At the apex of an upright lung, ventilation-perfusion (V/Q) ratio is approximately 3.3, while at the base it is 0.6. Which statement correctly predicts the gas exchange consequences of this gradient?
- A Apical alveoli have high PO₂ and low PCO₂; basal alveoli have lower PO₂ and higher PCO₂ ✓
- B Apical alveoli are poorly ventilated and act as shunt units
- C Basal alveoli have higher PO₂ because blood flow there is greater
- D V/Q mismatch does not affect arterial PO₂ because hemoglobin is already saturated
Correct answer: A. Apical alveoli have high PO₂ and low PCO₂; basal alveoli have lower PO₂ and higher PCO₂
Explanation
High V/Q at the apex means relative over-ventilation relative to perfusion, driving alveolar PO₂ up (~130 mmHg) and PCO₂ down (~28 mmHg). The base has relative over-perfusion (low V/Q), so alveolar PO₂ is lower (~89 mmHg) and PCO₂ higher (~42 mmHg). Because blood flow is much greater at the base, the poorly oxygenated basal blood dominates the mixed arterial sample, creating the normal physiologic A-a gradient.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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