Within hours of ascent to an altitude of 4000 m, a climber develops hyperventilation with a fall in PaCO2. Over the following days, renal bicarbonate excretion restores CSF pH toward normal, allowing the ventilatory drive to increase further. Which additional hematological adaptation contributes to improved tissue oxygen delivery during acclimatization?
- A Decreased red cell 2,3-BPG, shifting the curve leftward to improve loading
- B Increased red cell 2,3-BPG, shifting the oxyhemoglobin curve rightward and easing unloading ✓
- C Reduced erythropoietin secretion, lowering viscosity
- D Increased carboxyhemoglobin fraction, stabilizing the curve
Correct answer: B. Increased red cell 2,3-BPG, shifting the oxyhemoglobin curve rightward and easing unloading
Explanation
Chronic hypoxia raises red cell 2,3-bisphosphoglycerate, which binds deoxygenated hemoglobin and shifts the dissociation curve rightward, facilitating oxygen release in tissues. Combined with erythropoietin-driven polycythemia and sustained hyperventilatory alkalosis compensation, this improves delivery. B leftward shift would impair unloading, and erythropoietin rises rather than falls at altitude. Carboxyhemoglobin plays no adaptive role here.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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