Factors Affecting the Composition of Alveolar Air

The composition of alveolar air differs markedly from that of atmospheric air. While atmospheric air is almost entirely N₂ and O₂ with negligible CO₂ and water vapor, alveolar air contains a lower O₂ percentage, a much higher CO₂ percentage, and is fully saturated with water vapor. This difference is due to the following factors:

1. Humidification of inspired air As soon as air enters the respiratory passages, it is exposed to the fluid lining the airways and becomes almost completely humidified before reaching the alveoli. Water vapor pressure at body temperature (37°C) is 47 mm Hg, and since total alveolar pressure cannot exceed atmospheric pressure, this water vapor dilutes the partial pressures of all other gases (e.g., PO₂ falls from 159 mm Hg in dry atmospheric air to about 149 mm Hg after humidification).

2. Slow, incomplete replacement of alveolar air with each breath The functional residual capacity (about 2300 mL) is large compared with the tidal volume of fresh air entering with each normal breath (about 350 mL). So only about one-seventh of the alveolar air is replaced per breath, and multiple breaths are needed to exchange most of it. This slow renewal prevents sudden changes in alveolar gas concentrations and stabilizes respiratory control, buffering the blood and tissues against abrupt swings in O₂, CO₂, and pH when breathing is briefly interrupted.

3. Continuous absorption of O₂ into pulmonary blood O₂ diffuses constantly from alveolar air into pulmonary capillary blood. Alveolar PO₂ therefore depends on the balance between (a) the rate of O₂ absorption into blood (determined by metabolic O₂ consumption) and (b) the rate of entry of fresh O₂ via alveolar ventilation. Increased ventilation raises alveolar PO₂ (up to the ceiling of 149 mm Hg for air at sea level); increased O₂ uptake by tissues lowers it.

4. Continuous diffusion of CO₂ from blood into alveoli CO₂ produced by tissue metabolism is carried to the lungs and diffuses into the alveoli, from which it is removed by ventilation. Alveolar PCO₂ rises in direct proportion to the rate of CO₂ production/excretion and falls in inverse proportion to alveolar ventilation.

5. Rate and depth of alveolar ventilation Both hyperventilation and hypoventilation alter alveolar gas tensions - increased ventilation washes out CO₂ and raises O₂, while reduced ventilation does the reverse - making ventilation rate a key regulator of alveolar composition.

6. Mixing with dead space air during expiration Expired air (not identical to pure alveolar air) is a mixture of dead-space air (having the composition of inspired/humidified air) and true alveolar air, further modifying the gas sample obtained.

Typical values (sea level): Alveolar air - PO₂ ≈ 104 mm Hg, PCO₂ ≈ 40 mm Hg, PH₂O = 47 mm Hg, PN₂ ≈ 569 mm Hg - compared with atmospheric PO₂ ≈ 159 mm Hg and PCO₂ ≈ 0.3 mm Hg.

Recent evidence check: This is core physiology unlikely to be contradicted by new studies; a quick PubMed check found no recent systematic reviews/meta-analyses that alter this classical description of alveolar gas composition.