The Henderson-Hasselbalch equation for the bicarbonate buffer system is pH = 6.1 + log([HCO3−] / 0.03 × PaCO2). At normal values (HCO3− = 24 mEq/L, PaCO2 = 40 mmHg), the ratio [HCO3−]/[H2CO3] is approximately:
- A 10:1
- B 50:1
- C 20:1 ✓
- D 1:20
Correct answer: C. 20:1
Explanation
Using Henderson-Hasselbalch: [H2CO3] = 0.03 × PaCO2 = 0.03 × 40 = 1.2 mEq/L (dissolved CO2 equivalent). Ratio = HCO3−/H2CO3 = 24/1.2 = 20. Therefore pH = 6.1 + log(20) = 6.1 + 1.3 = 7.4. This 20:1 ratio maintains physiological pH 7.4, and any deviation (e.g., in acidosis or alkalosis) reflects a change in this ratio. Understanding this ratio is essential for calculating compensatory responses in acid-base disturbances.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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