
Oxygen diffuses out of the
arterial ends of tissue
capillaries and carbon
dioxide diffuses out of
the tissue because of
differences in partial
pressures.
As a result of diffusion at
the venous ends of tissue
capillaries, the P
O
2
in the
blood is equal to the PO
2
in the tissue and the PCO
2
in the blood is equal to the
PCO
2
in the tissue.
Go back to step 1.
1.
2.
3.
5.
4.
P
O
2
= 40
PO
2
= 40
PCO
2
= 45
P
CO
2
= 45 PCO
2
= 40
P
CO
2
= 45
PO
2
= 95
PO
2
= 40 PO
2
= 104
P
CO
2
= 40
P
O
2
= 104
P
CO
2
= 40
P
CO
2
= 0.3
P
O
2
= 104
P
O
2
= 160
P
CO
2
= 27
P
O
2
= 120
PO
2
= 40
P
O
2
= 20
PCO
2
= 40
P
CO
2
= 45
P
CO
2
= 46
Tissue capillary
Heart
Right Left
Pulmonary capillary
Blood in
pulmonary veins
Alveolus
Alveolus
Inspired air
Expired air
Interstitial
fluid
Tissue cells
P
CO
2
= 40
PO
2
= 95
1
2
3
5
4
ProcessFigure 23.16
Changesin the Partial Pressures of Oxygen and Carbon Dioxide
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23. Respiratory System
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15mm Hg because the skeletal muscle cells are using the oxygen in
aerobic respiration (see chapter 9).At a P
O
2
of15 mm Hg, approx-
imately 25% ofthe hemoglobin is saturated with oxygen, and it re-
leases 73% of the bound oxygen (figure 23.18). Thus, when the
oxygen needs of tissues increase, blood P
O
2
decreases, and more
oxygen is released for use by the tissues.
Effectof pH, P
CO
2
, and Temperature
In addition to P
O
2
,other factors influence the degree to which oxy-
gen binds to hemoglobin. As the pH of the blood declines, the
amount ofoxygen bound to hemoglobin at any given P
O
2
also de-
clines.This occurs because decreased pH results from an increase
in hydrogen ions,and the hydrogen ions combine with the protein
part ofthe hemoglobin molecule and change its three-dimensional
structure,causing a decrease in the ability of hemoglobin to bind
oxygen.Conversely,an increase in blood pH results in an increased
Part4 Regulationsand Maintenance840
ability of hemoglobin to bind oxygen.The effect of pH (hydrogen
ions) on the oxygen–hemoglobin dissociation curve is called the
Bohr effectafter its discoverer, Christian Bohr.
An increase in P
CO
2
also decreases the ability ofhemoglobin
to bind oxygen because of the effect of carbon dioxide on pH.
Within red blood cells,an enzyme called carbonic anhydrase cat-
alyzes this reversible reaction.
Carbonic
anhydrase
CO
2
⫹ H
2
O
→
←
H
2
CO
3
→
←
H
⫹
⫹ HCO
3
⫺
Carbon Water Carbonic Hydrogen Bicarbonate
dioxide acid ion ion
As carbon dioxide levels increase, more hydrogen ions are
produced,and the pH declines. As carbon dioxide levels decline,
the reaction proceeds in the opposite direction,resulting in a de-
crease in hydrogen ion concentration and an increase in pH.
0
20
20 40
Hemoglobin saturated with oxygen
in the lungs is like a nearly full
glass.
60
Oxygen released
to tissue
at rest: 23%
P
O
2
(mm Hg)
%O
2
saturation
P
O
2
in tissue
at rest
80 100 105
40
60
80
100
P
O
2
in lungs
In resting tissues, hemoglobin
releases some oxygen, which is
like partially emptying the glass.
23%
75%
98%
Figure 23.17
Oxygen-Hemoglobin Dissociation Curve atRest
(a) Atthe P
O
2
in the lungs, hemoglobin is98% saturated. At the P
O
2
ofresting tissues, hemoglobin is 75% saturated. Consequently23% of the oxygen picked up in
the lungsis released to the tissues. (b) The ability ofhemoglobin to pick up and release oxygen is like a glass filling and emptying.
(a)
(b)
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Chapter 23 RespiratorySystem 841
As blood passes through tissue capillaries, carbon dioxide
enters the blood from the tissues.As a consequence, blood carbon
dioxide levels increase,hemoglobin has less affinity for oxygen in
the tissue capillaries,and a greater amount of oxygen is released in
the tissue capillaries than would be released ifcarbon dioxide were
not present. When blood is returned to the lungs and passes
through the pulmonary capillaries,carbon dioxide leaves the capil-
laries and enters the alveoli.As a consequence, carbon dioxide lev-
els in the pulmonary capillaries are reduced, and the affinity of
hemoglobin for oxygen increases.
An increase in temperature also decreases the tendency for
oxygen to remain bound to hemoglobin.Elevated temperatures re-
sulting from increased metabolism,therefore, increase the amount
ofoxygen released into the tissues by hemoglobin. In less metabol-
ically active tissues in which the temperature is lower,less oxygen is
released from hemoglobin.
When the affinity of hemoglobin for oxygen decreases, the
oxygen–hemoglobin dissociation curve is shifted to the right, and
hemoglobin releases more oxygen (figure 23.19a).During exercise,
when carbon dioxide and acidic substances,such as lactic acid, ac-
cumulate and the temperature increases in the tissue spaces,the
oxygen–hemoglobin curve shifts to the right. Under these condi-
tions,as much as 75%–85% of the oxygen is released from the he-
moglobin.In the lungs, however,the curve shifts to the left because
of the lower carbon dioxide levels, lower temperature,and lower
lactic acid levels.The affinity of hemoglobin for oxygen, therefore,
increases,and it becomes easily saturated (figure 23.19b).
During resting conditions,approximately 5 mL of oxygen is
transported to the tissues in each 100 mL ofblood, and cardiac out-
put is approximately 5000 mL/min.Consequently, 250 mL of oxy-
gen is delivered to the tissues each minute.During conditions of
exercise,this value can increase up to 15 times. Oxygen transport
P
O
2
(mm Hg)
P
O
2
in lungs
0
20
20 40 60
%O
2
saturation
80 100 105
40
60
80
100
Oxygen released
to tissue during
exercise: 73%
In exercising tissues, hemoglobin
releases more oxygen, which is like
emptying most of the glass.
P
O
2
in tissue
during exercise
Hemoglobin saturated with oxygen
in the lungs is like a nearly full
glass.
98%
25%
73%
Figure 23.18
Oxygen-Hemoglobin Dissociation Curve During Exercise
(a) Atthe P
O
2
in the lungs, hemoglobin is98% saturated. At the P
O
2
ofexercising tissues, hemoglobin is 25% saturated. Consequently73% of the oxygen picked up
in the lungsis released to the tissues. (b) The ability ofhemoglobin to pick up and release oxygen is like a glass filling and emptying.
(a)
(b)
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can be increased threefold because ofa greater degree of oxygen re-
lease from hemoglobin in the tissue spaces,and the rate of oxygen
transport is increased another five times because of the increase in
cardiac output.Consequently,the volume of oxygen delivered to the
tissues can be as high as 3750 mL/min (15 ⫻250 mL/min). Highly
trained athletes can increase this volume to as high as 5000 mL/min.
Part4 Regulationsand Maintenance842
40. Name two ways that oxygen is transported in the blood, and
state the percentage of total oxygen transportfor which
each isresponsible.
41. How does the oxygen-hemoglobin dissociation curve
explain the uptake of oxygen in the lungsand the release of
oxygen in tissues?
42. What is the Bohr effect? How is it related to blood carbon
dioxide?
43. Why is it advantageous for the oxygen-hemoglobin
dissociation curve to shiftto the left in the lungs and to the
rightin tissues?
PREDICT
In carbon monoxide (CO) poisoning, CO bindsto hemoglobin, thereby
preventing the uptake ofoxygen by hemoglobin. In addition, when CO
bindsto hemoglobin, the oxygen–hemoglobin dissociation curve
shiftsto the left. What are the consequencesof this shift on the ability
oftissues to get oxygen? Explain.
Effectof BPG
As red blood cells break down glucose for energy,they produce a
substance called 2,3-bisphosphoglycerate (BPG; formerly called
diphosphoglycerate). BPG binds to hemoglobin and increases its
ability to release oxygen.When BPG levels increase, hemoglobin
releases more oxygen.When BPG levels decrease, hemoglobin re-
leases less oxygen.For example, people living at high altitudes have
increased levels ofBPG, which increases oxygen delivery to tissues
by causing hemoglobin to release more oxygen.On the other hand,
when blood is removed from the body and stored in a blood bank,
the BPG levels in the stored blood gradually decrease.As BPG lev-
els decrease, the blood becomes unsuitable for transfusion pur-
poses because the hemoglobin releases less oxygen to the tissues.
44. How does BPG affect the release of oxygen from
hemoglobin?
PREDICT
Ifa person lacks the enzyme necessary for BPG synthesis, would she
exhibitanemia (lower-than-normal number of red blood cells) or
erythrocytosis(higher-than-normal number of red blood cells)?
Explain.
Fetal Hemoglobin
As fetal blood circulates through the placenta,oxygen is released
from the mother’s blood into the fetal blood and carbon dioxide is
released from fetal blood into the mother’s blood.Fetal blood is
very efficient at picking up oxygen for several reasons.
1. The concentration offetal hemoglobin is approximately
50% greater than the concentration ofmaternal
hemoglobin.
2. Fetal hemoglobin is different from maternal hemoglobin.It
has an oxygen–hemoglobin dissociation curve that’s to the
left ofthe maternal oxygen–hemoglobin dissociation curve.
Thus,for a given P
O
2
fetal hemoglobin can hold more
oxygen than maternal hemoglobin.
3. BPG has little effect on fetal hemoglobin.That is,BPG does
not cause fetal hemoglobin to release oxygen.
0
20
20 40 60
Increased
oxygen
release
to tissues
P
O
2
in tissue
Curve shifts to right
as pH , CO
2
, temperature
Curve before
shift
P
O
2
(mm Hg)
%O
2
saturation
80 100 105
40
60
80
100
0
20
20 40 60
Increased
uptake of
oxygen in
lungs
P
O
2
in lungs
Curve shifts to left
as pH , CO
2
, temperature
Curve before
shift
P
O
2
(mm Hg)
%O
2
saturation
80 100 105
40
60
80
100
Figure 23.19
Effectsof Shifting the Oxygen-Hemoglobin
Dissociation Curve
(a) In the tissues, aspH decreases, P
CO
2
increases, or temperature increases,
the curve (black) shiftsto the right (red ), resulting in an increased release of
oxygen. (b) In the lungs, aspH increases, P
CO
2
decreases, or temperature
decreases, the curve (black) shiftsto the left (red), resulting in an increased
abilityof hemoglobin to pick up oxygen.
