At high altitude (4500 m), a climber's PaO2 is 45 mmHg. Comparing his Hb-O2 dissociation curve to a sea-level resident, the climber's curve is shifted:

• A Left, due to alkalosis from hyperventilation reducing 2,3-DPG effect
• B Right, due to increased 2,3-DPG from hypoxic erythropoiesis
• C Unchanged; altitude does not alter the ODC
• D Biphasically: left initially, then right after 2 weeks ✓

Explanation

At high altitude, acclimatisation involves two competing shifts: (1) Acute hyperventilation causes respiratory alkalosis, left-shifting the ODC (increases O2 affinity, better loading at low PO2 but impaired unloading). (2) Within 2–3 days, hypoxia stimulates red cell 2,3-DPG synthesis, which right-shifts the ODC (improving O2 unloading to tissues). The net result after several weeks is a right-shifted curve. The initial transient left-shift helps during the first hours of acute altitude exposure, explaining why option D (biphasic) is most physiologically accurate.

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

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