The Haldane effect describes an important physiological property of hemoglobin. Which statement BEST describes the Haldane effect and its physiological significance?

• A Deoxygenated hemoglobin binds CO2 more avidly as carbamino-Hb and can carry more CO2, facilitating CO2 transport from tissues to lungs ✓
• B Increased CO2 in tissues reduces hemoglobin's oxygen affinity, facilitating oxygen release — this is the Haldane effect
• C Oxyhemoglobin is a stronger acid than deoxyhemoglobin, causing H+ release at the lungs which drives CO2 off hemoglobin as bicarbonate is reformed
• D Deoxygenated hemoglobin in tissues absorbs H+ ions as a buffer, shifting the bicarbonate equilibrium to produce more CO2 for exhalation

Explanation

The Haldane effect states that deoxygenated hemoglobin (deoxy-Hb) has greater affinity for CO2 (forming carbamino-Hb) than oxyhemoglobin, and is also a better H+ buffer (pKa shifts). At tissue capillaries where O2 is released, Hb becomes deoxygenated and can carry ~50% more CO2 than oxyHb at the same PCO2. At the lungs, O2 binding converts Hb to oxyHb, which releases CO2 (carbamino dissociation) and H+ (Bohr effect), driving CO2 exhalation. Option B describes the Bohr effect (not Haldane). Option C is partially true but describes Bohr-related H+ changes. Option D conflates H+ buffering with CO2 production.

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

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