(a)
(b)
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Chapter 23 RespiratorySystem 843
4. The movement ofcarbon dioxide out of the fetal blood causes
the fetal oxygen–hemoglobin dissociation curve to shift to the
left.At the same time, the movement of carbon dioxide into
the mother’s blood causes the maternal oxygen–hemoglobin
dissociation curve to shift to the right.Thus, the mother’s
blood releases more oxygen and the fetal blood picks up more
oxygen.This is called the double Bohr effect.
45. How does the affinity for oxygen of fetal hemoglobin
compare to maternal hemoglobin?
46. What is the double Bohr effect?
Transportof Carbon Dioxide
Carbon dioxide is transported in the blood in three major ways:
approximately 7% is transported as carbon dioxide dissolved in the
plasma, approximately 23% is transported in combination with
blood proteins (mostly hemoglobin), and 70% is transported in
the form ofbicar bonate ions.
The most abundant protein to which carbon dioxide binds
in the blood is hemoglobin. Carbon dioxide binds in a reversible
fashion to the globin part of the hemoglobin molecule, and many
carbon dioxide molecules can combine to a single hemoglobin
molecule.
Hemoglobin that has released its oxygen binds more readily
to carbon dioxide than hemoglobin that has oxygen bound to it.
This is called the Haldane effect.In tissues, after hemoglobin has
released oxygen,the hemoglobin has an increased ability to pick up
carbon dioxide.In the lungs, as hemoglobin binds to oxygen, the
hemoglobin more readily releases carbon dioxide.
Chloride Shift
Carbon dioxide from tissues diffuses into red blood cells within the
capillaries (figure 23.20a).Some of the carbon dioxide binds to he-
moglobin,but most of it reacts with water inside the red blood cells to
form carbonic acid,a reaction catalyzed by carbonic anhydrase. The
carbonic acid then dissociates to form bicarbonate and hydrogen ions.
Thus,most of the carbon dioxide becomes part of a bicarbonate ion.
Lowering the amount of bicarbonate and hydrogen ions in-
side red blood cells promotes carbon dioxide transport,because as
these reaction products are removed and their ion concentrations
decrease,more carbon dioxide combines with water to form addi-
tional bicarbonate and hydrogen ions (see section on “Reversible
Reactions”on p. 36).In a process called the chloride shift (see fig-
ure 23.20a),bicarbonate ion concentrations inside red blood cells
are lowered by exchanging them for chloride ions (Cl
⫺
).As bicar-
bonate ions are produced,carrier molecules in red blood cell mem-
branes move bicarbonate ions out of the red blood cells and
chloride ions into the red blood cells.The exchange of negatively
charged ions maintains electrical balance in the red blood cells and
the plasma.Hemoglobin, which binds hydrogen ions,decreases the
concentration of hydrogen ions inside the red blood cells.Thus,
hemoglobin functions as a buffer and resists an increase in pH
within the red blood cells.
PREDICT
How isthe ability of hemoglobin to release oxygen and pickup carbon
dioxide in tissuesaffected by the change in the concentration of
hydrogen ionsinside red blood cells? Explain.
The reverse of the previous events occurs in the lungs (figure
23.20b). Carbon dioxide diffuses from the red blood cells into the
alveoli.As carbon dioxide levels in the red blood cells decrease, car-
bonic acid is converted to carbon dioxide and water.In response,bi-
carbonate ions join with hydrogen ions to form carbonic acid.As the
bicarbonate and hydrogen ions decrease because ofthis reaction, they
are replaced.Bicarbonate ions enter the red blood cell in exchange for
chloride ions,and hydrogen ions are released from hemoglobin.
Carbon Dioxide and Blood pH
Blood pH refers to the pH in plasma,not inside red blood cells. In
plasma,carbon dioxide can combine with water to form carbonic
acid,a reaction that is catalyzed by carbonic anhydrase on the sur-
face of capillary endothelial cells. The carbonic acid then dissoci-
ates to form bicarbonate and hydrogen ions. Thus, as plasma
carbon dioxide levels increase, hydrogen ion levels increase,and
blood pH decreases.An important function of the respiratory sys-
tem is to regulate blood pH by changing plasma carbon dioxide
levels (see chapter 27).Hyperventilation decreases plasma carbon
dioxide,and hypoventilation increases it.
47. List three ways that carbon dioxide is transported in the
blood, and state the percentage of total carbon dioxide
transportfor which each is responsible.
48. What is the Haldane effect?
49. Where and why does the chloride shift take place?
PREDICT
Whateffect does hyperventilation and holding one’s breath have on
blood pH? Explain.
Rhythmic Ventilation
Objective
■ Describe the brainstem structuresthat regulate respiration,
and explain howrhythmic ventilation is produced.
The generation of the basic rhythm of ventilation is con-
trolled by neurons within the medulla oblongata that stimulate the
muscles of respiration. Recruitment of muscle fibers and the in-
creased frequency ofstimulation of muscle fibers result in stronger
contractions ofthe muscles and an increased depth of respiration.
The rate of respiration is determined by how frequently the respi-
ratory muscles are stimulated.
RespiratoryAreas in the Brainstem
The classic view of respiratory areas held that distinct inspiratory
and expiratory centers were located in the brainstem.This view is
now known to be too simplistic.Although neurons involved with
respiration are aggregated in certain parts of the brainstem, neu-
rons that are active during inspiration are intermingled with neu-
rons that are active during expiration. Modern imaging
techniques, such as positron emission tomography (PET), also
confirm that much ofthe historical work on animals doesn’t apply
to humans.
The medullary respiratory center consists of two dorsal
respiratory groups, each forming a longitudinal column of cells
located bilaterally in the dorsal part of the medulla oblongata,and
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two ventral respiratory groups, each forming a longitudinal col-
umn of cells located bilaterally in the ventral part of the medulla
oblongata (figure 23.21).Although the dorsal and ventral respira-
tory groups are bilaterally paired,cross communication exists be-
tween the pairs so that respiratory movements are symmetric. In
addition, communication exists between the dorsal and ventral
respiratory groups.
Each dorsal respiratory group is a collection ofneurons that
are most active during inspiration,but some are active during ex-
piration. The dorsal respiratory groups are primarily responsible
for stimulating contraction of the diaphragm. They receive input
from other parts of the brain and peripheral receptors that allows
modification ofrespiration.
Each ventral respiratory group is a collection ofneurons that
are active during inspiration and expiration. These neurons pri-
marily stimulate the external intercostal,internal intercostal, and
abdominal muscles.
Part4 Regulationsand Maintenance844
The pontine respiratory group, formerly called the pneu-
motaxic center,is a collection of neurons in the pons (see figure
23.21). Some of the neurons are only active during inspiration,
some only during expiration, and some during both inspiration
and expiration. The precise function of the pontine respiratory
group is unknown,but it has connections with the medullary res-
piratory center and appears to play a role in switching between in-
spiration and expiration.It’s not considered to be essential for the
generation ofthe respirator y rhythm.
Generation ofRhythmic Ventilation
The exact locations ofneurons in the medullary respirator y center
responsible for rhythmic ventilation are unknown.Nor is it well
understood how they generate the basic pattern of spontaneous,
rhythmic ventilation at rest.One explanation involves integration
ofstimuli that start and stop inspiration.
CO
2
produced
CO
2
HHb
HHb
Hb
–
Hb
–
CO
2
+ H
2
O
H
2
CO
3
Capillary wall
Red blood cell
Tissue cells
Carbonic
anhydrase
Carbonic
anhydrase
Plasma
HCO
3
–
+ H
+
Cl
–
Cl
–
Cl
–
Cl
–
HCO
3
–
HCO
3
–
H
2
CO
3
H
+
+ HCO
3
–
H
2
O + CO
2
CO
2
CO
2
Alveoli of the lung
Capillary wall
(a) In the tissue capillaries, carbon dioxide
enters red blood cells and reacts with
water to form carbonic acid, which
dissociates to form bicarbonate and
hydrogen ions. Bicarbonate ions are
exchanged for chloride ions in the
chloride shift. Hydrogen ions combine
with hemoglobin. Lowering the
concentration of bicarbonate and
hydrogen ions inside red blood cells
promotes the conversion of carbon
dioxide to bicarbonate ions.
(b) In the pulmonary capillaries, carbon
dioxide leaves red blood cells, resulting in
the formation of additional carbon dioxide
from carbonic acid. Bicarbonate and
hydrogen ions combine to replace the
carbonic acid. The bicarbonate ions are
exchanged for chloride ions, and the
hydrogen ions are released from
hemoglobin.
Chloride
shift
Figure 23.20
Carbon Dioxide Transportand Chloride Movement
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Chapter 23 RespiratorySystem 845
1. Starting inspiration.Certain neurons in the medullary
respiratory center that promote inspiration are
continuously active.The medullary respiratory center
constantly receives stimulation from receptors that monitor
blood gas levels,blood temperature, and movements of
muscles and joints.In addition, stimulation from parts of
the brain concerned with voluntary respiratory movements
and emotions can occur.Inspiration starts when the
combined input from all these sources causes the
production ofaction potentials in the neurons that
stimulate respiratory muscles.
2. Increasing inspiration.Once inspiration begins,more and
more neurons are gradually activated.The result is
progressively stronger stimulation ofthe respiratory
muscles that lasts for approximately 2 seconds.
3. Stopping inspiration.The neurons stimulating the muscles
ofrespiration also stimulate other neurons in the medullary
respiratory center that are responsible for stopping
inspiration.The neurons responsible for stopping
inspiration also receive input from the pontine respiratory
group,stretch receptors in the lungs, and probably other
sources.When these inhibitory neurons are activated, they
cause the neurons stimulating respiratory muscles to be
inhibited.Relaxation of respiratory muscles results in
expiration,which lasts approximately 3 seconds. For the
next inspiration,go back to step 1.
50. Name the three respiratory groups and describe their main
functions.
51. How is rhythmic ventilation generated?
Modification of Ventilation
Objective
■ Describe the different ways by which rhythmic ventilation
can be altered.
Although the medullary neurons establish the basic rate and
depth of breathing, their activities can be influenced by input
from other parts of the brain and by input from peripherally lo-
cated receptors.
Cerebraland Limbic System Control
Through the cerebral cortex,it’s possible to consciously or uncon-
sciously increase or decrease the rate and depth of the respiratory
movements (figure 23.22).For example, during talking or singing,
air movement is controlled to produce sounds as well as to facili-
tate gas exchange.
