tors to the respiratory center, and breathing
The increase in breathing rate during exer-
cise requires increased blood flow to skeletal
muscles. Thus, exercise increases demand on both
the respiratory and the cardiovascular systems. If
either of these systems fails to keep pace with cel-
lular demands, the person will begin to feel out
of breath. This sensation, however, is usually due
to the inability of the cardiovascular system to
move enough blood between the lungs and the
cells, rather than to the inability of the respiratory
system to provide enough air.
ectiveness in obtaining oxygen and releasing
to the outside.
The cerebral cortex and the propriocep-
tors associated with muscles and joints are also
implicated in the increased breathing rate asso-
ciated with exercise (see chapter 12, p. 444). The
cortex transmits stimulating impulses to the
respiratory center whenever it signals the skel-
etal muscles to contract. At the same time, mus-
cular movements stimulate the proprioceptors,
triggering a
joint reflex.
In this reflex, sensory
impulses are transmitted from the propriocep-
oderate to heavy exercise greatly
increases the amount of oxygen skel-
etal muscles use. A young man at rest
uses about 250 milliliters of oxygen per minute
but may require 3,600 milliliters per minute dur-
ing maximal exercise. While oxygen use is increas-
ing, CO
production increases also. Decreased
blood oxygen and increased blood CO
the respiratory center, so exercise is accompanied
by increased breathing rate. However, blood oxy-
gen and CO
levels usually do not change during
exercise—this reflects the respiratory system’s
Exercise and Breathing
ers make up the
respiratory membrane
membrane), through which gas exchange occurs between
the alveolar air and the blood
gs. 19.34
Exposure to high oxygen concentration (hyperoxia) for a prolonged
time may damage lung tissues, particularly capillary walls. Excess ±
may escape the capillaries and ±
ood the alveolar air spaces, interfer-
ing with gas exchange, which can be lethal. Similarly, hyperoxia can
damage the retinal capillaries of premature infants, causing retrolen-
tal ²
broplasia (RL³), a condition that may lead to blindness.
Diffusion Through the Respiratory
Molecules diffuse from regions where they are in higher
concentration toward regions where they are in lower con-
centration. Thus, in determining the direction of diffusion
of a solute, we must know the concentration gradient. In
the case of gases, it is more convenient to think in terms of
a partial pressure gradient, such that a gas will diffuse from
an area of higher partial pressure to an area of lower partial
To Chapter 3, Di±±usion, pages 90–92.
When a mixture of gases dissolves in blood, the result-
ing concentration of each dissolved gas is proportional to
its partial pressure. Each gas diffuses between blood and
its surroundings from areas of higher partial pressure to
areas of lower partial pressure until the partial pressures in
the two regions reach equilibrium. For example, the P
blood entering the pulmonary capillaries is 45 mm Hg, but
airborne agents, including bacteria, thereby cleaning the
g. 19.33)
Respiratory Membrane
Part of the wall of an alveolus is made up of cells (type II
cells) that secrete pulmonary surfactant, described earlier,
but the bulk of the wall of an alveolus consists of a layer
of simple squamous epithelium (type I cells). Each alveo-
lus is associated with a dense network of capillaries lined
with simple squamous epithelial cells (±
g. 19.33). Thin base-
ment membranes separate the layers of these fl attened cells,
and in the spaces between them are elastic and collagenous
± bers that help support the alveolar wall. Thus, two thick-
nesses of epithelial cells and basement membranes separate
the air in an alveolus and the blood in a capillary. These lay-
FIGURE 19.32
Alveolar pores (arrow) allow air to pass from one
alveolus to another (300×).
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