Maintaining a pH of 7.4 requires which approximate ratio of bicarbonate to carbonic acid?

Maintaining a pH of 7.4 requires which approximate ratio of bicarbonate to carbonic acid?

• A. 10:1
• B. 20:1
• C. 1:20
• D. 2:1

Answer: (B)

The balance in blood is described by the Henderson-Hasselbalch equation for the bicarbonate buffer: pH = pKa + log([HCO3-]/[H2CO3]). In blood, the pKa for carbonic acid is about 6.1. The carbonic acid concentration is tied to dissolved CO2; with normal alveolar CO2 (PaCO2) around 40 mmHg, [H2CO3] is roughly 1.2 mEq/L. To achieve a pH of 7.4, you need log([HCO3-]/[H2CO3]) ≈ 1.3, since 7.4 − 6.1 ≈ 1.3. 10^1.3 is about 20, so the required ratio of bicarbonate to carbonic acid is roughly 20:1. In other words, maintaining pH 7.4 means the bicarbonate level is about twenty times the carbonic acid level. This reflects how the lungs (CO2 removal) and kidneys (bicarbonate handling) work together to keep the ratio, and thus the pH, near normal. If CO2 rises (more carbonic acid), the ratio drops and pH falls; if bicarbonate rises, the ratio increases and pH rises. If you consider the other options: a 10:1 ratio would yield a pH closer to 7.1, 1:20 would be far too acidic (pH well below 7), and 2:1 would give around pH 6.4.

The balance in blood is described by the Henderson-Hasselbalch equation for the bicarbonate buffer: pH = pKa + log([HCO3-]/[H2CO3]). In blood, the pKa for carbonic acid is about 6.1. The carbonic acid concentration is tied to dissolved CO2; with normal alveolar CO2 (PaCO2) around 40 mmHg, [H2CO3] is roughly 1.2 mEq/L. To achieve a pH of 7.4, you need log([HCO3-]/[H2CO3]) ≈ 1.3, since 7.4 − 6.1 ≈ 1.3. 10^1.3 is about 20, so the required ratio of bicarbonate to carbonic acid is roughly 20:1.

In other words, maintaining pH 7.4 means the bicarbonate level is about twenty times the carbonic acid level. This reflects how the lungs (CO2 removal) and kidneys (bicarbonate handling) work together to keep the ratio, and thus the pH, near normal. If CO2 rises (more carbonic acid), the ratio drops and pH falls; if bicarbonate rises, the ratio increases and pH rises.

If you consider the other options: a 10:1 ratio would yield a pH closer to 7.1, 1:20 would be far too acidic (pH well below 7), and 2:1 would give around pH 6.4.