Apnea (ap⬘ne¯-a˘) is the absence ofbreathing. A person may
stop breathing voluntarily.As the period of voluntary apnea in-
creases,a greater and greater urge to breathe develops. That urge is
primarily associated with increasing P
CO
2
levels in the arterial
I
n
t
e
r
c
o
s
t
a
l
n
e
r
v
e
s
t
o
i
n
t
e
r
n
a
l
i
n
t
e
r
c
o
s
t
a
l
m
u
s
c
l
e
s
I
n
t
e
r
c
o
s
t
a
l
n
e
r
v
e
s
t
o
e
x
t
e
r
n
a
l
i
n
t
e
r
c
o
s
t
a
l
m
u
s
c
l
e
s
Pons
Pontine respiratory
group
Dorsal
respiratory group
Ventral
respiratory group
Medulla oblongata
Spinal cord
Diaphragm
(involved in inspiration)
Internal intercostal muscles
(involved in expiration)
External intercostal muscles
(involved in inspiration)
Medullary
respiratory
center
P
h
r
e
n
i
c
n
e
r
v
e
t
o
d
i
a
p
h
r
a
g
m
Figure 23.21
RespiratoryStructures in the Brainstem
The relationship ofrespiratory structures to each other and to the nerves innervating the muscles of respiration.
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blood. Finally,the P
CO
2
reaches levels that cause the respiratory
center to override the conscious influence from the cerebrum.Oc-
casionally,people are able to hold their breath until the blood P
O
2
declines to a level low enough that they lose consciousness.After
consciousness is lost, the respiratory center resumes its normal
function in automatically controlling respiration.
Voluntary hyperventilation can decrease blood P
CO
2
levels
sufficiently to cause vasodilation of the peripheral blood vessels
and a decrease in blood pressure (see chapter 21).Dizziness or a
giddy feeling can result because ofdecreased delivery of oxygen to
the brain caused by the decreased rate ofblood flow to the brain af-
ter blood pressure drops.
Part4 Regulationsand Maintenance846
Emotions acting through the limbic system ofthe br ain can
also affect the respiratory center (see figure 23.22). For example,
strong emotions can cause hyperventilation or produce the sobs
and gasps ofcr ying.
ChemicalControl of Ventilation
The respiratory system maintains blood oxygen and carbon dioxide
concentrations and blood pH within a normal range ofvalues. A de-
viation by any of these parameters from their normal range has a
marked influence on respiratory movements.The effect of changes
in oxygen and carbon dioxide concentrations and in pH is superim-
posed on the neural mechanisms that establish rhythmic ventilation.
Higher centers of
the brain (speech,
emotions, voluntary
control of breathing,
and action potentials
in motor pathways)
Hering-Breuer reflex
(stretch receptors
in lungs)
Carotid and
aortic body
chemoreceptors
Carotid
body
Aortic
body
Proprioceptors
in muscles
and joints
Receptors for
touch, temperature,
and pain stimuli
Input to respiratory
centers in the
medulla oblongata
and pons modifies
respiration
+
+
+
+
+
Medullary
chemoreceptors
pH, CO
2
O
2
Figure 23.22
Modifying Respiration
Voluntarycontrol; emotions; changesin blood pH, carbon dioxide, and oxygen levels; stretch of the lungs; movements of the limbs (proprioception); and stimuli
such astouch, temperature, and pain can affectthe respirator ycenter and modify respiration. A plus sign (⫹) indicates an increase in respiration, and a minus (⫺)
signindicates a decrease in respiration.
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Chapter 23 RespiratorySystem 847
Chemoreceptors
Chemoreceptors are specialized neurons that respond to
changes in chemicals in solution. The chemoreceptors involved
with the regulation of respiration respond to changes in hydro-
gen ion concentrations or changes in P
O
2
(or both) (see figures
23.22 and 23.23).Central chemoreceptors are located bilaterally
and ventrally in the chemosensitive areaof the medulla oblon-
gata,and they are connected to the respiratory center.Peripheral
chemoreceptors are found in the carotid and aortic bodies.
These structures are small vascular sensory organs,which are en-
capsulated in connective tissue and located near the carotid si-
nuses and the aortic arch (see chapter 21).The respiratory center
is connected to the carotid body chemoreceptors through the
glossopharyngeal nerve (IX) and to the aortic body chemorecep-
tors by the vagus nerve (X).
Effectof pH
The chemosensitive area is bathed by cerebrospinal fluid and is
sensitive to changes in the pH of the fluid. Because the
blood–brain barrier separates the chemosensitive area from the
blood, this area doesn’t directly detect changes in blood pH.
Changes in blood pH can alter cerebrospinal fluid pH,however,so
the chemosensitive area responds indirectly to changes in blood
pH.In addition, the carotid and aortic bodies have a rich vascular
supply and are directly sensitive to changes in blood pH.
Maintaining body pH levels within normal parameters is
necessary for the proper functioning of cells. Because changes in
carbon dioxide levels can change pH,the respiratory system plays
an important role in acid–base balance.For example, if blood pH
decreases,the respiratory center is stimulated, resulting in elimina-
tion ofcarbon dioxide and an increase in blood pH back to normal
Blood pH
(normal range)
Blood pH
decreases
Blood pH
increases
Blood pH
(normal range)
Blood pH
homeostasis
is maintained
A decrease in blood pH is caused
by the increase in blood CO
2
.
Decreased stimulation of the respiratory
muscles by the respiratory centers
results in decreased ventilation, which
decreases gas exchange.
• A decrease in blood pH (often caused by
an increase in blood CO
2
) is detected by
the medullary chemoreceptors.
• A decrease in blood O
2
is detected by
the carotid and aortic body
chemoreceptors.
Increased stimulation of the respiratory
centers results.
An increase in blood pH (often caused by a
decrease in blood CO
2
) is detected by the
medullary chemoreceptors.
Decreased stimulation of the
respiratory centers results.
Increased stimulation of the respiratory muscles
by the respiratory centers results in increased
ventilation, which increases gas exchange.
• An increase in blood pH is caused by the
decrease in blood CO
2
.
• Blood O
2
increases.
HomeostasisFigure 23.23
Regulation ofBlood pH and Gases
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levels. Conversely,if blood pH increases, the respiratory rate de-
creases, and carbon dioxide levels increase,causing blood pH to
decrease back to normal levels.The role of the respiratory system
in maintaining pH is considered in greater detail in chapter 27.
Effectof Carbon Dioxide
Blood carbon dioxide levels are a major regulator of respiration
during resting conditions and conditions when the carbon dioxide
levels are elevated, for example, during intense exercise.Even a
small increase in carbon dioxide in the circulatory system triggers
a large increase in the rate and depth ofrespiration. An increase in
P
CO
2
of5 mm Hg, for example, causes an increase in ventilation of
100%. A greater-than-normal amount of carbon dioxide in the
blood is called hypercapnia(hı¯-per-kap⬘ne¯-a˘). Conversely, lower-
than-normal carbon dioxide levels,a condition called hypocapnia
(hı¯-po¯-kap⬘ne¯-a˘),result in periods in which respiratory move-
ments are reduced or do not occur.
Carbon dioxide apparently doesn’t directly affect the
chemosensitive area.Instead,it exerts its effect by changing pH lev-
els,which can affect the chemosensitive area (see figure 23.23). For
example, if blood carbon dioxide levels increase, carbon dioxide
diffuses across the blood–brain barrier into the cerebrospinal
fluid. The carbon dioxide combines with water to form carbonic
acid, which dissociates into hydrogen ions and bicarbonate ions.
The increased concentration of hydrogen ions lowers the pH and
stimulates the chemosensitive area,which then stimulates the res-
piratory center,resulting in a greater rate and depth of breathing.
Consequently,carbon dioxide levels decrease as carbon dioxide is
eliminated from the body.
PREDICT
Explain whya person who breathes rapidly and deeply
(hyperventilates) for severalseconds experiences a short period
during which respiration doesnot occur (apnea) before normal
breathing resumes.
The chemoreceptors in the carotid and aortic bodies also re-
spond to changes in carbon dioxide because of the effects of car-
bon dioxide on blood pH.The carotid and aortic bodies, however,
are responsible for,at most, 15%–20% of the total response to
changes in P
CO
2
or pH. The chemosensitive area in the medulla
oblongata is far more important for the regulation ofP
CO
2
and pH
than are the carotid and aortic bodies. During intense exercise,
however,the carotid bodies respond more rapidly to changes in
blood pH than does the chemosensitive area ofthe medulla.
Effectof Oxygen
Changes in P
O
2
can affect respiration (see figure 23.23),although
P
CO
2
levels detected by the chemosensitive area are responsible for
most changes in respiration. A decrease in oxygen levels below
normal values is called hypoxia(hı¯-pok⬘se¯-a˘).If P
O
2
levels in the
arterial blood are markedly reduced while the pH and P
CO
2
are
held constant,an increase in ventilation occurs. Within a normal
range ofP
O
2
levels,however, the effect of oxygen on the regulation
ofrespiration is small. Only after arterial P
O
2
decreases to approx-
imately 50% ofits normal value does it beg in to have a large stim-
ulatory effect on respiratory movements.
Part4 Regulationsand Maintenance848
At first, it’s somewhat surprising that small changes in
P
O
2
don’t cause changes in respiratory rate.Consideration of the
oxygen–hemoglobin dissociation curve, however, provides an ex-
planation. Because of the S shape of the curve, at any P
O
2
above
80mm Hg nearly all of the hemoglobin is saturated with oxygen.
Consequently,until P
O
2
levels change significantly,the oxygen-
carrying capacity of the blood is unaffected.
The carotid and aortic body chemoreceptors respond to de-
creased P
O
2
by increased stimulation of the respiratory center,
which can keep it active,despite decreasing oxygen levels. If P
O
2
decreases sufficiently,however, the respiratory center can fail to
function,resulting in death.
Importance ofReduced P
O
2
Carbon dioxide ismuch more importantthan oxygen as a regulator of
normalalveolar ventilation, but under certain circumstancesa reduced P
O
2
in the arterialblood does play an important stimulatory role. During
conditionsof shockin which blood pressure is very low, the P
O
2
in arterial
blood can drop to levelssufficiently low to strongly stimulate carotid and
aorticbody sensory receptors. At high altitudes where barometric air
pressure islow, the P
O
2
in arterialblood can also drop to levels sufficiently
low to stimulate carotid and aorticbodies. Although P
O
2
levelsin the
blood are reduced, the abilityof the respiratory system to eliminate carbon
dioxide isnot greatly affected by low barometricair pressure. Thus, blood
carbon dioxide levelsbecome lower than normalbecause of the increased
alveolar ventilation initiated in response to low P
O
2
.
