Which term describes the volume of air participates in gas exchange?

• A. Alveolar ventilation
• B. Inspiratory capacity
• C. Minute ventilation
• D. Functional residual capacity

Answer: (A)

Air that actually participates in gas exchange is alveolar ventilation. It measures the amount of fresh air that reaches the alveoli each minute after the portion filling dead space is excluded. This is why breathing air that fills dead space doesn’t contribute to gas exchange, even though it moves in and out of the lungs.

Alveolar ventilation is calculated as (tidal volume minus dead space) multiplied by the respiratory rate. For example, with a tidal volume of about 500 mL and a dead space of about 150 mL, at 12 breaths per minute, alveolar ventilation is roughly (350 mL) × 12 ≈ 4,200 mL per minute.

This concept differs from:

• Minute ventilation, which is the total air moved in and out per minute (tidal volume times rate) and includes air that does not reach the alveoli.

• Inspiratory capacity, which is how much you can inhale after a normal exhale.

• Functional residual capacity, which is the volume left in the lungs after a normal exhale.