If tidal volume equals the dead space volume, what happens to alveolar ventilation?

• A. It becomes equal to respiratory rate
• B. It becomes zero
• C. It becomes equal to total minute ventilation
• D. It becomes infinite

Answer: (B)

Alveolar ventilation is the portion of each breath that actually reaches the gas-exchange surfaces. It depends on how much of the tidal volume bypasses dead space: alveolar ventilation per minute equals (tidal volume minus dead space) multiplied by the respiratory rate. Dead space is the air in the conducting airways that does not participate in gas exchange. If tidal volume equals dead space, nothing of the inhaled air reaches the alveoli, so alveolar ventilation is zero regardless of how fast you breathe. This is why tidal volumes must exceed dead space to achieve effective ventilation. For context, in an average adult, dead space is about 150 mL, so a typical tidal volume around 500 mL leaves roughly 350 mL per breath for alveolar ventilation.