A similar situation existsin people who have emphysema.
Because carbon dioxide diffusesacross the respiratory membrane more
readilythan oxygen, the decreased surface area ofthe respirator y
membrane caused bythe disease results in low arterial P
O
2
without
elevated arterialP
CO
2
. The elevated rate and depth ofrespiration are
due, to a large degree, to the stimulatoryeffect of low arterial P
O
2
levels
on carotid and aorticbodies. More severe emphysema, in which the
surface area ofthe respiratory membrane is reduced to a minimum, can
also resultin elevated P
CO
2
levelsin arterial blood.
Hering-Breuer Reflex
The Hering-Breuer (her⬘ing-broy⬘er) reflexlimits the degree to
which inspiration proceeds and prevents overinflation ofthe lungs
(see figure 23.22).This reflex depends on stretch receptors in the
walls ofthe bronchi and bronchioles of the lung. Action potentials
are initiated in these stretch receptors when the lungs are inflated
and are passed along sensory neurons within the vagus nerves to
the medulla oblongata.The action potentials have an inhibitory in-
fluence on the respiratory center and result in expiration.As expi-
ration proceeds,the stretch receptors are no longer stimulated,and
the decreased inhibitory effect on the respiratory center allows in-
spiration to begin again.
In infants,the Hering-Breuer reflex plays a role in regulating
the basic rhythm of breathing and in preventing overinflation of
the lungs.In adults, however,the reflex is important only when the
tidal volume is large,such as during exercise.
Effectof Exercise on Ventilation
The mechanisms by which ventilation is regulated during exercise are
controversial,and no one factor can account for all of the observed
responses.Ventilation during exercise is divided into two phases.
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Chapter 23 RespiratorySystem 849
1. Ventilation increases abruptly.At the onset ofexercise,
ventilation immediately increases.This initial increase can be as
much as 50% ofthe total increase that occurs during exercise.
The immediate increase in ventilation occurs too quickly to
be explained by changes in metabolism or blood gases.As
axons pass from the motor cortex ofthe cerebrum through
the motor pathways,numerous collateral fibers project into
the reticular formation ofthe brain. During exercise, action
potentials in the motor pathways stimulate skeletal muscle
contractions,and action potentials in the collateral fibers
stimulate the respiratory center (see figure 23.22).
Furthermore,during exercise, body movements
stimulate proprioceptors in the joints ofthe limbs. Action
potentials from the proprioceptors pass along sensory nerve
fibers to the spinal cord and along ascending nerve tracts
(the dorsal-column/medial-lemniscal system) ofthe spinal
cord to the brain.Collateral fibers project from these
ascending pathways to the respiratory center in the medulla
oblongata.Movement of the limbs has a strong stimulatory
influence on the respiratory center (see figure 23.22).
A learned component may also exist to the ventilation
response during exercise.After a period of training, the brain
“learns”to match ventilation with the intensity of the exercise.
Well-trained athletes match their respiratory movements
more efficiently with their level ofphysical activity than do
untrained individuals.Thus, centers of the brain involved in
learning have an indirect influence on the respiratory center,
but the exact mechanism for this kind ofregulation is unclear.
2. Ventilation increases gradually. After the immediate increase
in ventilation,a gradual increase occurs that levels off
within 4–6 minutes after the onset ofexercise. Factors
responsible for the immediate increase in ventilation may
play a role in the gradual increase as well.
Despite large changes in oxygen consumption and carbon
dioxide production during exercise,the average arterial P
O
2
,
P
CO
2
,and pH remain constant and close to resting levels as
long as the exercise is aerobic (see chapter9). This suggests
that changes in blood gases and pH do not play an
important role in regulating ventilation during aerobic
exercise.During exercise, however,the values of arterial P
O
2
,
P
CO
2
,and pH rise and fall more than at rest. Thus, even
though their average values don’t change,their oscillations
may be a signal for helping to control ventilation.
The highest level ofexercise that can be performed
without causing a significant change in blood pH is called
theanaerobic threshold. If the exercise intensity is high
enough to exceed the anaerobic threshold,then skeletal
muscles produce and release lactic acid into the blood.The
resulting change in blood pH stimulates the carotid bodies,
resulting in increased ventilation.In fact, ventilation can
increase so much that arterial P
CO
2
decreases below resting
levels and arterial P
O
2
increases above resting levels.
Other Modificationsof Ventilation
The activation oftouch, thermal, and pain receptors can also affect
the respiratory center (see figure 23.22).For example, irritants in
the nasal cavity can initiate a sneeze reflex, and irritants in the
lungs can stimulate a cough reflex.An increase in body tempera-
ture can stimulate increased ventilation.
52. Describe cerebral and limbic system control of ventilation.
53. Define central and peripheral chemoreceptors. Which are
mostimportant for the regulation of blood pH and carbon
dioxide?
54. Define hypercapnia and hypocapnia.
55. What effect does a decrease in blood pH or carbon dioxide
have on respiratoryrate?
56. Describe the Hering-Breuer reflex and its function.
57. Define hypoxia. Why must arterial P
O
2
change significantly
before itaffects respiratory rate?
58. What mechanisms regulate ventilation at the onset of
exercise and during exercise? Whatis the anaerobic
threshold?
PREDICT
Describe the respiratoryresponse when cold water is splashed onto a
person. In the past, newborn babieswere sometimes swatted on the
buttocks. Explain the rationale for thisprocedure.
Respiratory Adaptations
to Exercise
Objective
■ Describe respiratoryadaptations that occur in response to
training.
In response to training, athletic performance increases be-
cause the cardiovascular and respiratory systems become more ef-
ficient at delivering oxygen and picking up carbon dioxide.
Ventilation in most individuals does not limit performance be-
cause ventilation can increase to a greater extent than does cardio-
vascular function.
After training, vital capacity increases slightly and residual
volume decreases slightly.Tidal volume at rest and during sub-
maximal exercise does not change.At maximal exercise, however,
tidal volume increases.After training, the respiratory rate at rest
or during submaximal exercise is slightly lower than in an un-
trained person,but at maximal exercise respiratory rate is gener-
ally increased.
Minute ventilation is affected by the changes in tidal vol-
ume and respiratory rate.After training, minute ventilation is es-
sentially unchanged or slightly reduced at rest and is slightly
reduced during submaximal exercise. Minute ventilation is
greatly increased at maximal exercise.For example, an untrained
person with a minute ventilation of 120 L/min can increase to
150 L/min after training. Increases to 180 L/min are typical of
highly trained athletes.
Gas exchange between the alveoli and blood increases at
maximal exercise following training.The increased minute ventila-
tion results in increased alveolar ventilation.In addition, increased
cardiovascular efficiency results in greater blood flow through the
lungs,especially in the superior parts of the lungs.
59. What effect does training have on resting, submaximal, and
maximal tidal volumesand on minute ventilation?
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Clinical Focus Disordersof the Respiratory System
Bronchi and Lungs
Bronchitis(brong-kı¯⬘tis) isan inflammation
of the bronchi caused byirritants, such as
cigarette smoke, air pollution, or infections.
The inflammation resultsin swelling of the
mucous membrane lining the bronchi, in-
creased mucusproduction, and decreased
movementof mucus by cilia. Consequently,
the diameter of the bronchi is decreased,
and ventilation isimpaired. Bronchitis can
progressto emphysema.
Emphysema(em-fi-ze¯⬘ma˘) resultsin the
destruction ofthe alveolar walls. Many smok-
ers have both bronchitis and emphysema,
which are often referred to as chronicob-
structive pulmonarydisease (COPD).Chronic
inflammation of the bronchioles, usually
caused by cigarette smoke or air pollution,
probablyinitiates emphysema. Narrowing of
the bronchiolesrestricts air movement, and
air tendsto be retained in the lungs. Cough-
ing to remove accumulated mucusincreases
pressure in the alveoli, resulting in rupture
and destruction of alveolar walls. Loss of
alveolar walls has two important conse-
quences. The respiratory membrane has a
decreased surface area, which decreasesgas
exchange, and loss of elastic fibers de-
creasesthe ability of the lungs to recoil and
expel air. Symptomsof emphysema include
shortness ofbreath and enlargement of the
thoraciccavity. Treatment involves removing
sourcesof irritants (e.g., stopping smoking),
promoting the removal of bronchial secre-
tions, using bronchiodilators, retraining peo-
ple to breathe so that expiration of air is
maximized, and using antibioticsto prevent
infections. The progressof emphysema can
be slowed, butno cure exists.
Cystic fibrosis is an inherited disease
that affects the secretory cells lining the
lungs, pancreas, sweatglands, and salivary
glands. The defect produces an abnormal
chloride transportprotein that doesn’t reach
the cellsurface or doesn’t function normallyif
itdoes reach the cell surface. The resultis de-
creased chloride ion secretion outofcells and
increased sodium ion movement into cells.
Normally, the presence of chloride and
sodium ionsoutside of the cells causeswater
to move to the outside by osmosis. In the
lungs, the water formsa thin fluid layer over
which mucusismoved by ciliated cells. In cys-
tic fibrosis, the decreased chloride and
sodium ionsoutside the cells results in dehy-
drated respiratory secretions. The mucusis
more viscous, resisting movement by cilia,
and itaccumulates in the lungs. For reasons
notcompletely understood, the mucus accu-
mulation increases the likelihood of infec-
tions. Chronic airflow obstruction causes
difficultyin breathing, and coughing in an at-
tempt to remove the mucus can result in
pneumothoraxand bleeding within the lungs.
Once fatalduring early childhood, many vic-
tims ofcystic fibrosis are now surviving into
young adulthood. Future treatmentscould in-
clude the development ofdrugs that correct
or assistthe normal ion transport mechanism.
Alternatively, cysticfibrosis may someday be
cured through geneticengineering by insert-
ing a functional copyof the defective gene
into a person with the disease. Research on
thisexciting possibility is currently underway.
Pulmonaryfibrosis is the replacement
of lung tissue with fibrousconnective tis-
sue, thereby making the lungsless elastic
and breathing more difficult. Exposure to
asbestos, silica (silicosis), or coaldust is
the mostcommon cause.
Lung, or bronchiogenic, cancer arises
from the epithelium ofthe respiratory tract.
Cancersarising from tissues other than res-
piratoryepithelium are not called lung can-
cer, even though they occur in the lungs.
