A trekker ascends rapidly to 4500 m. Within days he hyperventilates, his arterial pH normalizes, and his red cell 2,3-bisphosphoglycerate concentration rises. Which pair correctly matches the compensatory mechanism to its timescale?

• A Increased erythropoietin corrects alkalosis over days; 2,3-BPG left-shifts the curve
• B Renal bicarbonate retention raises pH immediately; 2,3-BPG falls over weeks
• C Carotid body resetting alone restores pH within minutes; 2,3-BPG is unchanged
• D Renal bicarbonate excretion corrects the respiratory alkalosis over days; raised 2,3-BPG right-shifts the oxyhemoglobin curve over hours to days ✓

Explanation

Hypoxia drives hyperventilation via carotid bodies, lowering PaCO2 and causing respiratory alkalosis. Over days the kidney excretes bicarbonate and restores pH, which also permits further hyperventilation. Concurrently, rising 2,3-bisphosphoglycerate in red cells right-shifts the dissociation curve, easing tissue oxygen unloading. Erythropoietin raises hematocrit but takes weeks and does nothing for acid-base status.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.