The Henderson-Hasselbalch equation for the bicarbonate buffer system is pH = pKa + log [HCO3-]/[H2CO3]. The pKa of this system is approximately 6.1. At normal blood pH of 7.4, the HCO3-:H2CO3 ratio is approximately:

• A 10:1
• B 100:1
• C 1:1
• D 20:1 ✓

Explanation

pH = pKa + log [HCO3-]/[H2CO3]; 7.4 = 6.1 + log(ratio); log(ratio) = 1.3; ratio = 10^1.3 ≈ 20:1. At pH 7.4, plasma [HCO3-] ≈ 24 mEq/L and dissolved CO2 + H2CO3 = 1.2 mEq/L, giving a ratio of 20:1. This far-from-pKa ratio makes the bicarbonate system physiologically effective because any acid is rapidly buffered and the system can be regulated by both lungs (CO2) and kidneys (HCO3-).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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