Lung cancer isthe most common cause of
cancer death in malesand females in the
United States, and almostall cases occur in
smokers. Because of the rich lymph and
blood supplyin the lungs, cancer in the lung
can readilyspread to other parts of the lung
or body. In addition, the disease isoften ad-
vanced before symptoms become severe
enough for the victim to seekmedical aid.
Typical symptoms include coughing, spu-
tum production, and blockage of the air-
ways. Treatmentsinclude removal of part or
allof the lung, chemotherapy, and radiation.
NervousSystem
Sudden infant death syndrome (SIDS),or
crib death, is the most frequent cause of
death ofinfants between 2 weeksand 1 year
ofage. Death results when the infant stops
breathing during sleep. Although the cause
ofSIDS remains controversial, evidence ex-
ists that damage to the respiratorycenter
during development is a factor. No treat-
menthas yet been found, but at-risk babies
can be placed on a monitor thatsounds an
alarm ifthe baby stops breathing.
Paralysisof the respiratory muscles can
resultfrom damage of the spinal cord in the
cervicalor thoracicregions. The damage inter-
ruptsnerve tracts that transmit action poten-
tialsto the muscles of respiration. Transection
of the spinal cord can result from trauma,
Effects of Aging on the
Respiratory System
Objective
■ Describe the effectsof aging on the respiratory system.
Almost all aspects of the respiratory system are affected by ag-
ing.Even though vital capacity, maximum ventilation rates, and gas
exchange decrease with age,the elderly can engage in light to moder-
ate exercise because the respiratory system has a large reserve capacity.
Part4 Regulationsand Maintenance850
Vital capacity decreases with age because ofa decreased ability
to fill the lungs (decreased inspiratory reserve volume) and a de-
creased ability to empty the lungs (decreased expiratory reserve vol-
ume). As a result, maximum minute ventilation rates decrease,
which in turn decreases the ability to perform intense exercise.These
changes are related to weakening ofrespiratory muscles and to de-
creased compliance ofthe thoracic cage caused by stiffening of carti-
lage and ribs.Lung compliance actually increases with age, but this
effect is offset by the decreased thoracic cage compliance. Lung com-
pliance decreases because alveoli become shallower with age,which
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such asautomobile accidents or diving into
water that istoo shallow. Another cause of
paralysisis poliomyelitis, a viral infection that
damagesneurons of the respiratory center or
motor neuronsthat stimulate the muscles of
respiration. Anesthetics or central nervous
system depressants can also depress the
function of the respiratory center ifthey are
taken or administered in large enough doses.
Diseasesof the Upper
RespiratoryTract
Strep throat is caused by a streptococcal
bacteria (Streptococcus pyogenes) and is
characterized byinflammation of the phar-
ynxand by fever. Frequently, inflammation of
the tonsilsand middle ear is involved. With-
outa throat analysis, the infection cannot be
distinguished from viralcauses of pharyn-
gealinflammation. Current techniques allow
rapid diagnosiswithin minutes to hours, and
antibioticsare an effective treatment.
Diphtheria (dif-the¯⬘re¯-a˘) was once a
major cause of death among children. It
iscaused by a bacterium (Corynebacterium
diphtheriae). A grayish membrane forms
in the throat and can blockthe respiratory
passagestotally. A vaccine againstdiphthe-
ria ispart of the normal immunization pro-
gram for children in the United States.
Thecommon cold is the result of a viral
infection. Symptoms include sneezing, ex-
cessive nasalsecretions, and congestion.
The infection can easilyspread to sinus cav-
ities, lower respiratory passages, and the
middle ear. Laryngitisand middle ear infec-
tionsare common complications. The com-
mon cold usuallyruns its course to recovery
in about1 week.
Diseasesof the Lower
RespiratoryTract
Laryngitis (lar-in-jı¯⬘tis) isan inflammation
ofthe larynx, especially the vocal folds, and
bronchitis is an inflammation of the
bronchi. Bacterial or viral infections can
move from the upper respiratory tract to
cause laryngitisor bronchitis. Bronchitis is
also often caused bycontinually breathing
air containing harmful chemicals, such as
those found in cigarette smoke.
Whooping cough(pertussis; per-tu˘s⬘is)
isa bacterial infection (Bordetella pertussis)
thatcauses a loss of cilia of the respiratory
epithelium. Mucusaccumulates, and the in-
fected person attemptsto cough up the mu-
cous accumulations. The coughing can be
severe. A vaccine for whooping cough ispart
ofthe normal vaccination procedure for chil-
dren in the United States.
Tuberculosis (tu¯ -ber⬘kyu¯-lo¯⬘sis) is
caused by a tuberculosis bacterium
(Mycobacterium tuberculosis). In the lung,
the bacteria form lesions called tubercles.
The small lumps contain degenerating
macrophagesand tuberculosis bacteria. An
immune reaction isdirected against the tu-
bercles, which causesthe formation of larger
lesionsand inflammation. The tubercles can
rupture and release bacteria thatinfect other
partsof the lung or body. Recently, a strain of
the tuberculosisbacteria has developed that
isresistant to treatment, and this strain isin-
creasing concern thattuberculosis will again
become a widespread infectiousdisease.
Pneumonia(noo-mo¯⬘ne¯-a˘) is a general
term that refers to many infections ofthe
lung. Mostpneumonias are caused by bac-
teria, butsome result from viral, fungal, or
protozoan infections. Symptoms include
fever, difficulty in breathing, and chest
pain. Inflammation of the lungs results in
the accumulation of fluid within alveoli
(pulmonary edema) and poor inflation of
the lungswith air. A fungal infection (Pneu-
mocystiscarinii) that results in pneumocys-
tosispneumonia is rare, except in persons
who have a compromised immune system.
Thistype of pneumonia has become one of
the infections commonly suffered byper-
sonswho have AIDS.
Flu (influenza)is a viral infection of the
respiratory system and doesnot affect the
digestive system asis commonly assumed.
Flu is characterized bychills, fever, head-
ache, and muscular aches, in addition to
coldlike symptoms. Several strains offlu
viruseshave been identified. The mortality
rate from flu isapproximately 1%, and most
of those deaths occur among the very old
and veryyoung. During a flu epidemic, the
infection rate isso rapid and the disease so
widespread thatthe total number of deaths
issubstantial, even though the percentage
ofdeaths is relatively low. Flu vaccines can
provide some protection againstthe flu.
A number of fungal diseases, such as
histoplasmosis(his⬘to¯-plaz-mo¯⬘sis) and coc-
cidioidomycosis(kok-sid-e¯-oy⬘do¯-mı¯-ko¯⬘sis),
affect the respiratory system. The fungal
spores (Histoplasma capsulatum; Coccid-
ioides immitis) usually enter the respiratory
system through dustparticles. Spores in soil
and fecesof certain animals make the rate of
infection higher in farm workersand in gar-
deners. The infectionsusually result in minor
respiratoryinfections, but in some cases they
can cause infectionsthroughout the body.
Chapter 23 RespiratorySystem 851
reduces the surface tension ofthe water lining the alveoli. There are
no significant age-related changes in lung elastic fibers or surfactant.
Residual volume increases with age as the alveolar ducts and
many ofthe larger bronchioles increase in diameter.This increases the
dead space,which decreases the amount of air available for gas ex-
change (alveolar ventilation).In addition,gas exchange across the res-
piratory membrane is reduced because parts of the alveolar walls are
lost,which decreases the surface area available for gas exchange,and the
remaining walls thicken,which decreases diffusion of gases.A gradual
increase in resting tidal volume with age compensates for these changes.
With age, mucus accumulates within the respiratory pas-
sageways.The mucus-cilia escalator is less able to move the mucus
because it becomes more viscous and because the number ofcilia
and their rate ofmovement decrease. As a consequence, the elderly
are more susceptible to respiratory infections and bronchitis.
60. Why do vital capacity, alveolar ventilation, and diffusion of
gasesacross the respiratory membrane decrease with age?
61. Why are the elderly more likely to develop respiratory
infectionsand bronchitis?
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Mr. Wis an 18-year-old track athlete in seemingly good health. One
dayhe came down with a common cold, resulting in the typical symp-
tomsof nasal congestion and discomfort. After several days, he began
to cough and wheeze, and he thoughtthat his cold had progressed to
hislungs. Determined not to get “out of shape” because of his cold,
Mr. W tooka few aspirins to relieve his discomfort and went to the
trackto jog. After a few minutes of exercise, he began to wheeze very
forcefullyand rapidly, and he felt that he could hardly get enough air.
Even though he stopped jogging, hiscondition did not improve (figure
A). Fortunately, a concerned friend who wasalso at the tracktook him
to the emergencyroom.
Although Mr. Whad no previous history of asthma, careful eval-
uation bythe emergency room doctor convinced her that he probably
washaving an asthma attack. Mr. W inhaled a bronchiodilator drug,
which resulted in rapid improvementin his condition. He wasreleased
from the emergency room and referred to hispersonal physician for
further treatmentand education about asthma.
Background Information
Asthma(az⬘ma˘) isa disease characterized byincreased constriction of
the trachea and bronchi in response to variousstimuli, resulting in a
narrowing ofthe air passagewaysand decreased ventilation efficiency.
Symptoms include wheezing, coughing, and shortnessof breath. In
contrastto many other respiratory disorders, however, the symptoms
ofasthma typically reverse either spontaneously or with therapy.
It’sestimated that the prevalence ofasthma in the United States
is from 3%–6% of the general population. Approximatelyhalf the
cases first appear before age 10, and twice asmany boys as girls
develop asthma. Anywhere from 25%–50% ofchildhood asthmatics
are symptom-free from adolescence onward.
The exactcause or causes of asthma are unknown, but asthma
and allergies run stronglyin some families. No definitive pathologic
feature or diagnostictest for asthma has been discovered, but three
importantfeatures of the disease are chronicair wayinflammation, air-
way hyperreactivity, and airflow obstruction. The inflammatory re-
sponse resultsin tissue damage, edema, and mucous buildup, which
can blockairflow through the bronchi. Airway hyperreactivityis greatly
increased contraction of the smooth muscle in the trachea and
bronchi in response to a stimulus. Asa result of airway hyperactivity,
the diameter of the airway decreases, and resistance to airflow in-
creases. The effectsof inflammation and airway hyperreactivity com-
bine to cause airflow obstruction.
Manycases of asthma appear to be associated with a chronic in-
flammatory response bythe immune system. The number of immune
cellsin the bronchi increases, including mast cells, eosinophils, neu-
trophils, macrophages, and lymphocytes. These cellsrelease chemical
mediators, such asinterleukins, leukotrienes, prostaglandins, platelet-
activating factor, thromboxanes, and chemotacticfactors. These chemi-
calmediators promote inflammation, increase mucous secretion, and
attractadditional immune cells to the bronchi, resulting in chronic air-
wayinflammation. Airway hyperreactivity and inflammation appear to
be linked bysome of the chemical mediators, which increase the sensi-
tivityof the airway to stimulation and cause smooth muscle contraction.
