In a standing individual, alveolar PO2 is highest and ventilation-perfusion ratio (V/Q) is greatest in which lung zone?
- A Zone 2 (mid-lung)
- B Zone 3 (base)
- C Zone 1 (apex) ✓
- D V/Q ratio is uniform throughout the lung
Correct answer: C. Zone 1 (apex)
Explanation
In the upright posture, gravity causes both ventilation and perfusion to be greater at the lung bases, but the increase in perfusion is proportionally much larger. Therefore the V/Q ratio is highest at the apex (Zone 1), where perfusion is least. High V/Q at the apex means less CO2 removal relative to ventilation and higher alveolar PO2 (~130 mmHg at apex vs ~89 mmHg at base). This regional heterogeneity explains why reactivation tuberculosis typically occurs at the apex.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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