The Henderson-Hasselbalch equation underlies blood pH regulation. At physiological pH of 7.40, what is the ratio of bicarbonate to dissolved CO2 (HCO3-:dCO2) assuming pKa of carbonic acid = 6.1?

• A 10:1
• B 1:20
• C 1:10
• D 20:1 ✓

Explanation

Using Henderson-Hasselbalch: pH = pKa + log([HCO3-]/[dCO2]). Substituting: 7.40 = 6.10 + log(ratio), therefore log(ratio) = 1.30, so ratio = antilog(1.30) = 20. The normal bicarbonate:dissolved CO2 ratio is 20:1 (typically 24 mEq/L HCO3- and 1.2 mEq/L dissolved CO2 from PaCO2 40 mmHg × 0.03). This ratio must be maintained at 20:1 to maintain pH 7.40. Acid-base disturbances and compensations all aim to restore this 20:1 ratio. This foundational relationship explains why changes in either HCO3- (kidneys) or CO2 (lungs) alter pH.

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

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