alveolar air
Summary
Alveolar air refers to the gas mixture present in the alveoli of the lungs, which has a different composition than atmospheric air due to gas exchange with pulmonary capillary blood. It contains less oxygen (~14%) and more carbon dioxide (~5.5%) compared to inspired air.
Detail
Alveolar air represents the final gas mixture that participates in gas exchange at the respiratory membrane. Unlike atmospheric air (21% O2, 0.04% CO2), alveolar air contains approximately 14% oxygen, 5.5% carbon dioxide, 80% nitrogen, and is saturated with water vapor at body temperature. This composition results from continuous gas exchange with mixed venous blood in pulmonary capillaries, where oxygen diffuses from alveoli to blood while carbon dioxide moves from blood to alveoli. The alveolar air equation (PAO2 = FiO2(Patm - PH2O) - PaCO2/RQ) calculates ideal alveolar oxygen partial pressure. Alveolar ventilation, not total ventilation, determines alveolar gas composition because only fresh air reaching alveoli (not anatomical dead space) participates in gas exchange. Normal alveolar partial pressures are PAO2 ~100 mmHg and PACO2 ~40 mmHg at sea level.
Sources
- Guyton and Hall Textbook of Medical Physiology
- West's Respiratory Physiology
- First Aid for the USMLE Step 1
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