A sea-level resident ascends to 4500 m. Within the first week, which change in the oxyhemoglobin dissociation curve is observed?

• A Leftward shift due to respiratory alkalosis, increasing oxygen loading in the lungs
• B Biphasic shift: initial rightward shift followed immediately by a leftward shift
• C No shift occurs; the curve remains identical to the sea-level baseline
• D Rightward shift due to increased 2,3-BPG, facilitating oxygen unloading in tissues ✓

Explanation

Within days of altitude exposure, erythrocyte 2,3-BPG increases, causing a rightward shift of the oxyhemoglobin dissociation curve. This reduces hemoglobin's oxygen affinity, facilitating oxygen unloading at tissues despite lower arterial PO2. Respiratory alkalosis alone causes a leftward shift, but the 2,3-BPG effect dominates within the first week, producing net rightward shift.

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

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