Systems Pathology
Asthma
Part4 Regulationsand Maintenance852
Figure A
Jogger with Asthma
Respiration includes the movement ofair into and out of the lungs, the ex-
change ofgases between the air and the blood, the transport of gases in the
blood,and the exchange of gases between the blood and tissues.
Functionsof the Respiratory System
(p. 814)
Major functions associated with the respiratory system include gas
exchange, regulation of blood pH, voice production, olfaction, and
protection against some microorganisms.
SUMMARY
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The stimuli thatprompt airflow obstruction vary from one individ-
ualto another. Some asthmatics have reactions to particular allergens,
which are foreign substancesthatevoke an inappropriate immune system
response (see chapter 22). Examplesinclude inhaled pollen, animaldan-
der, and dustmites. Many cases of asthma may be caused by an allergic
reaction to substancesin the droppings and carcasses of cockroaches,
which mayexplain the higher rate of asthma in poor, urban areas.
On the other hand, inhaled substances, such aschemicals in the
workplace or cigarette smoke, can provoke an asthma attackwithout
stimulating an allergicreaction. Over 200 substances have been associ-
ated with occupationalasthma. An asthma attackcan also be stimulated
byingested substanceslike aspirin, nonsteroidal anti-inflammatory com-
poundslike ibuprofen (i-boo⬘pro¯-fen), sulfites in food preservatives, and
tartrazine (tar⬘tra˘-ze¯n) in food colorings. Asthmatics can substitute aceta-
minophen (as-et-a˘-me¯ ⬘no¯-fen; Tylenol) for aspirin.
Other stimuli, such as strenuous exercise, especiallyin cold
weather, can precipitate an asthma attack. Such episodescan often
be avoided byusing a bronchiodilator drug prior to exercise. Viral in-
fections, emotionalupset, stress, and even reflux of stomach acid into
the esophagusare known to elicit an asthma attack.
Treatmentof asthma involves avoiding the causative stimulus
and administering drug therapy. Steroidsand mast cell–stabilizing
agents, which prevent the release ofchemical mediators from mast
cells, are used to reduce airwayinflammation. Theophylline (the¯-of⬘i-
le¯n, the¯-of⬘i-lin) and -adrenergicagents (see chapter 16) are com-
monly used to cause bronchiolar dilation. Although treatment is
generally effective in controlling asthma, in rare casesdeath by as-
phyxiation mayoccur. Earlier and more intensive therapy will in most
casesprevent death by asphyxiation.
PREDICT
Itis not usually necessary to assessarterial blood gases in the
diagnosisand treatment of asthma. Thisinformation, however, can
sometimesbe useful in cases of severe asthma attacks. Suppose that
Mr. Whad a P
O
2
of60 mm Hg and a P
CO
2
of30 mm Hg when he first
came to the emergencyroom. Explain how that could happen.
Chapter 23 RespiratorySystem 853
System Interactions
System Effect of Asthma on Other Systems
Integumentary Cyanosis, a bluish skin color, results from a decreased blood oxygen content.
Muscular Skeletal muscles are necessary for respiratory movements and the cough reflex. Increased muscular work during a severe
asthma attack can cause metabolic acidosis because of anaerobic respiration and excessive lactic acid production.
Skeletal Red bone marrow is the site of production of many of the immune cells responsible for the inflammatory response of asthma.
The thoracic cage is necessary for respiration.
Nervous Emotional upset or stress can evoke an asthma attack. Peripheral and central chemoreceptor reflexes affect ventilation. The
cough reflex helps to remove mucus from respiratory passages. Pain, anxiety, and death from asphyxiation can result from
the altered gas exchange caused by asthma. One theory of the cause of asthma is an imbalance in the autonomic nervous
system (ANS) control of bronchiolar smooth muscle, and drugs that enhance sympathetic effects or block parasympathetic
effects are used in asthma treatment.
Endocrine Steroids from the adrenal gland play a role in regulating inflammation, and they are used in asthma therapy.
Cardiovascular Increased vascular permeability of lung blood vessels results in edema. Blood carries ingested substances that provoke an
asthma attack to the lungs. Blood carries immune cells from the red bone marrow to the lungs. Tachycardia commonly
occurs, and the normal effects of respiration on venous return of blood to the heartare exaggerated, resulting in large
fluctuations in blood pressure.
Lymphatic and immune Immune cells release chemical mediators that promote inflammation, increase mucous production, and cause bronchiolar
constriction (believed to be a major factor in asthma). Ingested allergens, such as aspirin or sulfites in food, can evoke an
asthma attack.
Digestive Reflux of stomach acid into the esophagus can evoke an asthma attack.
Urinary Modifying hydrogen ion secretion into the urine helps to compensate for acid–base imbalances caused by asthma.
Anatomyand Histology of the Respiratory System
(p.814)
Nose
1. The nose consists ofthe external nose and the nasal cavity.
2. The bridge ofthe nose is bone, and most of the external nose is
cartilage.
3. Openings ofthe nasal cavity
• The nares open to the outside, and the choane lead to the pharynx.
• The paranasal sinuses and the nasolacrimal duct open into the
nasal cavity.
4. Parts ofthe nasal cavity
• The nasal cavity is divided by the nasal septum.
• The anterior vestibule contains hairs that trap debris.
• The nasal cavity is lined with pseudostratified ciliated columnar
epithelium that traps debris and moves it to the pharynx.
• The superior part of the nasal cavity contains the olfactory
epithelium.
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Pharynx
1. The nasopharynx joins the nasal cavity through the internal nares and
contains the openings to the auditory tube and the pharyngeal tonsils.
2. The oropharynx joins the oral cavity and contains the palatine and
lingual tonsils.
3. The laryngopharynx opens into the larynx and the esophagus.
Larynx
1. Cartilage
• Three unpaired cartilages exist. The thyroid cartilage and cricoid
cartilage form most ofthe lar ynx.The epiglottis covers the
opening ofthe larynx during swallowing .
• Six paired cartilages exist. The vocal folds attach to the arytenoid
cartilages.
2. Sounds are produced as the vocal folds vibrate when air passes
through the larynx.Tightening the folds produces sounds of
different pitches by controlling the length ofthe fold, which is
allowed to vibrate.
Trachea
The trachea connects the larynx to the primary bronchi.
TracheobronchialTree
1. The conducting zone,from the trachea to the terminal bronchioles,
is a passageway for air movement.
• The area from the trachea to the terminal bronchioles is ciliated to
facilitate removal ofdebris.
• Cartilage helps to hold the tube system open (from the trachea to
the bronchioles).
• Smooth muscle controls the diameter of the tubes (terminal
bronchioles).
2. The respiratory zone,from the respiratory bronchioles to the alveoli,
is a site ofgas exchange.
3. The components ofthe respiratory membrane include a film of water,
the walls ofthe alveolus and the capillary, and an interstitial space.
Lungs
1. The body contains two lungs.
2. The lungs are divided into lobes,bronchopulmonary segments, and
lobules.
ThoracicWall and Muscles of Respiration
1. The thoracic wall consists ofvertebrae, ribs, sternum,and muscles
that allow expansion ofthe thoracic cavity.
2. Contraction ofthe diaphragm increases thoracic volume.
3. Muscles can elevate the ribs and increase thoracic volume or can
depress the ribs and decrease thoracic volume.
Pleura
The pleural membranes surround the lungs and provide protection
against friction.
Blood Supply
1. Deoxygenated blood is transported to the lungs through the
pulmonary arteries,and oxygenated blood leaves through the
pulmonary veins.
2. Oxygenated blood is mixed with a small amount ofdeoxygenated
blood from the bronchi.
LymphaticSupply
The superficial and deep lymphatic vessels drain lymph from the lungs.
Ventilation
(p. 828)
Pressure Differencesand Airflow
1. Ventilation is the movement ofair into and out ofthe lungs.
Part4 Regulationsand Maintenance854
2. Air moves from an area ofhigher pressure to an area of lower pressure.
Pressure and Volume
Pressure is inversely related to volume.
Airflow into and outof Alveoli
1. Inspiration results when barometric air pressure is greater than
alveolar pressure.
2. Expiration results when barometric air pressure is less than alveolar
pressure.
Changing Alveolar Volume
1. Lung recoil causes alveoli to collapse.
• Lung recoil results from elastic fibers and water surface tension.
• Surfactant reduces water surface tension.
2. Pleural pressure is the pressure in the pleural cavity.
• A negative pleural pressure can cause the alveoli to expand.
• Pneumothorax is an opening between the pleural cavity and the
air that causes a loss ofpleural pressure.
3. Changes in thoracic volume cause changes in pleural pressure,
resulting in changes in alveolar volume,alveolar pressure,and airflow.
Measuring Lung Function
(p. 833)
Compliance ofthe Lungs and the Thorax
1. Compliance is a measure oflung expansion caused by alveolar
pressure.
2. Reduced compliance means that it’s more difficult than normal to
expand the lungs.
PulmonaryVolumes and Capacities
1. Four pulmonary volumes exist:tidal volume,inspir atory reserve
volume,expiratory reserve volume, and residual volume.
2. Pulmonary capacities are the sum oftwo or more pulmonary
volumes and include inspiratory capacity,functional residual
capacity,vital capacity, and total lung capacity.
3. The forced expiratory vital capacity measures vital capacity as the
individual exhales as rapidly as possible.
Minute Ventilation and Alveolar Ventilation
1. The minute ventilation is the total amount ofair moved in and out
ofthe respiratory system per minute.
2. Dead space is the part ofthe respiratory system in which gas
exchange does not take place.
3. Alveolar ventilation is how much air per minute enters the parts of
the respiratory system in which gas exchange takes place.
PhysicalPrinciples of Gas Exchange
(p. 835)
PartialPressure
1. Partial pressure is the contribution ofa gas to the total pressure of a
mixture ofgases (Dalton’s law).
2. Water vapor pressure is the partial pressure produced by water.
3. Atmospheric air,alveolar air,and expired air have different
compositions.
Diffusion ofGases Through Liquids
The concentration ofa gas in a liquid is determined by its partial pressure
and by its solubility coefficient (Henry’s law).
Diffusion ofGases Through the Respiratory Membrane
1. The respiratory membrane is thin and has a large surface area that
facilitates gas exchange.
2. The rate ofdiffusion of gases through the respiratory membrane
depends on its thickness,the diffusion coefficient of the gas, the
surface area ofthe membrane, and the partial pressure of the gases
in the alveoli and the blood.
