CHA PT ER
The Respiratory System
CONTENTS AT A GLANCE
Respiratory Anatomy
Cellular and external respiration
Respiratory Anatomy
Anatomy of respiratory system, thorax, and pleura e primary function of respiration is to obtain O2 for use by
Respiratory Mechanics the body cells and to eliminate the CO2 the cells produce.
Pressure considerations
Respiratory cycle The respiratory system does not participate
Airway resistance in all steps of respiration.
Elastic behavior of the lungs Most people think of respiration as the process of breathing in and
Work of breathing breathing out. In physiology, however, respiration has a much
Lung volumes and capacities broader meaning. Respiration encompasses two separate but re-
Pulmonary and alveolar ventilation lated processes: cellular respiration and external respiration.
Local controls to match airflow to blood flow
CELLULAR RESPIRATION e term cellular respiration refers to
Gas Exchange
the intracellular metabolic processes carried out within the mito-
Concept of partial pressure
chondria, which use O2 and produce CO2 while deriving energy
Gas exchange across the pulmonary and systemic from nutrient molecules (see p. 33). e respiratory quotient
capillaries
(RQ), the ratio of CO2 produced to O2 consumed, varies depend-
Gas Transport ing on the foodstu consumed. When carbohydrate is being
Oxygen transport used, the RQ is 1; that is, for every molecule of O2 consumed, one
Carbon dioxide transport molecule of CO2 is produced. For fat utilization, the RQ is 0.7; for
Abnormalities in blood-gas content protein, it is 0.8. On a typical American diet consisting of a mix-
Control of Respiration ture of these three nutrients, resting O2 consumption averages
Respiratory control centers in the brain stem about 250 ml/min, and CO2 production averages about 200 ml/
Generation of respiratory rhythm
min, for an average RQ of 0.8:
Chemical inputs affecting the magnitude of ventilation CO2 produced (200 ml/min)
RQ 0.8
O2 consumed (250 ml/min)
EXTERNAL RESPIRATION e term external respiration refers
to the entire sequence of events in the exchange of O2 and CO2
between the external environment and the cells of the body.
External respiration, the topic of this chapter, encompasses four
steps (● Figure 13-1):
1. Air is alternately moved into and out of the lungs so that air
can be exchanged between the atmosphere (external environ-
ment) and air sacs (alveoli) of the lungs. is exchange is ac-
complished by the mechanical act of breathing, or ventilation.
e rate of ventilation is regulated to adjust the ow of air be-
tween the atmosphere and alveoli according to the body’s meta-
Log on to CengageNOW at http://www bolic needs for O2 uptake and CO2 removal.
.cengage.com/sso/ for an opportunity to explore a learning
module that illustrates diffi cult concepts with self-study 2. Oxygen and CO2 are exchanged between air in the alveoli
tutorials, animations, and interactive quizzes to help you learn, and blood within the pulmonary (pulmonary means “lung”)
review, and master physiology concepts. capillaries by the process of di usion.
461
, Atmosphere STEPS OF EXTERNAL RESPIRATION ■ It is a route for water loss
O2 CO2 and heat elimination. Inspired
(inhaled) atmospheric air is
1 Ventilation or gas exchange between humidi ed and warmed by
the atmosphere and air sacs (alveoli)
the respiratory airways before
in the lungs
it is expired. Moistening of
inspired air is essential to pre-
vent the alveolar linings from
Alveoli O2 CO2 drying out. Oxygen and CO2
of lungs
cannot di use through dry
membranes.
2 Exchange of O2 and CO2 between air
■ It enhances venous return
CO2 in the alveoli and the blood in the
pulmonary capillaries (see the “respiratory pump,”
O2 p. 375).
Pulmonary ■ It helps maintain normal
circulation acid–base balance by altering
the amount of H -generating
CO2 exhaled (see p. 575).
■ It enables speech, singing,
and other vocalization.
3 Transport of O2 and CO2 by the blood ■ It defends against inhaled
between the lungs and the tissues
foreign matter (see p. 457).
■ It removes, modi es, acti-
vates, or inactivates various
Heart materials passing through the
pulmonary circulation. All
Systemic
blood returning to the heart
circulation
from the tissues must pass
CO2 through the lungs before being
O2 returned to the systemic circu-
lation. e lungs, therefore, are
4 Exchange of O2 and CO2 between the
blood in the systemic capillaries and uniquely situated to act on
the tissue cells speci c materials that have
Food O2 CO2 H2O ATP been added to the blood at the
CELLULAR RESPIRATION
tissue level before they have a
chance to reach other parts of
Tissue cells the body by means of the arte-
● FIGURE 13-1 External and cellular respiration. External respiration encompasses the steps rial system. For example, pros-
involved in the exchange of O2 and CO2 between the external environment and tissue cells (steps taglandins, a collection of
1 through 4). Cellular respiration encompasses the intracellular metabolic reactions involving the chemical messengers released
use of O2 to derive energy (ATP) from food, producing CO2 as a by-product. in many tissues to mediate
particular local responses (see
p. 758), may spill into the
3. e blood transports O2 and CO2 between the lungs and blood, but they are inactivated during passage through the lungs
tissues. so that they cannot exert systemic e ects. By contrast, the lungs (ACE)
4. Oxygen and CO2 are exchanged between the tissue cells and activate angiotensin II, a hormone that plays an important role in
blood by the process of di usion across the systemic (tissue) regulating the concentration of Na in the ECF (see p. 527).
