The buffering capacity of blood is maintained by a reversible exchange between bicarbonate and which anion?

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

Answer: (D)

Blood buffering relies on the bicarbonate system, and this is balanced by a reversible exchange of bicarbonate with chloride across red blood cell membranes—the chloride shift. As carbon dioxide from tissues enters red blood cells, carbonic anhydrase converts it to carbonic acid, which rapidly dissociates to bicarbonate and hydrogen ions. The bicarbonate then moves out into the plasma to help carry CO2 back to the lungs, and chloride moves in to maintain electrical neutrality. When blood reaches the lungs, the process reverses: bicarbonate reenters the cells in exchange for chloride, CO2 is released, and both buffering and CO2 transport are restored. Sodium, potassium, and calcium don’t participate in this major buffering exchange; chloride is the key partner.