1. Unit Seven: Respiration
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

2. 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

3. Physics of Gas Diffusion and Gas Partial Pressure
Gas Pressures in a Mixture of Gases- “partial pressures of individual gases
Pressure is directly proportional to the concentration of all the gas molecules
Rate of diffusion of each gas is directly proportional to the pressure caused by that gas alone (called the partial pressure)

4. Physics of Gas Diffusion and Gas Partial Pressure
Pressure of Gases Dissolved in Water and Tissues
Solubility Coefficient
Henry’s Law
Diffusion of gases between the alveoli and pulmonary blood
Vapor pressure of water

5. 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

6. Table 39.1 Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)
Atm.
Air %
Hum.
Alveo.
Expir. 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 40
5.3 27
3.6
H2O
3.7
0.50
47.0
6.20 47
6.2
TOTAL
760
100

7. Rate at Which Alveolar Air is Renewed by Atmospheric Air
Composition (cont.)
Rate at Which Alveolar Air is Renewed by Atmospheric Air
Fig Expiration of a gas from an alveolus with
successive breaths
Fig Rate of removal of excess gas from alveoli

8. Composition (cont.)
Oxygen Concentration and Partial Pressure
in the Alveoli
Oxygen concentration in the alveoli, as well as partial pressure, is controlled by
The rate of absorption of oxygen into the blood
The rate of entry of new oxygen into the lungs by ventilation

9. Composition (cont.)
Fig Effect of alveolar ventilation on the alveolar PO2 at two rates of oxygen
absorption from the aveoli

10. Composition (cont.)
CO2 Concentration and Partial Pressure in
the Alveoli
Alveolar PCO2 increases directly in proportion
to the rate of carbon dioxide excretion
Alveolar PCO2 decreases in inverse proportion
to alveolar ventilation

11. Composition (cont.)
Fig Effect of alveolar ventilation on the alveolar PCO2 at two rates of
carbon dioxide excretion from the blood

12. Expired Air is a Combination of Dead Space Air and Alveolar Air
Composition (cont.)
Expired Air is a Combination of Dead Space
Air and Alveolar Air
Fig. 39.6

13. Diffusion of Gases Through the Respiratory Membrane
Respiratory Unit- composed of
A respiratory bronchiole
Alveolar ducts
Atria
Alveoli

14. Respiratory Membrane (cont.)
Fig Respiratory Unit Fig Surface view of capillaries
in an alveolar wall

15. Respiratory Membrane (cont.)
Respiratory Membrane- has the following layers:
Layer of fluid lining the alveolus and containing
surfactant to reduce surface tension
Alveolar membrane (layer of thin epithelial cells)
Epithelial basement membrane
Thin interstitial space between the alveolar and
capillary membranes
Capillary basement membrane (fuses with the
alveolar membrane
f. Capillary endothelial membrane

16. Respiratory Membrane (cont.)
Fig Ultrastructure of the alveolar respiratory membrane

17. Respiratory Membrane (cont.)
Factors Affecting Diffusion Rate Through
the Respiratory Membrane
Thickness of the membrane
Surface area of the membrane
Diffusion coefficient of gases
Partial pressure difference of the gas between
the two sides of the membrane

18. 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
For oxygen, the normal: 21 ml/min/mm Hg
Increases during exercise (better ventilation-
perfusion ratio)
c. Not measured but approx. 20x that of oxygen

19. Respiratory Membrane (cont.)
Fig Diffusing capacities for carbon monoxide, oxygen,
and carbon dioxide in normal lungs under resting
conditions and exercise