The Henderson-Hasselbalch equation for the bicarbonate buffer system is pH = pKa + log([HCO3⁻]/[H2CO3]). In blood at physiologic pH 7.4, the ratio of [HCO3⁻] to [H2CO3] is approximately:
- A 20:1 ✓
- B 10:1
- C 1:20
- D 1:1
Correct answer: A. 20:1
Explanation
The pKa of carbonic acid (H2CO3) is 6.1. At pH 7.4: 7.4 = 6.1 + log([HCO3⁻]/[H2CO3]), so log([HCO3⁻]/[H2CO3]) = 1.3, meaning [HCO3⁻]/[H2CO3] = 10^1.3 ≈ 20:1. Physiologically, [HCO3⁻] ≈ 24 mEq/L and dissolved CO2 (≈ H2CO3) ≈ 1.2 mEq/L (0.03 × PaCO2 40 mmHg), giving ratio of 24/1.2 = 20. Despite this system operating far from its pKa (making it a relatively poor buffer in vitro), it is the most effective physiological buffer because both components are independently regulated — CO2 by lungs and HCO3⁻ by kidneys.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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