capillaries. ■ e nose, a part of the respiratory system, serves as the or-
gan of smell (see p. 230).
e respiratory system does not accomplish all the steps of
respiration; it is involved only with ventilation and the ex-
change of O2 and CO2 between the lungs and blood (steps 1
The respiratory airways conduct air between
and 2 ). e circulatory system carries out the remaining the atmosphere and alveoli.
steps. e respiratory system includes the respiratory airways lead-
ing into the lungs, the lungs themselves, and the structures of
NONRESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM the thorax (chest) involved in producing movement of air
e respiratory system also lls these nonrespiratory functions: through the airways into and out of the lungs. e respiratory
462 Chapter 13
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, Terminal Smoooth
bronchiole muscle
Branch of
pulmonary
Branch of vein
pulmonary
artery
Nasal
passages
Mouth
Pharynx
Larynx
Alveolus Pulmonary
Trachea capillaries
Cartilaginous Alveolar
ring Pores of Kohn sac
(b) Enlargement of alveoli (air
Right Left sacs) at terminal ends of airways
bronchus bronchus
Bronchiole
Terminal bronchiole
Alveolar sac
Terminal
bronchiole
(a) Respiratory airways
● FIGURE 13-2 Anatomy of the respiratory system. (a) The respiratory airways include the
nasal passages, pharynx, larynx, trachea, bronchi, and bronchioles. (b) Most alveoli (air sacs) are
clustered in grapelike arrangements at the end of the terminal bronchioles.
airways are tubes that carry air between the atmosphere and air
Kay Pentax, a division of Pentax
sacs, the latter being the only site where gases can be exchanged
between air and blood. e airways begin with the nasal pas- Vocal fold
sages (nose) (● Figure 13-2a). e nasal passages open into the Glottis
pharynx (throat), which serves as a common passageway for
Medical
both the respiratory and digestive systems. Two tubes lead from
the pharynx—the trachea (windpipe), through which air is
conducted to the lungs, and the esophagus, the tube through (a) Glottis open (b) Glottis
closed
which food passes to the stomach. Air normally enters the
pharynx through the nose, but it can enter by the mouth as well ● FIGURE 13-3 Vocal folds. Photograph of the vocal folds as
when the nasal passages are congested; that is, you can breathe viewed from above at the laryngeal opening, showing the vocal
folds (a) positioned apart when the glottis is open and (b) in tight
through your mouth when you have a cold. Because the phar-
apposition when the glottis is closed.
ynx serves as a common passageway for food and air, re ex
mechanisms close o the trachea during swallowing so that
food enters the esophagus and not the airways. e esophagus
stays closed except during swallowing to keep air from entering passes into the larynx through the space between the vocal folds.
the stomach during breathing. is laryngeal opening is known as the glottis. As air moves
e larynx, or voice box, is located at the entrance of the through the open glottis past the variably positioned, taut vocal
trachea. e anterior protrusion of the larynx forms the “Adam’s folds, they vibrate to produce the many di erent sounds of
apple.” e vocal folds, two bands of elastic tissue that lie across speech. e lips, tongue, and so palate modify the sounds into
the opening of the larynx, can be stretched and positioned in recognizable sound patterns. During swallowing, the vocal folds
di erent shapes by laryngeal muscles (● Figure 13-3a). Air assume a function not related to speech: ey close the glottis.
The Respiratory System 463
, at is, laryngeal muscles bring the vocal folds into tight apposi- (● Figure 13-4a). ese cells secrete pulmonary surfactant, a
tion to each other to close o the entrance to the trachea so that phospholipoprotein complex that facilitates lung expansion
food does not get into the airways (● Figure 13-3b). (described later). Furthermore, defensive alveolar macrophages
Beyond the larynx, the trachea divides into two main stand guard within the lumen of the air sacs (see p. 457).
branches, the right and le bronchi, which enter the right and Minute pores of Kohn exist in the walls between adjacent
le lungs, respectively. Within each lung, the bronchus continues alveoli (see ● Figure 13-2b). eir presence permits air ow be-
to branch into progressively narrower, shorter, and more numer- tween adjoining alveoli, a process known as collateral ventila-
ous airways, much like the branching of a tree. e smaller tion. ese passageways are especially important in allowing
branches are known as bronchioles. Clustered at the ends of the fresh air to enter an alveolus whose terminal conducting airway
terminal bronchioles are the alveoli, the tiny air sacs where gases is blocked because of disease.
are exchanged between air and blood (see ● Figure 13-2b).
