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

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

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

Table 39.1 Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)

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 proportionto the rate of carbon dioxide excretion

b. Alveolar PCO2 decreases in inverse proportionto 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 SpaceAir and Alveolar Air

Fig. 39.6

Diffusion of Gases Through the Respiratory Membrane

• Respiratory Unit- composed of

a. A respiratory bronchioleb. Alveolar ductsc. Atriad. 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 containingsurfactant to reduce surface tension

b. Alveolar membrane (layer of thin epithelial cells)c. Epithelial basement membraned. Thin interstitial space between the alveolar and

capillary membranese. Capillary basement membrane (fuses with the

alveolar membranef. 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 betweenthe two sides of the membrane

Respiratory Membrane (cont.)

• Diffusing Capacity of the Membrane- the volumeof gas that will diffuse through the membraneeach minute for a partial pressure differenceof 1 mm Hg

a. For oxygen, the normal: 21 ml/min/mm Hgb. Increases during exercise (better ventilation-

perfusion 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