Chapter 39: Physical Principles of Gas Exchange;
Diffusion of Oxygen and Carbon Dioxide Through
the Respiratory Membrane
Guyton and Hall, Textbook of Medical Physiology, 12 edition
Physics of Gas Diffusion and Gas Partial Pressure
• Molecular Basis of Gas Diffusion- kinetic motion
of the molecules
• Effect of a Concentration Gradient- net diffusion
from areas of high concentration to areas of lower
concentration
Fig. 39.1
Physics of Gas Diffusion and Gas Partial Pressure
• Gas Pressures in a Mixture of Gases- “partial
pressures of individual gases
a. Pressure is directly proportional to the
concentration of all the gas molecules
b. Rate of diffusion of each gas is directly
proportional to the pressure caused by that gas
alone (called the partial pressure)
Physics of Gas Diffusion and Gas Partial Pressure
• Pressure of Gases Dissolved in Water and Tissues
a. Solubility Coefficient
b. Henry’s Law
c. Diffusion of gases between the alveoli and
pulmonary blood
d. Vapor pressure of water
Physics of Gas Diffusion and Gas Partial Pressure
• Diffusion of Gases Through Fluids- pressure
difference causes net diffusion
•
Composition of Alveolar Air and Atmospheric
Air Are Different
Table 39.1 Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)
Atm.
Air
%
Hum.
Air
%
Alveo.
Air
%
Expir.
Air
%
N2
597
78.62
563.4
74.09
569
74.9
566
74.5
O2
159
20.84
149.3
19.67
104
13.6
120
15.7
CO2
0.3
0.04
0.3
0.04
40
5.3
27
3.6
H2O
3.7
0.50
47.0
6.20
47
6.2
47
6.2
TOTAL
760
100
760
100
760
100
760
100
Composition (cont.)
• Rate at Which Alveolar Air is Renewed
by Atmospheric Air
Fig. 39.2 Expiration of a gas from an alveolus with
successive breaths
Fig. 39.3 Rate of removal of excess gas from alveoli
Composition (cont.)
• Oxygen Concentration and Partial Pressure
in the Alveoli
a. Oxygen concentration in the alveoli, as well
as partial pressure, is controlled by
1. The rate of absorption of oxygen into the
blood
2. The rate of entry of new oxygen into the
lungs by ventilation
Composition (cont.)
Fig. 39.4 Effect of alveolar ventilation on the alveolar PO2 at two rates of oxygen
absorption from the aveoli
Composition (cont.)
• CO2 Concentration and Partial Pressure in
the Alveoli
a. Alveolar PCO2 increases directly in proportion
to the rate of carbon dioxide excretion
b. Alveolar PCO2 decreases in inverse proportion
to alveolar ventilation
Composition (cont.)
Fig. 39.5 Effect of alveolar ventilation on the alveolar PCO2 at two rates of
carbon dioxide excretion from the blood
Composition (cont.)
• Expired Air is a Combination of Dead Space
Air and Alveolar Air
Fig. 39.6
Diffusion of Gases Through the Respiratory Membrane
• Respiratory Unit- composed of
a.
b.
c.
d.
A respiratory bronchiole
Alveolar ducts
Atria
Alveoli
Respiratory Membrane (cont.)
Fig. 39.7 Respiratory Unit
Fig. 39.7 Surface view of capillaries
in an alveolar wall
Respiratory Membrane (cont.)
• Respiratory Membrane- has the following layers:
a. Layer of fluid lining the alveolus and containing
surfactant to reduce surface tension
b. Alveolar membrane (layer of thin epithelial cells)
c. Epithelial basement membrane
d. Thin interstitial space between the alveolar and
capillary membranes
e. Capillary basement membrane (fuses with the
alveolar membrane
f. Capillary endothelial membrane
Respiratory Membrane (cont.)
Fig. 39.9 Ultrastructure of the alveolar respiratory membrane
Respiratory Membrane (cont.)
• Factors Affecting Diffusion Rate Through
the Respiratory Membrane
a. Thickness of the membrane
b. Surface area of the membrane
c. Diffusion coefficient of gases
d. Partial pressure difference of the gas between
the two sides of the membrane
Respiratory Membrane (cont.)
• Diffusing Capacity of the Membrane- the volume
of gas that will diffuse through the membrane
each minute for a partial pressure difference
of 1 mm Hg
a. For oxygen, the normal: 21 ml/min/mm Hg
b. Increases during exercise (better ventilationperfusion ratio)
c. Not measured but approx. 20x that of oxygen
Respiratory Membrane (cont.)
Fig. 39.10 Diffusing capacities for carbon monoxide, oxygen,
and carbon dioxide in normal lungs under resting
conditions and exercise