The Haldane effect refers to the influence of oxygenation on CO₂ carrying capacity of blood. In the pulmonary capillary, oxygenation of hemoglobin increases CO₂ release via which specific molecular mechanism?
- A Oxy-Hb is a stronger acid (lower pKa) than deoxy-Hb, releasing H⁺ that shifts the carbonate equilibrium toward CO₂ production
- B Both: oxy-Hb releases H⁺ (Bohr effect in reverse) that drives HCO₃⁻ + H⁺ → CO₂ + H₂O, AND oxy-Hb releases carbamino-CO₂ ✓
- C Oxy-Hb has reduced affinity for carbamino carriage, releasing CO₂ from carbamino-Hb compounds
- D Oxygenation activates carbonic anhydrase in red blood cells, speeding dehydration of H₂CO₃ to CO₂
Explanation
The Haldane effect encompasses two complementary mechanisms. First, oxygenated hemoglobin is a stronger acid (higher ionization of –NH and –COOH groups) and releases H⁺ ions, which combine with HCO₃⁻ to form H₂CO₃ → CO₂ + H₂O (carbonate buffer shift driven by H⁺ release). Second, carbamino compounds (CO₂ bound to –NH₂ groups of Hb) dissociate more readily from oxy-Hb than deoxy-Hb, releasing additional CO₂. Together, these account for ~50% of CO₂ unloading in the lungs; the Bohr and Haldane effects are interlinked expressions of the same underlying allosteric changes in hemoglobin.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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