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Chapter 23 RespiratorySystem 855
Relationship Between Ventilation and Pulmonary
CapillaryBlood Flow
1. Increased ventilation or increased pulmonary capillary blood flow
increases gas exchange.
2. The physiologic shunt is the deoxygenated blood returning from the
lungs.
Oxygen and Carbon Dioxide Transportin
the Blood
(p. 838)
Oxygen Diffusion Gradients
1. Oxygen moves from the alveoli (P
O
2
⫽104 mm Hg) into the blood
(P
O
2
⫽40 mm Hg). Blood is almost completely saturated with
oxygen when it leaves the capillary.
2. The P
O
2
in the blood decreases (P
O
2
⫽95 mm Hg) because of
mixing with deoxygenated blood.
3. Oxygen moves from the tissue capillaries (P
O
2
⫽95 mm Hg) into
the tissues (P
O
2
⫽40 mm Hg).
Carbon Dioxide Diffusion Gradients
1. Carbon dioxide moves from the tissues (P
CO
2
⫽45 mm Hg) into
tissue capillaries (P
CO
2
⫽40 mm Hg).
2. Carbon dioxide moves from the pulmonary capillaries (P
CO
2
⫽45
mm Hg) into the alveoli (P
CO
2
⫽40 mm Hg).
Hemoglobin and Oxygen Transport
1. Oxygen is transported by hemoglobin (98.5%) and is dissolved in
plasma (1.5%).
2. The oxygen–hemoglobin dissociation curve shows that hemoglobin
is almost completely saturated when P
O
2
is 80 mm Hg or above.At
lower partial pressures,the hemoglobin releases oxygen.
3. A shift ofthe oxygen–hemoglobin dissociation curve to the right
because ofa decrease in pH (Bohr effect), an increase in carbon
dioxide,or an increase in temperature results in a decrease in the
ability ofhemoglobin to hold oxygen.
4. A shift ofthe oxygen–hemoglobin dissociation curve to the left
because ofan increase in pH (Bohr effect), a decrease in carbon
dioxide,or a decrease in temperature results in an increase in the
ability ofhemoglobin to hold oxygen.
5. The substance 2,3-bisphosphoglycerate increases the ability of
hemoglobin to release oxygen.
6. Fetal hemoglobin has a higher affinity for oxygen than does
maternal hemoglobin.
Transportof Carbon Dioxide
1. Carbon dioxide is transported as bicarbonate ions (70%),in
combination with blood proteins (23%),and in solution in plasma
(7%).
2. Hemoglobin that has released oxygen binds more readily to carbon
dioxide than hemoglobin that has oxygen bound to it (Haldane effect).
3. In tissue capillaries,carbon dioxide combines with water inside the
red blood cells to form carbonic acid,which dissociates to form
bicarbonate ions and hydrogen ions.
4. The chloride shift is the movement ofchloride ions into red blood
cells as bicarbonate ions move out.
5. In lung capillaries,bicarbonate ions and hydrogen ions move into
red blood cells,and chloride ions move out. Bicarbonate ions
combine with hydrogen ions to form carbonic acid.The carbonic
acid is converted to carbon dioxide and water.The carbon dioxide
diffuses out ofthe red blood cells.
6. Increased plasma carbon dioxide lowers blood pH.The respiratory
system regulates blood pH by regulating plasma carbon dioxide levels.
RhythmicVentilation
(p. 843)
RespiratoryAreas in the Brainstem
1. The medullary respiratory center consists ofthe dorsal and ventral
respiratory groups.
• The dorsal respiratory groups stimulate the diaphragm.
• The ventral respiratory groups stimulate the intercostal and
abdominal muscles.
2. The pontine respiratory group is involved with switching between
inspiration and expiration.
Generation ofRhythmic Ventilation
1. When stimuli from receptors or other parts ofthe brain exceed a
threshold level,inspiration begins.
2. At the same time that respiratory muscles are stimulated,neurons
that stop inspiration are stimulated.When the stimulation of these
neurons exceeds a threshold level,inspiration is inhibited.
Modification ofVentilation
(p. 845)
Cerebraland Limbic System Control
Respiration can be voluntarily controlled and can be modified by emotions.
ChemicalControl of Ventilation
1. Carbon dioxide is the major regulator ofrespiration. An increase in
carbon dioxide or a decrease in pH can stimulate the chemosensitive
area,causing a greater rate and depth of respiration.
2. Oxygen levels in the blood affect respiration when a 50% or greater
decrease from normal levels exists.Decreased oxygen is detected by
receptors in the carotid and aortic bodies,which then stimulate the
respiratory center.
Hering-Breuer Reflex
Stretch of the lungs during inspiration can inhibit the respiratory center
and contribute to a cessation ofinspiration.
Effectof Exercise on Ventilation
1. Collateral fibers from motor neurons and from proprioceptors
stimulate the respiratory centers.
2. Chemosensitive mechanisms and learning fine-tune the effects
produced through the motor neurons and proprioceptors.
Other Modificationsof Ventilation
Touch,thermal,and pain sensations can modify ventilation.
RespiratoryAdaptations to Exercise
(p. 849)
Tidal volume,respiratory rate, minute ventilation, and gas exchange be-
tween the alveoli and blood remain unchanged or slightly lower at rest or
during submaximal exercise but increase at maximal exercise.
Effectsof Aging on the Respiratory System
(p. 850)
1. Vital capacity and maximum minute ventilation decrease with age
because ofweakening of respiratory muscles and decreased thoracic
cage compliance.
2. Residual volume and dead space increase because ofincreased
diameter ofrespiratory passageways. As a result, alveolar ventilation
decreases.
3. An increase in resting tidal volume compensates for decreased
alveolar ventilation,loss of alveolar walls (surface area), and
thickening ofalveolar walls.
4. The ability to remove mucus from the respiratory passageways
decreases with age.
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Part4 Regulationsand Maintenance856
1. The nasal cavity
a. has openings for the paranasal sinuses.
b. has a vestibule,which contains the olfactory epithelium.
c. is connected to the pharynx by the nares.
d. has passageways called conchae.
e. is lined with squamous epithelium,except for the vestibule.
2. The nasopharynx
a. is lined with moist stratified squamous epithelium.
b. contains the pharyngeal tonsil.
c. opens into the oral cavity through the fauces.
d. extends to the tip ofthe epiglottis.
e. is an area that food,drink, and air pass through.
3. The larynx
a. connects the oropharynx to the trachea.
b. has three unpaired and six paired cartilages.
c. contains the vocal folds.
d. contains the vestibular folds.
e. all ofthe above.
4. The trachea contains
a. skeletal muscle.
b. pleural fluid glands.
c. C-shaped pieces ofcartilage.
d. all ofthe above.
5. The conducting zone ofthe tracheobronchial tree ends at the
a. alveolar duct.
b. alveoli.
c. bronchioles.
d. respiratory bronchioles.
e. terminal bronchioles.
6. During an asthma attack,the patient has difficulty breathing
because ofconstriction of the
a. trachea.
b. bronchi.
c. terminal bronchioles.
d. alveoli.
e. respiratory membrane.
7. During quiet expiration,the
a. abdominal muscles relax.
b. diaphragm moves inferiorly.
c. external intercostal muscles contract.
d. thorax and lungs passively recoil.
e. all ofthe above.
8. The parietal pleura
a. covers the surface ofthe lung.
b. covers the inner surface ofthe thoracic cavity.
c. is the connective tissue partition that divides the thoracic cavity
into right and left pleural cavities.
d. covers the inner surface ofthe alveoli.
e. is the membrane across which gas exchange occurs.
9. Contraction ofthe bronchiolar smooth muscle has which of these
effects?
a. a smaller pressure gradient is required to get the same rate of
airflow when compared to normal bronchioles
b. increases airflow through the bronchioles
c. increases resistance to airflow
d. increases alveolar ventilation
10. During the process ofexpiration, the alveolar pressure is
a. greater than the pleural pressure.
b. greater than the barometric pressure.
c. less than the barometric pressure.
d. unchanged.
11. The lungs do not normally collapse because of
a. surfactant.
b. pleural pressure.
c. elastic recoil.
d. both a and b.
12. Immediately after the creation ofan opening through the thorax
into the pleural cavity,
a. air flows through the hole and into the pleural cavity.
b. air flows through the hole and out ofthe pleural cavity.
c. air flows neither out nor in.
d. the lung protrudes through the hole.
13. Compliance ofthe lungs and thorax
a. is the volume by which the lungs and thorax change for each unit
change ofalveolar pressure.
b. increases in emphysema.
c. decreases because oflack of surfactant.
d. all ofthe above.
14. Given these lung volumes:
1. tidal volume= 500 mL
2. residual volume= 1000 mL
3. inspiratory reserve volume= 2500 mL
4. expiratory reserve volume= 1000 mL
5. dead space= 1000 mL
The vital capacity is
a. 3000 mL.
b. 3500 mL.
c. 4000 mL.
d. 5000 mL.
e. 6000 mL.
15. The alveolar ventilation is the
a. tidal volume times respiratory rate.
b. minute ventilation plus the dead space.
c. amount ofair available for gas exchange in the lungs.
d. vital capacity divided by respiratory rate.
e. inspiratory reserve volume times minute ventilation.
16. Ifthe total pressure of a gas is 760 mm Hg and its composition is
20% oxygen,0.04% carbon dioxide, 75% nitrogen, and 5% water
vapor,the partial pressure of oxygen is
a. 15.2 mm Hg.
b. 20 mm Hg.
c. 118 mm Hg.
d. 152 mm Hg.
e. 740 mm Hg.
17. The rate ofdiffusion of a gas across the respiratory membrane
increases as the
a. respiratory membrane becomes thicker.
b. surface area ofthe respiratory membrane decreases.
c. partial pressure difference ofthe gas across the respiratory
membrane increases.
d. diffusion coefficient ofthe gas decreases.
e. all ofthe above.
18. In which ofthese sequences does P
O
2
progressively decrease?
a. arterial blood,alveolar air,body tissues
b. body tissues,arterial blood,alveolar air
c. body tissues,alveolar air,arterial blood
d. alveolar air,arterial blood,body tissues
e. arterial blood,body tissues, alveolar air
19. The partial pressure ofcarbon dioxide in the venous blood is
a. greater than in the tissue spaces.
b. less than in the tissue spaces.
c. less than in the alveoli.
d. less than in arterial blood.
