458
Chapter 13
pressure of the mixture is simply the sum of the individual
pressures. These individual pressures, termed
partial pres-
sures,
are denoted by a
P
in front of the symbol for the gas.
For example, the partial pressure of oxygen is expressed as
P
O
2
. The partial pressure of a gas is directly proportional to its
concentration. Net diffusion of a gas will occur from a region
where its partial pressure is high to a region where it is low.
Atmospheric air consists of approximately 79 percent
nitrogen and approximately 21 percent oxygen, with very small
quantities of water vapor, carbon dioxide, and inert gases. The
sum of the partial pressures of all these gases is termed atmo-
spheric pressure, or barometric pressure. It varies in different
parts of the world as a result of local weather conditions and
gravitational differences due to altitude, but at sea level it is
760 mmHg. Because the partial pressure of any gas in a
mixture is the fractional concentration of that gas times the
total pressure of all the gases, the
P
O
2
of atmospheric air is
0.21
×
760 mmHg = 160 mmHg at sea level.
Diffusion of Gases in Liquids
When a liquid is exposed to air containing a particular gas, mol-
ecules of the gas will enter the liquid and dissolve in it.
Henry’s
law
states that the amount of gas dissolved will be directly pro-
portional to the partial pressure of the gas with which the liquid
is in equilibrium. A corollary is that, at equilibrium, the partial
pressures of the gas molecules in the liquid and gaseous phases
must be identical. Suppose, for example, that a closed container
contains both water and gaseous oxygen. Oxygen molecules
from the gas phase constantly bombard the surface of the water,
some entering the water and dissolving. The number of mol-
ecules striking the surface is directly proportional to the
P
O
2
of
the gas phase, so the number of molecules entering the water
and dissolving in it is also directly proportional to the
P
O
2
. As
long as the
P
O
2
in the gas phase is higher than the
P
O
2
in the
liquid, there will be a net diffusion of oxygen into the liquid.
Diffusion equilibrium will be reached only when the
P
O
2
in the
liquid is equal to the
P
O
2
in the gas phase, and there will then be
no further net diffusion between the two phases.
Conversely, if a liquid containing a dissolved gas at high
partial pressure is exposed to a lower partial pressure of that
same gas in a gas phase, a net diffusion of gas molecules will
occur out of the liquid into the gas phase until the partial
pressures in the two phases become equal.
The exchanges
between
gas and liquid phases described
in the preceding two paragraphs are precisely the phenomena
occurring in the lungs between alveolar air and pulmonary
capillary blood. In addition,
within
a liquid, dissolved gas
molecules also diffuse from a region of higher partial pressure
Begin
Begin
Tissue capillaries
Lung capillaries
200 ml/min
Alveoli
840 ml/min
590 ml/min
200 ml CO
2
250 ml O
2
Cells
250 ml O
2
Alveolar ventilation = 4 L/min
Air
Cardiac output
=
5 L/min
Tissue capillaries
Lung capillaries
Cells
200 ml CO
2
O
2
O
2
CO
2
Figure 13–20
Summary of typical oxygen and carbon dioxide exchanges between atmosphere, lungs, blood, and tissues
during 1 min
in a resting individual.
Note that the values in this fi gure for oxygen and carbon dioxide in blood are
not
the values per liter of blood, but rather the amounts
transported
per minute
in the cardiac output (5 L in this example). The volume of oxygen in 1 L of arterial blood is 200 ml O
2
/L of blood—
that is, 1000 ml O
2
/5 L of blood.
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