474
Chapter 13
fore arterial
P
CO
2
do not change. In fact, in very strenuous
exercise the alveolar ventilation increases relatively more than
carbon dioxide production. In other words, during strenuous
exercise, a person may hyperventilate, and thus alveolar and
systemic arterial
P
CO
2
may actually decrease (
Figure 13–41
)!
Decreased P
O
2
as the Stimulus?
The story is similar for oxygen. Although systemic
venous
P
O
2
decreases during exercise due to an increase in oxygen con-
sumption in the tissues, alveolar
P
O
2
and, therefore, systemic
arterial
P
O
2
usually remain unchanged (see Figure 13–41).
This is because cellular oxygen consumption and alveolar ven-
tilation increase in exact proportion to each other, at least dur-
ing moderate exercise.
This is a good place to recall an important point made in
Chapter 12. In healthy individuals, ventilation is not the limiting
factor in endurance exercise—cardiac output is. Ventilation can,
as we have just seen, increase enough to maintain arterial
P
O
2
.
Increased H
+
Concentration as the Stimulus?
Because the arterial
P
CO
2
does not change during moderate exer-
cise and decreases during strenuous exercise, there is no accumu-
lation of excess H
+
resulting from carbon dioxide accumulation.
However, during strenuous exercise, there
is
an increase in
of normal arterial H
+
is necessary because most enzymes of
the body function best at physiological pH.
Figure 13–40
summarizes the control of ventilation by
P
O
2
,
P
CO
2
, and H
+
concentration.
Control of Ventilation During Exercise
During exercise, the alveolar ventilation may increase as much
as 20-fold. On the basis of our three variables—
P
O
2
,
P
CO
2
, and
H
+
concentration—it might seem easy to explain the mecha-
nism that induces this increased ventilation. Such is not the
case, however, and the major stimuli to ventilation during
exercise, at least moderate exercise, remain unclear.
Increased P
CO
2
as the Stimulus?
It would seem logical that, as the exercising muscles produce
more carbon dioxide, blood
P
CO
2
would increase. This is true,
however, only for systemic
venous
blood but not for systemic
arterial
blood. Why doesn’t arterial
P
CO
2
increase during exer-
cise? Recall two facts from the section on alveolar gas pres-
sures: (1) arterial
P
CO
2
is determined by alveolar
P
CO
2
, and
(2) alveolar
P
CO
2
is determined by the ratio of carbon dioxide
production to alveolar ventilation. During moderate exercise,
the alveolar ventilation increases in exact proportion to the
increased carbon dioxide production, so alveolar and there-
Diaphragm and inspiratory
intercostals
Contractions
Ventilation
Firing of neurons
to diaphragm and
inspiratory intercostals
Firing of medullary
inspiratory neurons
Peripheral chemoreceptors
Firing
Central chemoreceptors
Firing
Brain extracellular fluid [H
+
]
Arterial [H
+
]
Brain extracellular fluid
P
CO
2
Production of non-CO
2
acids
Arterial
P
O
2
Arterial
P
CO
2
Figure 13–40
Summary of the major chemical inputs that stimulate ventilation. This is
a combination of Figures 13–35, 13–37, and 13–39. When arterial
P
O
2
increases or when
P
CO
2
or hydrogen ion concentration decreases, ventilation is
refl exly decreased.
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