As venous blood passes through systemic capillaries, bicarbonate formed inside erythrocytes exits into plasma in exchange for chloride entry. This exchange is known as:

• A Donnan equilibrium across the red cell membrane
• B Sodium-potassium cotransport driven by carbonic anhydrase
• C The Bohr effect operating on haemoglobin
• D The chloride shift or Hamburger phenomenon ✓

Explanation

Carbonic anhydrase inside the red cell converts CO2 to H2CO3, which dissociates to HCO3- and H+. Bicarbonate leaves the erythrocyte down its concentration gradient through the AE1 (band 3) exchanger, and chloride enters in exchange, maintaining electroneutrality. This is the chloride shift or Hamburger phenomenon. The Bohr effect describes altered oxygen affinity with changing pH, and Donnan equilibrium refers to unequal distribution of diffusible ions across a membrane with fixed charges.

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

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