What is dead space and how does it affect gas exchange?

• A. Dead space is air in the alveoli; it increases gas exchange efficiency
• B. Dead space is air not involved in gas exchange; large dead space reduces effective ventilation
• C. Dead space is air breathed exclusively through the nose
• D. Dead space is air in the atmosphere; it does not affect ventilation

Answer: (B)

Dead space is the portion of each breath that does not participate in gas exchange. It includes the air in the conducting airways (and any alveoli that are ventilated but not perfused). Because this air never reaches alveoli where O2 and CO2 exchange with blood occurs, the effective ventilation is reduced. In other words, for a given tidal volume, only the air minus the dead space actually ventilates functional alveoli. If dead space is large, alveolar ventilation drops, making gas exchange less efficient and CO2 removal harder unless ventilation is increased. The key idea is that gas exchange depends on air reaching alveoli; air in dead space doesn’t contribute to that process.