Bicarbonate leaving RBCs to plasma is exchanged with which anion to maintain electroneutrality?

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

Answer: (C)

This item hinges on the chloride shift, the process by which bicarbonate is exchanged for chloride across red blood cell membranes to keep electrical balance during CO2 transport. In tissues, carbon dioxide diffuses into red blood cells and carbonic anhydrase converts it to carbonic acid, which dissociates to bicarbonate and a proton. The bicarbonate is then transported out of the cell into plasma, and to maintain electroneutrality, a chloride ion moves into the red blood cell in exchange. This exchange is mediated by the anion exchanger protein (band 3). Because bicarbonate carries a negative charge, its exit would disrupt charge balance unless a negative ion accompanies the process; chloride moving in restores that balance. In the lungs, the opposite occurs, helping to release CO2. The chloride ion is the correct partner for this exchange, whereas the other ions listed do not participate in this specific transport mechanism.