The Haldane effect states that deoxygenation of blood increases its capacity to transport carbon dioxide. This occurs because deoxyhemoglobin:

• A Increases the solubility coefficient of CO2 in plasma
• B Releases bicarbonate into plasma through band 3 channels
• C Forms carbamino compounds and buffers hydrogen ions more readily than oxyhemoglobin ✓
• D Activates carbonic anhydrase inside the erythrocyte

Explanation

Deoxyhemoglobin is a weaker acid and a better proton acceptor than oxyhemoglobin, so it buffers the hydrogen ions liberated when CO2 is hydrated, driving more CO2 carriage as bicarbonate. Its terminal amino groups also form carbamino compounds with CO2 more readily. Both effects promote CO2 uptake in systemic capillaries and CO2 release in the lungs when hemoglobin takes up oxygen. Band 3 mediates exchange of already formed ions and does not create capacity, while carbonic anhydrase activity is unchanged.

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

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