The bicarbonate buffer system is the principal extracellular buffer. The Henderson-Hasselbalch equation for this system is pH = pKa + log([HCO3−]/[H2CO3]). The normal ratio of [HCO3−]/[H2CO3] maintained at pH 7.4 is approximately:
- A 20:1 ✓
- B 1:20
- C 10:1
- D 40:1
Correct answer: A. 20:1
Explanation
At physiological pH 7.4 with pKa of carbonic acid/bicarbonate system = 6.1: pH = pKa + log([HCO3−]/[H2CO3]); 7.4 = 6.1 + log(ratio); log(ratio) = 1.3; ratio = 10^1.3 ≈ 20. Thus [HCO3−]/[H2CO3] = 20:1. Normal values: [HCO3−] = 24 mEq/L, [H2CO3] = 1.2 mEq/L (derived from PaCO2 40 mmHg × 0.03 solubility coefficient). Despite the bicarbonate system's pKa being far from physiological pH, its effectiveness as a physiological buffer is enhanced by the ability to control both components independently — HCO3− by kidneys and CO2 by lungs.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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