To permit air ow into and out of the gas-exchanging por- The lungs occupy much of the thoracic cavity.
tions of the lungs, the continuum of conducting airways from ere are two lungs, each divided into several lobes and each
the entrance through the terminal bronchioles to the alveoli supplied by one of the bronchi. e lung tissue itself consists of
must remain open. e trachea and larger bronchi are fairly the series of highly branched airways, the alveoli, the pulmo-
rigid, nonmuscular tubes encircled by a series of cartilaginous
nary blood vessels, and large quantities of elastic connective
rings that prevent these tubes from compressing. e smaller tissue. e only muscle within the lungs is the smooth muscle
bronchioles have no cartilage to hold them open. eir walls in the walls of the arterioles and the walls of the bronchioles,
contain smooth muscle that is innervated by the autonomic
both of which are subject to control. No muscle is present
nervous system and is sensitive to certain hormones and local within the alveolar walls to cause them to in ate and de ate
chemicals. ese factors, by varying the degree of contraction during the breathing process. Instead, changes in lung volume
of bronchiolar smooth muscle and hence the caliber of these (and accompanying changes in alveolar volume) are brought
small terminal airways, regulate the amount of air passing be- about through changes in the dimensions of the thoracic cavity.
tween the atmosphere and each cluster of alveoli.
You will learn about this mechanism a er we complete our
discussion of respiratory anatomy.
The gas-exchanging alveoli are thin-walled, e lungs occupy most of the volume of the thoracic
inflatable air sacs encircled by pulmonary (chest) cavity, the only other structures in the chest being the
capillaries. heart and associated vessels, the esophagus, the thymus, and
some nerves. e outer chest wall (thorax) is formed by
e lungs are ideally structured for gas exchange. According to 12 pairs of curved ribs, which join the sternum (breastbone)
Fick’s law of di usion, the shorter the distance through which anteriorly and the thoracic vertebrae (backbone) posteriorly.
di usion must take place, the greater the rate of di usion is.
e rib cage provides bony protection for the lungs and heart.
Also, the greater the surface area across which di usion can e diaphragm, which forms the oor of the thoracic cavity,
take place, the greater the rate of di usion is (see p. 62). is a large, dome-shaped sheet of skeletal muscle that com-
e alveoli are clusters of thin-walled, in atable, grapelike sacs
pletely separates the thoracic cavity from the abdominal cav-
at the terminal branches of the conducting airways. e alveolar ity. It is penetrated only by the esophagus and blood vessels
walls consist of a single layer of attened, fried-egg appearing traversing the thoracic and abdominal cavities. At the neck,
Type I alveolar cells (● Figure 13-4a). Each alveolus is sur- muscles and connective tissue enclose the thoracic cavity. e
rounded by a network of pulmonary capillaries, the walls of which only communication between the thorax and the atmosphere
are also only one cell thick (● Figure 13-4b). e interstitial space
is through the respiratory airways into the alveoli. Like the
between an alveolus and the surrounding capillary network forms lungs, the chest wall contains considerable amounts of elastic
an extremely thin barrier, with only 0.5 m separating air in the connective tissue.
alveoli from blood in the pulmonary capillaries. (A sheet of trac-
ing paper is about 50 times thicker than this air-to-blood barrier.)
e thinness of this barrier facilitates gas exchange. A pleural sac separates each lung
Furthermore, the alveolar air–blood interface presents a from the thoracic wall.
tremendous surface area for exchange. e lungs contain about A double-walled, closed sac called the pleural sac separates each
500 million alveoli, each about 300 m in diameter. So dense lung from the thoracic wall and other surrounding structures
are the pulmonary capillary networks that each alveolus is en- (● Figure 13-5). e interior of the pleural sac is known as the
circled by an almost continuous sheet of blood (● Figure 13-4c). pleural cavity. In the illustration, the dimensions of the pleural
e total surface area thus exposed between alveolar air and cavity are greatly exaggerated to aid visualization; in reality the
pulmonary capillary blood is about 75 m2 (about the size of a layers of the pleural sac are in close contact with one another.
tennis court). In contrast, if the lungs consisted of a single hol- e surfaces of the pleura secrete a thin intrapleural uid (intra
low chamber of the same dimensions instead of being divided means “within”), which lubricates the pleural surfaces as they
into a myriad alveolar units, the total surface area would be only slide past each other during respiratory movements.
about 0.01 m2. Pleurisy, an in ammation of the pleural sac, is accom-
In addition to the thin, wall-forming Type I cells, 5% of the panied by painful breathing, because each in ation and
alveolar surface epithelium is covered by Type II alveolar cells each de ation of the lungs cause a “friction rub.”
464 Chapter 13
