In order to maintain electrical neutrality in the red blood cell, bicarbonate leaves the red blood cell and enters the plasma through an exchange mechanism with what electrolyte?

• A. Sodium
• B. Potassium
• C. Chloride
• D. Phosphate

Answer: (C)

Bicarbonate leaving the red blood cell is balanced by the uptake of chloride ions, maintaining electrical neutrality. This chloride-bicarbonate exchange, known as the chloride shift, uses the anion exchanger (Band 3) on the red blood cell membrane: bicarbonate moves out of the cell while chloride moves in. Carbon dioxide is converted to bicarbonate inside the cell by carbonic anhydrase, and as bicarbonate exits, the influx of chloride keeps the charge balance across the membrane. The process reverses in the lungs, with bicarbonate re-entering in exchange for chloride to be expelled as CO2. So the electrolyte involved in this exchange is chloride.