Diffusion is a net movement of gas molecules from an area of high partial pressure to low partial pressure. Pulmonary diffusion largely refers to the passive movement of O2 and CO2 along their pressure gradients in the lungs. At the level of the alveolus, where pulmonary diffusion occurs, inhaled anesthetic agents move according to partial pressure differences.
Lung respiratory zones include bronchioles leading to alveolar ducts, then sacs, and finally alveoli. The alveolus is composed of three primary cell types: Type 1 cells, Type 2 cells, and alveolar macrophages. Type 1 pneumocytes cover 95% of the alveolar septal surface and join one another by tight junctions. They are approximately 0.3-0.4 μm thick and allow for gas exchange between the alveolus and the pulmonary capillaries. Type 2 pneumocytes contain characteristic lamellar inclusions for surfactant production. Surfactant is responsible for decreasing pulmonary surface tension. These cells are also mitotically active and can differentiate into Type 1 cells. Lastly, Type 2 cells secrete a variety of substances in defense of the structure, including fibronectin and alpha1-antitrypsin. The third type of cells is macrophages that perform cleansing and defense functions.
The atmosphere is composed of permanent gases whose percentage remains relatively constant, and variable gases which change in concentration over time. Table 138-1 indicates typical values for atmospheric gases.
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TABLE 138-1 Atmospheric Gases
|Type ||Gas ||Percentage |
|Permanent ||Nitrogen ||78.1 |
| ||Oxygen ||20.9 |
| ||Argon ||0.9 |
| ||Neon ||0.002 |
| ||Helium ||0.0005 |
| ||Krypton ||0.0001 |
| ||Hydrogen ||0.00005 |
|Variable ||Water vapor ||0-4 |
| ||Carbon dioxide ||0.035 |
| ||Methane ||0.0002 |
| ||Ozone ||0.000004 |
Alveolar air differs in composition of gases from the atmosphere. Alveolar air is only partially replaced with atmospheric air with each breath (Table 138-2). Oxygen is constantly being absorbed into the pulmonary bloodstream from alveolar air, while CO2 moves down its concentration gradient from bloodstream to alveolus.
Another important factor contributing to the difference between alveolar and atmospheric air is humidification. Atmospheric air becomes 100% humidified by the time it reaches the alveolus. The partial pressure of water vapor at the normal body temperature of 37°C is 47 mm Hg, the partial pressure of water vapor in alveolar air. Total pressure in the alveoli can never be greater than atmospheric pressure. Therefore, alveolar water vapor dilutes all inspired gases, and partial pressure of O2 and CO2 in the alveolus is about 149 and 0.3 mm Hg, respectively.
Respiratory Gas Composition
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TABLE 138-2 Respiratory Gas Composition
| ||Atmospheric air (mm Hg) ||Alveolar humidified air prior to gas exchange (mm Hg) ||Alveolar air after gas exchange (mm Hg) ||Expired air (mm Hg) |
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