460
Chapter 13
carbon dioxide in inspired air and so we can ignore that factor.
A decreased alveolar ventilation will increase the alveolar
P
CO
2
(see Figure 13–22) because there is less inspired fresh air to
dilute the carbon dioxide entering the alveoli from the blood.
Increased production of carbon dioxide will also increase the
alveolar
P
CO
2
because more carbon dioxide will be entering
the alveoli from the blood per unit time. Recall that in the
steady state, the volume of carbon dioxide entering the alveoli
per unit time is always equal to the volume produced by the
tissues. Just reverse the direction of the changes to decrease
alveolar
P
CO
2
.
For simplicity, we assumed only one factor would change
at a time, but if more than one factor changes, the effects will
either add to or subtract from each other. For example, if oxy-
gen consumption and alveolar ventilation both increase at the
same time, their opposing effects on alveolar
P
O
2
will tend to
cancel each other out, and alveolar
P
O
2
will not change.
This last example emphasizes that, at any particular atmo-
spheric
P
O
2
, it is the
ratio
of oxygen consumption to alveolar ven-
tilation that determines alveolar
P
O
2
—the higher the ratio, the
lower the alveolar
P
O
2
. Similarly, alveolar
P
CO
2
is determined by
the ratio of carbon dioxide production to alveolar ventilation—
the higher the ratio, the higher the alveolar
P
CO
2
.
We can now defi ne two terms that denote the adequacy
of ventilation—that is, the relationship between metabolism
and alveolar ventilation. Physiologists state these defi nitions in
terms of carbon dioxide rather than oxygen.
Hypoventilation
exists when there is an increase in the ratio of carbon dioxide
production to alveolar ventilation. In other words, a person is
hypoventilating if the alveolar ventilation cannot keep pace with
the carbon dioxide production. The result is that alveolar
P
CO
2
rises above the normal value.
Hyperventilation
exists when
there is a decrease in the ratio of carbon dioxide production to
alveolar ventilation—that is, when alveolar ventilation is actually
too great for the amount of carbon dioxide being produced. The
result is that alveolar
P
CO
2
decreases below the normal value.
Note that “hyperventilation” is not synonymous with
“increased ventilation.” Hyperventilation represents increased
ventilation
relative to metabolism.
Thus, for example, the
increased ventilation that occurs during moderate exercise is
not hyperventilation because, as we will see, the increase in
production of carbon dioxide in this situation is proportional
to the increased ventilation.
Gas Exchange Between Alveoli and Blood
The blood that enters the pulmonary capillaries is systemic
venous blood pumped to the lungs through the pulmonary
arteries. Having come from the tissues, it has a relatively high
P
CO
2
(46 mmHg in a normal person at rest) and a relatively low
P
O
2
(40 mmHg) (see Figure 13–21 and
Table 13–7
). The dif-
ferences in the partial pressures of oxygen and carbon dioxide
Table 13–6
Effects of Various Conditions on Alveolar Gas Pressures
Condition
Alveolar
P
O
2
Alveolar
P
CO
2
Breathing air with low
P
O
2
Decreases
No change*
Alveolar ventilation and unchanged metabolism
Increases
Decreases
Alveolar ventilation and unchanged metabolism
Decreases
Increases
Metabolism and unchanged alveolar ventilation
Decreases
Increases
Metabolism and unchanged alveolar ventilation
Increases
Decreases
Proportional increases in metabolism and alveolar ventilation
No change
No change
*Breathing air with low
P
O
2
has no direct effect on alveolar
P
CO
2
. However, as described later in the text, people in this situation will refl exly increase their ventilation, and that will lower
P
CO
2
.
1.0
4.0
8.0
50
100
150
0
Normal resting
values
O
2
CO
2
Alveolar ventilation (L/min)
Hypoventilation
Hyperventilation
Alveolar partial pressure (mmHg)
Figure 13–22
Effects of increasing or decreasing alveolar ventilation on alveolar
partial pressures in a person having a constant metabolic rate
(cellular oxygen consumption and carbon dioxide production). Note
that alveolar
P
O
2
approaches zero when alveolar ventilation is about
1 L/min. At this point, all the oxygen entering the alveoli crosses
into the blood, leaving virtually no oxygen in the alveoli.
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