Pulmonary gas pressures

Following is a list of average partial pressures (in torr) for a human at rest:

Factors affecting alveolar gas pressures

The alveolar oxygen pressure is lower than the atmospheric O₂ partial pressure for two reasons.

• Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.
• The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO₂ out of the capillaries into the alveoli.

The factors that determine the values for alveolar PO₂ and PCO₂ are:

• The pressure of outside air
• The partial pressures of inspired oxygen and carbon dioxide
• The rates of total body oxygen consumption and carbon dioxide production
• The rates of alveolar ventilation and perfusion

The alveolar pO₂ is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation.

Pathology

Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values.

Hyperventilation exists when the same ratio decreases.

See also

• Pulmonary alveolus
• Alveolar-arterial gradient

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