The Henderson-Hasselbalch equation for the bicarbonate buffer system is pH = 6.1 + log [HCO3-]/[H2CO3]. In clinical practice, carbonic acid concentration [H2CO3] is replaced by 0.03 × PaCO2. The factor 0.03 represents:

• A The fraction of CO2 that undergoes conversion to carbonic acid by carbonic anhydrase
• B The dissociation constant of carbonic acid at physiological ionic strength
• C The Donnan equilibrium ratio of bicarbonate across the erythrocyte membrane
• D The solubility coefficient (Henry's constant) of CO2 in plasma at 37°C (0.0301 mmol/L/mmHg) ✓

Explanation

In the Henderson-Hasselbalch equation, the denominator [H2CO3] is replaced by [dissolved CO2], which is in rapid equilibrium with H2CO3. The concentration of dissolved CO2 in plasma = α × PaCO2, where α is Henry's solubility constant for CO2 in plasma at 37°C = 0.0301 mmol/L per mmHg (approximately 0.03). Thus [dissolved CO2] (mmol/L) = 0.03 × PaCO2 (mmHg). At normal PaCO2 of 40 mmHg, dissolved CO2 = 1.2 mmol/L. This allows the equation: pH = 6.1 + log (24/1.2) = 6.1 + log 20 = 6.1 + 1.3 = 7.4. The solubility coefficient is a physical property of CO2 in aqueous solution, not a catalytic fraction or equilibrium constant.

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

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