Respiratory Physiology
443
Nasal
cavity
Nostril
Mouth
Larynx
Trachea
Right
lung
Left main
bronchus
Left lung
Diaphragm
Right
main
bronchus
Pharynx
Figure 13–1
Organization of the respiratory system. The ribs have been removed
in front, and the lungs are shown in a way that makes visible the
major airways within them.
structures for the entire animal to exchange oxygen and carbon dioxide with the external environment.
In humans and other mammals, the
respiratory system
includes the oral and nasal cavities, the lungs,
the series of tubes leading to the lungs, and the chest structures responsible for moving air into and out
of the lungs during breathing.
In addition to mediating gas exchange with the environment, the respiratory system serves other
functions, as listed in
Table 13–1
.
Organization of the
Respiratory System
There are two lungs, the right and left, each divided into lobes.
The lungs consist mainly of tiny air-containing sacs called
alve-
oli
(singular,
alveolus
), which number approximately 300 mil-
lion in an adult. The alveoli are the sites of gas exchange with
the blood. The
airways
are the tubes that air fl ows through
from the external environment to the alveoli and back.
Inspiration
(inhalation) is the movement of air from
the external environment through the airways into the alve-
oli during breathing.
Expiration
(exhalation) is movement
in the opposite direction. An inspiration and an expiration
constitute a
respiratory cycle.
During the entire respiratory
cycle, the right ventricle of the heart pumps blood through
the pulmonary arteries and arterioles and into the capillaries
surrounding each alveolus. In a normal adult at rest, approxi-
mately 4 L of fresh air enters and leaves the alveoli per min-
ute, while 5 L of blood, virtually the entire cardiac output,
fl ows through the pulmonary capillaries. During heavy exer-
cise, the air fl ow can increase twentyfold, and the blood fl ow
fi ve- to sixfold.
The Airways and Blood Vessels
During inspiration, air passes through either the nose or mouth
into the
pharynx,
a passage common to both air and food
(
Figure 13–1
). The pharynx branches into two tubes: the
esophagus, through which food passes to the stomach, and
the
larynx,
which is part of the airways. The larynx houses the
vocal cords,
two folds of elastic tissue stretched horizontally
across its lumen. The fl ow of air past the vocal cords causes
them to vibrate, producing sounds. The nose, mouth, phar-
ynx, and larynx are collectively termed the
upper airways.
The larynx opens into a long tube, the
trachea,
which
in turn branches into two
bronchi
(singular,
bronchus
), one
of which enters each lung. Within the lungs, there are more
than 20 generations of branchings, each resulting in narrower,
shorter, and more numerous tubes; their names are summa-
rized in
Figure 13–2
. The walls of the trachea and bronchi
contain rings of cartilage, which give them their cylindri-
cal shape and support them. The fi rst airway branches that
Table 13–1
Functions of the Respiratory System
1. Provides oxygen.
2. Eliminates carbon dioxide.
3. Regulates the blood’s hydrogen ion concentration (pH) in
coordination with the kidneys.
4. Forms speech sounds (phonation).
5. Defends against microbes.
6. Infl uences arterial concentrations of chemical messengers
by removing some from pulmonary capillary blood and
producing and adding others to this blood.
7. Traps and dissolves blood clots arising from systemic veins
such as those in the legs.
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