The most important buffer pair in plasma is the:

• A. Phosphate/biphosphate
• B. Hemoglobin/imidazole
• C. Bicarbonate/carbonic acid
• D. Sulfate/bisulfate

Answer: (C)

The important idea here is which system keeps the pH of the blood plasma stable. The bicarbonate/carbonic acid pair serves as the main extracellular buffer because it directly ties to the carbon dioxide produced by metabolism and transported in the blood. Using the Henderson-Hasselbalch relationship, pH = pKa + log([HCO3-]/[CO2]), and with carbonic acid having a pKa around 6.1, the plasma maintains its pH around 7.4 by adjusting this ratio. In practice, the lungs control CO2 (and thus carbonic acid) quickly, while the kidneys regulate bicarbonate reabsorption and regeneration, providing a coordinated system that buffers most extracelluar fluid.

Phosphate/biphosphate buffers are more important inside cells and in the renal tubules, not in plasma. Hemoglobin/imidazole buffers work inside red blood cells to buffer CO2 and H+ during gas transport. Sulfate/bisulfate does not play a major buffering role in plasma.