Question

Why is alveolar air not of the same composition as that of atmospheric air?

• A. Moisture dilutes ${\mathrm{pO}}_2$ of alveolar air
• B. ${\mathrm{pO}}_2$ in alveolar air is less than atmospheric air
• C. Moisture increases ${\mathrm{pO}}_2$ of alveolar air
• D. ${\mathrm{pO}}_2$ in atmospheric air is less than atmospheric air

Answer 1

Answer: (A), (B)

The correct options are
A Moisture dilutes ${\mathrm{pO}}_2$ of alveolar air
B ${\mathrm{pO}}_2$ in alveolar air is less than atmospheric air

Alveolar air not of the same composition at the inspired atmospheric air for 2 reasons:

1. There is a lot of moisture and water vapour that also exerts partial pressure. Humidification of inspired air in effect “dilutes” the partial pressure of the inspired gases.
2. Aveolar ${\mathrm{pO}}_2$ is lower than atmospheric ${\mathrm{pO}}_2$because fresh inspired air is mixed with large volume of old air that remained in the lungs as well as the dead space at the end of the preceeding expiration.