REVIEW AND COMPREHENSION
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
23. Respiratory System
© The McGraw−Hill
Companies, 2004
Chapter 23 RespiratorySystem 857
20. Oxygen is mostly transported in the blood
a. dissolved in plasma.
b. bound to blood proteins.
c. within bicarbonate ions.
d. bound to the heme portion ofhemoglobin.
21. The oxygen–hemoglobin dissociation curve is adaptive because it
a. shifts to the right in the pulmonary capillaries and to the left in
the tissue capillaries.
b. shifts to the left in the pulmonary capillaries and to the right in
the tissue capillaries.
c. doesn’t shift.
22. Carbon dioxide is mostly transported in the blood
a. dissolved in plasma.
b. bound to blood proteins.
c. within bicarbonate ions.
d. bound to the heme portion ofhemoglobin.
e. bound to the globin portion ofhemoglobin.
23. When blood passes through the tissues,the hemoglobin in blood is
better able to combine with carbon dioxide because ofthe
a. Bohr effect.
b. Haldane effect.
c. chloride shift.
d. Boyle effect.
e. Dalton effect.
24. The chloride shift
a. occurs primarily in pulmonary capillaries.
b. occurs when chloride ions replace bicarbonate ions within
erythrocytes.
c. decreases the formation ofbicarbonate ions.
d. decreases the number ofhydrogen ions.
25. Which ofthese parts of the brainstem is correctly matched with its
main function?
a. ventral respiratory groups—stimulate the diaphragm
b. dorsal respiratory groups—limit inflation ofthe lungs
c. pontine respiratory group—switching between inspiration and
expiration
d. all ofthe above
26. The chemosensitive area
a. stimulates the respiratory center when blood carbon dioxide
levels increase.
b. stimulates the respiratory center when blood pH increases.
c. is located in the pons.
d. stimulates the respiratory center when blood oxygen levels
increase.
e. all ofthe above.
27. Blood oxygen levels
a. are more important than carbon dioxide in the regulation of
respiration.
b. need to change only slightly to cause a change in respiration.
c. are detected by sensory receptors in the carotid and aortic
bodies.
d. all ofthe above.
28. The Hering-Breuer reflex
a. limits inspiration.
b. limits expiration.
c. occurs in response to changes in carbon dioxide levels in the
blood.
d. is stimulated when oxygen decreases in the blood.
e. does not occur in infants.
29. At the onset ofexercise,respiration rate and depth increases
primarily because of
a. increased blood carbon dioxide levels.
b. decreased blood oxygen levels.
c. decreased blood pH.
d. input to the respiratory center from the cerebral motor cortex
and proprioceptors.
30. In response to exercise training,
a. the tidal volume at rest does not change.
b. minute ventilation during maximal exercise increases.
c. the brain learns to match ventilation to exercise intensity.
d. all ofthe above.
Answers in Appendix F
1. What effect does rapid (respiratory rate equals 24 breaths per minute),
shallow (tidal volume equals 250 mL per breath) breathing have on
minute ventilation,alveolar ventilation,and alveolar P
O
2
and P
CO
2
?
2. A person’s vital capacity is measured while standing and while lying
down.What difference, if any,in the measurement do you predict
and why?
3. Ima Diver wanted to do some underwater exploration.She didn’t
want to buy expensive SCUBA equipment,however.Instead, she
bought a long hose and an inner tube.She attached one end of the
hose to the inner tube so that the end was always out ofthe water,
and she inserted the other end ofthe hose in her mouth and went
diving.What happened to her alveolar ventilation and why? How
would she compensate for this change? How would diving affect
lung compliance and the work ofventilation?
4. The bacteria that cause gangrene (Clostridium perfringens)are
anaerobic microorganisms that don’t thrive in the presence ofoxygen.
Hyperbaric oxygenation (HBO) treatment places a person in a chamber
that contains oxygen at three to four times normal atmospheric
pressure.Explain how HBO helps in the treatment of gangrene.
5. Cardiopulmonary resuscitation (CPR) has replaced older,less
efficient methods ofsustaining respiration. The back-pressure/arm-
lift method is one such technique that’s no longer used.This
procedure is performed with the victim lying face down.The rescuer
presses firmly on the base ofthe scapulae for several seconds and
then grasps the arms and lifts them.The sequence is then repeated.
Explain why this procedure results in ventilation ofthe lungs.
6. Another technique for artificial respiration is mouth-to-mouth
resuscitation.The rescuer takes a deep breath, blows air into the
victim’s mouth,and then lets air flow out of the victim. The process is
repeated.Explain the following: (1) Why do the victim’s lungs expand?
(2) Why does air move out ofthe victim’s lungs? and (3) What effect
do the P
O
2
and the P
CO
2
ofthe rescuer’s air have on the victim?
7. During normal quiet respiration,when does the maximum rate of
diffusion ofoxygen in the pulmonary capillaries occur? The
maximum rate ofdiffusion of carbon dioxide?
8. Is the oxygen–hemoglobin dissociation curve in humans who live at
high altitudes to the left or to the right ofa person who lives at low
altitudes?
9. Predict what would happen to tidal volume ifthe vagus nerves were
cut.The phrenic nerves? The intercostal nerves?
10. You and your physiology instructor are trapped in an overturned
ship.To escape,you must swim underwater a long distance. You tell
your instructor it would be a good idea to hyperventilate before
making the escape attempt.Your instructor calmly replies,“What
good would that do,since your pulmonary capillaries are already
100% saturated with oxygen?”What would you do and why?
Answers in Appendix G
CRITICAL THINKING
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
23. Respiratory System
© The McGraw−Hill
Companies, 2004
Part4 Regulationsand Maintenance858
1. Air moving through the mouth is not as efficiently warmed and
moistened as air moving through the nasal cavity,and the throat or
lung tissue can become dehydrated or damaged by the cold air.
2. When food moves down the esophagus,the normally collapsed
esophagus expands.If the cartilage rings were solid, expansion of
the esophagus,and, therefore, swallowing, would be more difficult.
3. A foreign object is more likely to become lodged in the right
primary bronchus because it has a larger diameter and is more
directly in line with the trachea.
4. Respiratory distress syndrome results from inadequate surfactant,
which results in increased water surface tension.Consequently,lung
recoil is increased.At the end of expiration, pleural pressure is lower
than normal because ofthe increased lung recoil. Although the
decreased pleural pressure increases the tendency for the alveoli to
expand,the alveoli don’t expand because the increased force of
expansion is only counteracting the increased lung recoil.The
alveoli collapse ifthe lung recoil becomes larger than the force of
expansion caused by the difference between alveolar and pleural
pressure.During inspiration, pleural pressure has to be lower than
normal to overcome the effect ofthe larger-than-normal lung recoil.
A larger-than-normal increase in thoracic volume can cause a
greater-than-normal decrease in pleural pressure.The effort of
overcoming the increased lung recoil,however,can cause muscular
fatigue and death.
5. The alveolar ventilation is 4200 mL/min (12
⫻[500 ⫺ 150]).During
exercise,the alveolar ventilation is 88,800 mL/min (24
⫻[4000 ⫺
300]),a 21-fold increase. The increased air exchange increases P
O
2
and decreases P
CO
2
in the alveoli,thus increasing gas exchange
between the alveoli and the blood.
6. The air the diver is breathing has a greater total pressure than
atmospheric pressure at sea level.Consequently, the partial pressure
ofeach gas in the air increases. According to Henry’s law,as the
partial pressure ofa gas increases, the amount (concentration) of
gas dissolved in the liquid (e.g.,body fluids) with which the gas is in
contact increases.When the diver suddenly ascends,the partial
pressure ofgases in the body returns toward sea level barometric
pressure.As a result, the amount (concentration) of gas that can be
dissolved in body fluids suddenly decreases.When the fluids can no
longer hold all the gas,gas bubbles form.
7. At high altitudes,the atmospheric P
O
2
decreases because ofa
decrease in atmospheric pressure.The decreased atmospheric P
O
2
results in a decrease in alveolar P
O
2
and less oxygen diffusion into
lung tissue.If the person’s arterioles are especially sensitive to the
decreased oxygen levels,constriction of the arterioles reduces blood
flow through the lungs,and the ability to oxygenate blood decreases.
Such generalized hypoxemia can also be caused by certain
respiratory diseases,such as emphysema and cystic fibrosis.
8. Remember that the oxygen–hemoglobin dissociation curve
normally shifts to the right in tissues.The shift of the curve to the
left caused by CO reduces the ability ofhemoglobin to release
oxygen to tissues,which contributes to the detrimental effects of CO
poisoning.In the lungs, the shift to the left could slightly increase
the ability ofhemoglobin to pick up oxygen, but this effect is offset
by the decreased ability ofhemoglobin to release oxygen to tissues.
9. A person who cannot synthesize BPG has mild erythrocytosis.Her
hemoglobin releases less oxygen to tissues.Consequently,one would
expect increased erythropoietin release from the kidneys and
increased red blood cell production in red bone marrow.
10. In tissues,carbon dioxide moves into red blood cells,resulting in an
increase in hydrogen ions.According to the Bohr effect,as hydrogen
ions bind to hemoglobin the oxygen–hemoglobin dissociation
curve shifts to the right and there is increased release ofoxygen.
According to the Haldane effect,hemoglobin that has released
oxygen picks up more carbon dioxide.
11. Hyperventilation decreases blood carbon dioxide levels,causing an
increase in blood pH.Holding one’s breath increases blood carbon
dioxide levels and decreases blood pH.
12. When a person hyperventilates,P
CO
2
in the blood decreases.
Consequently,carbon dioxide moves out of cerebrospinal fluid into
the blood.As carbon dioxide levels in cerebrospinal fluid decrease,
hydrogen ions and bicarbonate ions combine to form carbonic acid,
which forms carbon dioxide.The result is a decrease in hydrogen ion
concentration in cerebrospinal fluid and decreased stimulation ofthe
respiratory center by the chemosensitive area.Until blood P
CO
2
levels
increase,the chemosensitive area is not stimulated, and apnea results.
13. Through touch,thermal,or pain receptors, the respiratory center
can be stimulated to cause a sudden inspiration ofair.
14. A P
O
2
of60 mm Hg and a P
CO
2
of30 mm Hg are both below
normal.The movement of air into and out of the lungs is restricted
because ofthe asthma and a mismatch occurs between ventilation of
the alveoli and blood flow to the alveoli.Consequently,because of
the ineffective ventilation,blood oxygen levels decrease.Mr. W
hyperventilates,which helps to maintain blood oxygen levels but
also results in lower-than-normal blood carbon dioxide levels.(If no
hyperventilation occurred,one would expect decreased blood
oxygen but increased blood carbon dioxide levels.)
ANSWERS TO PREDICT QUESTIONS
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