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MBBS QUESTION #10300
Question 1
The bicarbonate buffer system is the most important extracellular fluid buffer despite having a pKa (6.1) relatively far from physiological pH (7.4). This apparent paradox is best explained by the fact that it is:
  • A closed system with a fixed, unchangeable amount of CO2
  • An open system — CO2 can be rapidly eliminated via the lungs and bicarbonate can be regenerated/excreted by the kidneys, allowing effective buffering despite a non-ideal pKa✔️
  • A buffer whose pKa exactly matches plasma pH
  • Active only within the intracellular compartment
Correct Answer Explanation
Because the bicarbonate system is open — the CO2 (volatile acid) side can be rapidly adjusted by ventilation and the $HCO_3^-$ side by the kidney — it can effectively resist pH change over a wide range of acid loads, unlike a closed buffer whose capacity would be limited by the fixed pKa-pH mismatch.