Respiratory Physiology
459
to a region of lower partial pressure, an effect that underlies
the exchange of gases between cells, extracellular fl
uid, and
capillary blood throughout the body.
Why must the diffusion of gases into or within liquids
be presented in terms of partial pressures rather than “con-
centrations,” the values used to deal with the diffusion of all
other solutes? The reason is that the concentration of a gas
in a liquid is proportional not only to the partial pressure of
the gas but also to the solubility of the gas in the liquid. The
more soluble the gas, the greater its concentration will be at
any given partial pressure. Thus, if a liquid is exposed to two
different gases having the same partial pressures, at equilib-
rium the
partial pressures
of the two gases will be identical in
the liquid, but the
concentrations
of the gases in the liquid will
differ, depending upon their solubilities in that liquid.
With these basic gas properties as the foundation, we
can now discuss the diffusion of oxygen and carbon dioxide
across alveolar and capillary walls and plasma membranes.
The partial pressures of these gases in air and in various sites
of the body for a resting person at sea level appear in
Figure
13–21
. We start our discussion with the alveolar gas pressures
because their values set those of systemic arterial blood. This
fact cannot be emphasized too strongly: The alveolar
P
O
2
and
P
CO
2
determine the systemic arterial
P
O
2
and
P
CO
2
.
Alveolar Gas Pressures
Normal alveolar gas pressures are
P
O
2
= 105 mmHg and
P
CO
2
= 40 mmHg. (We do not deal with nitrogen, even though it is
the most abundant gas in the alveoli, because nitrogen is bio-
logically inert under normal conditions and does not undergo
any net exchange in the alveoli.) Compare these values with
the gas pressures in the air being breathed:
P
O
2
= 160 mmHg
and
P
CO
2
= 0.3 mmHg, a value so low that we will simply
treat it as zero. The alveolar
P
O
2
is lower than atmospheric
P
O
2
because some of the oxygen in the air entering the alveoli
leaves them to enter the pulmonary capillaries. Alveolar
P
CO
2
is higher than atmospheric
P
CO
2
because carbon dioxide enters
the alveoli from the pulmonary capillaries.
The factors that determine the precise value of alveolar
P
O
2
are (1) the
P
O
2
of atmospheric air, (2) the rate of alveolar
ventilation, and (3) the rate of total-body oxygen consump-
tion. Although equations exist for calculating the alveolar gas
pressures from these variables, we will describe the interac-
tions in a qualitative manner (
Table 13–6
). To start, we will
assume that only one of the factors changes at a time.
First, a decrease in the
P
O
2
of the inspired air, such as
would occur at high altitude, will decrease alveolar
P
O
2
. A
decrease in alveolar ventilation will do the same thing (
Figure
13–22
) because less fresh air is entering the alveoli per unit
time. Finally, an increase in the oxygen consumption in the
cells will also lower alveolar
P
O
2
because a larger fraction of the
oxygen in the entering fresh air will leave the alveoli to enter
the blood for use by the tissues. (Recall that in the steady
state, the volume of oxygen entering the blood in the lungs
per unit time is always equal to the volume utilized by the tis-
sues.) This discussion has been in terms of things that lower
alveolar
P
O
2
; just reverse the direction of change of the three
factors to see how to increase
P
O
2
.
The story for
P
CO
2
is analogous, again assuming that only
one factor changes at a time. There is normally essentially no
Tissue capillaries
P
CO
2
= 40 mmHg
P
O
2
= 100 mmHg
P
O
2
= 100 mmHg
P
CO
2
= 40 mmHg
P
CO
2
= 46 mmHg
P
O
2
= 40 mmHg
P
O
2
= 40 mmHg
P
CO
2
= 46 mmHg
Lung capillaries
Right
heart
Left
heart
Alveoli
Cells
Pulmonary
arteries
Systemic
veins
Pulmonary
veins
Systemic
arteries
P
O
2
< 40 mmHg (mitochondrial
P
O
2
< 5 mmHg)
P
CO
2
> 46 mmHg
P
CO
2
=
40 mmHg
P
O
2
=
105 mmHg
Air
P
O
2
= 160 mmHg
P
CO
2
= 0.3 mmHg
Figure 13–21
Partial pressures of carbon dioxide and oxygen
in inspired air at sea level and in various places in
the body. The reason that the alveolar
P
O
2
and
pulmonary vein
P
O
2
are not exactly the same is
described later in the text. Note also that the
P
O
2
in the systemic arteries is shown as identical to
that in the pulmonary veins; for reasons involving
the anatomy of the blood fl ow to the lungs, the
systemic arterial value is actually slightly less, but
we have ignored this for the sake of clarity.
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