Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 35Pulmonary Function 601

PAO 2 can also be calculated from the alveolar gas equation:

PAO 2 = PIO 2 – PACO 2

FIO 2 +

1– FIO 2

R

where FIO 2 is the fraction of O 2 molecules in the dry gas, PIO 2
is the inspired PO 2 , and R is the respiratory exchange ratio;
that is, the flow of CO 2 molecules across the alveolar mem-
brane per minute divided by the flow of O 2 molecules across
the membrane per minute.

COMPOSITION OF ALVEOLAR AIR

Oxygen continuously diffuses out of the gas in the alveoli into
the bloodstream, and CO 2 continuously diffuses into the alve-
oli from the blood. In the steady state, inspired air mixes with
the alveolar gas, replacing the O 2 that has entered the blood
and diluting the CO 2 that has entered the alveoli. Part of this
mixture is expired. The O 2 content of the alveolar gas then
falls and its CO 2 content rises until the next inspiration. Be-
cause the volume of gas in the alveoli is about 2 L at the end of
expiration (functional residual capacity), each 350 mL incre-
ment of inspired and expired air has relatively little effect on
PO 2 and PCO 2. Indeed, the composition of alveolar gas re-
mains remarkably constant, not only at rest but also under a
variety of other conditions.

DIFFUSION ACROSS THE

ALVEOLOCAPILLARY MEMBRANE

Gases diffuse from the alveoli to the blood in the pulmonary

capillaries or vice versa across the thin alveolocapillary

membrane made up of the pulmonary epithelium, the capil-

lary endothelium, and their fused basement membranes

(Figure 35–3). Whether or not substances passing from the al-

veoli to the capillary blood reach equilibrium in the 0.75 s that

blood takes to traverse the pulmonary capillaries at rest de-

pends on their reaction with substances in the blood. Thus, for

example, the anesthetic gas nitrous oxide (N 2 O) does not react

and reaches equilibrium in about 0.1 s (Figure 35–19). In this

situation, the amount of N 2 O taken up is not limited by diffu-

sion but by the amount of blood flowing through the pulmo-

nary capillaries; that is, it is flow-limited. On the other hand,

carbon monoxide (CO) is taken up by hemoglobin in the red

blood cells at such a high rate that the partial pressure of CO

in the capillaries stays very low and equilibrium is not reached

in the 0.75 s the blood is in the pulmonary capillaries. There-

fore, the transfer of CO is not limited by perfusion at rest and

instead is diffusion-limited. O 2 is intermediate between N 2 O

and CO; it is taken up by hemoglobin, but much less avidly

than CO, and it reaches equilibrium with capillary blood in

about 0.3 s. Thus, its uptake is perfusion-limited.

The diffusing capacity of the lung for a given gas is directly

proportionate to the surface area of the alveolocapillary mem-

brane and inversely proportionate to its thickness. The diffus-

ing capacity for CO (DLCO) is measured as an index of

diffusing capacity because its uptake is diffusion-limited.

DLCO is proportionate to the amount of CO entering the

blood (VCO) divided by the partial pressure of CO in the alve-

oli minus the partial pressure of CO in the blood entering the

pulmonary capillaries. Except in habitual cigarette smokers,

FIGURE 35–18 Partial pressures of gases (mm Hg) in various
parts of the respiratory system and in the circulatory system.

O 2

CO 2

H 2 O

N 2

158.0

0.3

5.7

596.0

O 2

CO 2

H 2 O

N 2

116.0

32.0

47.0

565.0

O 2

CO 2

H 2 O

N 2

100.0

40.0

47.0

573.0

Inspired air Expired gas

O 2

CO 2

H 2 O

N 2

95.0

40.0

47.0

573.0

O 2

CO 2

H 2 O

N 2

40.0−

46.0+

47.0

573.0

O 2

CO 2

H 2 O

N 2

40.0

46.0

47.0

573.0

Tissues

Capillaries

Dead space Physiologic

shunt

Left heart

Veins Arteries

Right heart

Alveoli

FIGURE 35–19 Uptake of various substances during the

0.75 s they are in transit through a pulmonary capillary. N 2 O is

not bound in blood, so its partial pressure in blood rises rapidly to its

partial pressure in the alveoli. Conversely, CO is avidly taken up by red

blood cells, so its partial pressure reaches only a fraction of its partial

pressure in the alveoli. O 2 is intermediate between the two.

0 0.25 0.50 0.75

CO

Alveolar

level

Time in capillary (s)

O 2

N 2 O

Partial pressure

(

(