Which ion pair is involved in maintaining the buffering capacity of blood via reversible exchange?

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

Answer: (D)

The buffering of blood relies on the bicarbonate buffer system and a reversible exchange of ions across red blood cell membranes, known as the chloride shift. When CO2 enters red blood cells, it is converted to bicarbonate (HCO3−) which then moves into the plasma. To keep electrical neutrality as bicarbonate leaves the cell, chloride (Cl−) moves into the red blood cell in exchange for bicarbonate. This exchange is reversible: in the lungs, bicarbonate re-enters the cells and chloride exits, helping convert bicarbonate back to CO2 for exhalation. This chloride-bicarbonate interaction maintains the capacity of the blood to buffer added acids, keeping pH within a narrow range.