A 75-year-old woman lies supine for an abdominal surgery. Her closing capacity is measured to be 1.8 L, while her functional residual capacity is 1.6 L. What is the primary physiological consequence of this relationship?

• A Uniform ventilation-perfusion matching throughout the lung
• B Decreased pulmonary vascular resistance due to alveolar hypoxia
• C Increased diffusing capacity for carbon monoxide
• D Airway closure during tidal breathing and hypoxemia ✓

Explanation

When closing capacity exceeds functional residual capacity, small airways close during tidal breathing rather than only at residual volume. This occurs in elderly individuals, obesity, and the supine position. The resulting ventilation-perfusion mismatch and right-to-left shunting cause hypoxemia. Therefore, the primary consequence is hypoxemia from airway closure. The other options do not describe the physiological effect of airway closure during normal breathing.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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