At high altitude (4500 m), acclimatized mountaineers show a rightward-then-leftward shift of the oxyhemoglobin dissociation curve during the acclimatization process. Which sequential physiological changes explain this temporal pattern?

• A Initial rightward shift: increased 2,3-DPG from hypoxia-driven glycolysis; later leftward shift: respiratory alkalosis from hyperventilation overcomes the 2,3-DPG effect (Bohr effect dominant at high pH) ✓
• B Initial leftward shift: hyperventilation-induced respiratory alkalosis; later rightward shift: increased erythropoietin raising RBC 2,3-DPG
• C Initial rightward shift: increased temperature at altitude due to solar radiation; later leftward shift: polycythemia diluting 2,3-DPG
• D Initial rightward shift: sympathetic activation releasing epinephrine; later leftward shift: increased HbF production at high altitude

Explanation

Within hours of high-altitude exposure, hypoxia stimulates increased 2,3-DPG synthesis (via depletion of DPGA phosphatase activity) and hyperventilation-induced respiratory alkalosis occurs. The alkalosis (leftward shift, Bohr) and 2,3-DPG increase (rightward shift) compete. Initially, 2,3-DPG rises but is not yet overwhelming, and net rightward shift improves O2 unloading in tissues. Over days, erythropoiesis increases and 2,3-DPG remains elevated. The alkalosis may partially compensate, but the dominant acclimatized state is a net slightly rightward curve — ensuring adequate oxygen delivery to tissues despite lower alveolar PO2. The exact temporal balance gives a biphasic appearance in some studies. Option B reverses the sequence; options C and D are physiologically incorrect.

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

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