548 Chapter 16
on (1) the solubility of the gas in the fluid, which is a physical
constant; (2) the temperature of the fluid—more gas can be dis-
solved in cold water than warm water; and (3) the partial pressure
of the gas. Because solubility is a constant and the temperature
of the blood does not vary significantly, the concentration of a
gas dissolved in a fluid (such as plasma) depends directly on its
partial pressure in the gas mixture. When water—or plasma—
is brought into equilibrium with air at a P^ O 2 of 100 mmHg, for
example, the fluid will contain 0.3 ml of O 2 per 100 ml fluid at
37 8 C. If the P^ O 2 of the gas were reduced by half, the amount of
dissolved oxygen would also be reduced by half.Blood Gas Measurements
Measurement of the oxygen content of blood (in ml O 2 per
100 ml blood) is a laborious procedure. Fortunately, an oxygen
electrode that produces an electric current in proportion to the
concentration of dissolved oxygen has been developed. If this
electrode is placed in a fluid while oxygen is artificially bub-
bled into it, the current produced by the oxygen electrode will
increase up to a maximum value. This maxiumum is attained
when the fluid is saturated with oxygen—when it contains
all of the dissolved oxygen possible (given the low solubility
constant of oxygen) at a particular temperature and P^ O 2. If the
temperature is constant, the maximum amount of oxygen that
can be dissolved (and thus the reading of the oxygen electrode)
depends directly on the P^ O 2 of the gas.
As a matter of convenience, it can now be said that the
fluid has the same P^ O 2 as the gas. If it is known that the gas
has a P^ O 2 of 152 mmHg, for example, the deflection of a nee-
dle by the oxygen electrode can be calibrated on a scale at
152 mmHg ( fig. 16.21 ). The actual amount of dissolved oxygen
under these circumstances is not particularly important (it can
be looked up in solubility tables, if desired); it is simply a linear
function of the P^ O 2. A lower P^ O 2 indicates that less oxygen is
dissolved; a higher P^ O 2 indicates that more oxygen is dissolved.Partial Pressures of Gases in Blood
The enormous surface area of alveoli and the short diffusion dis-
tance between alveolar air and the capillary blood ( fig. 16.20 )
quickly help to bring oxygen and carbon dioxide in the blood
and air into equilibrium. This function is further aided by the
tremendous number of capillaries that surround each alveolus.
Actually, pulmonary capillaries may not have the tubular struc-
ture of systemic capillaries, but might instead form a sheet that
provides an even greater surface area for gas exchange with the
alveolar air.
When a liquid and a gas, such as blood and alveolar air, are
at equilibrium, the amount of gas dissolved in the fluid reaches a
maximum value. According to Henry’s law, this value depends
Table 16.5 | Effect of Altitude on Partial Oxygen Pressure ( P^ O 2 )
Altitude (Feet
Above Sea Level)*Atmospheric
Pressure (mmHg)P^ O 2 in Air
(mmHg)P^ O 2 in Alveoli
(mmHg)P^ O 2 in Arterial
Blood (mmHg)
0 760 159 105 100
2,000 707 148 97 92
4,000 656 137 90 85
6,000 609 127 84 79
8,000 564 118 79 74
10,000 523 109 74 69
20,000 349 73 40 35
30,000 226 47 21 19*For reference, Pike’s Peak (Colorado) is 14,110 feet; Mt. Whitney (California) is 14,505 feet; Mt. Logan (Canada) is 19,524 feet; Mt. McKinley (Alaska) is
20,320 feet; and Mt. Everest (Nepal and Tibet), the tallest mountain in the world, is 29,029 feet.
Figure 16.19 Partial pressures of gases in inspired
air and alveolar air at sea level. Notice that as air enters the
alveoli its oxygen content decreases and its carbon dioxide content
increases. Also notice that air in the alveoli is saturated with water
vapor (giving it a partial pressure of 47 mmHg), which dilutes the
contribution of other gases to the total pressure.
See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.
Inspired air
H 2 OTotal
pressureCO 2O 2N 2Variable000.3 mmHg159 mmHg601 mmHg760 mmHgAlveolar air40 mmHg105 mmHg568 mmHg760 mmHg47 mmHg