Which relation governs blood pH and determines the influence of CO2 and bicarbonate on acidity?

• A. Goldman equation
• B. Henderson–Hasselbalch relation; CO2 is acid and bicarbonate is base
• C. Nernst equation
• D. Bohr effect equation

Answer: (B)

Blood pH is set by the bicarbonate buffer system, and the key relationship is captured by the Henderson–Hasselbalch equation. It links pH to the ratio of bicarbonate to dissolved carbon dioxide (the latter reflecting carbonic acid in equilibrium with CO2). In blood, pH ≈ pKa + log([HCO3-]/(0.03 × PCO2)). Since carbon dioxide dissolves as carbonic acid, increasing CO2 raises H2CO3, releases more H+, and lowers pH (more acidic); increasing bicarbonate raises the buffer capacity and raises pH (reduces acidity). The pKa for carbonic acid in plasma is about 6.1, and the 0.03 factor converts PCO2 to the dissolved CO2 concentration used in the ratio. Thus the pH is governed by the ratio of bicarbonate to CO2, which is why the respiratory system (CO2) and kidneys (bicarbonate) are the primary regulators of blood acidity.

Other equations don’t describe this buffering balance. The Nernst equation deals with the equilibrium potential of individual ions, not the buffering interconversion between CO2, carbonic acid, and bicarbonate. The Bohr effect describes how pH and CO2 affect hemoglobin’s oxygen affinity, not the quantitative pH buffering. The Goldman equation relates to membrane potentials based on multiple ions, not the carbonate buffering system.