Which statement best defines final alveolar ventilation per breath?

• A. Alveolar ventilation per breath equals tidal volume
• B. Alveolar ventilation per breath equals tidal volume minus dead space
• C. Alveolar ventilation per breath equals dead space
• D. Alveolar ventilation per breath equals inspiratory flow rate

Answer: (B)

Alveolar ventilation per breath is the portion of each breath that actually reaches the gas-exchanging units in the lungs. When you inhale, part of the tidal volume fills the anatomical dead space—the air in the conducting airways that doesn’t participate in gas exchange. The remaining air moves into the alveoli, where gas exchange occurs. So, per breath, alveolar ventilation equals the tidal volume minus the dead space volume. In practice, multiplying by the respiratory rate gives alveolar ventilation per minute, (tidal volume minus dead space) times rate. The other options don’t reflect the air that actually reaches the alveoli: tidal volume alone includes wasted air, dead space alone is air that doesn’t reach the alveoli, and inspiratory flow rate describes how fast air is drawn in rather than how much reaches the alveoli per breath.