At blood pH 7.40, what is the typical ratio of bicarbonate to carbonic acid?

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Multiple Choice

At blood pH 7.40, what is the typical ratio of bicarbonate to carbonic acid?

Explanation:
The key idea is the Henderson–Hasselbalch relationship for the bicarbonate buffer in blood. For the carbonic acid/bicarbonate system, pH = pKa + log([HCO3-]/[CO2]). The pKa of carbonic acid is about 6.1. At normal arterial pH of 7.40, the ratio becomes [HCO3-]/[CO2] = 10^(7.40 − 6.10) ≈ 10^1.30 ≈ 20. So the typical bicarbonate to carbonic acid ratio is about 20:1. In practical terms, with arterial CO2 around 40 mmHg, [CO2] ≈ 0.03 × 40 = 1.2 mM, and with [HCO3-] about 24 mEq/L, the ratio is 24/1.2 ≈ 20, matching the calculation. The other ratios would correspond to different pH values, so 20:1 is the normal value at pH 7.40.

The key idea is the Henderson–Hasselbalch relationship for the bicarbonate buffer in blood. For the carbonic acid/bicarbonate system, pH = pKa + log([HCO3-]/[CO2]). The pKa of carbonic acid is about 6.1. At normal arterial pH of 7.40, the ratio becomes [HCO3-]/[CO2] = 10^(7.40 − 6.10) ≈ 10^1.30 ≈ 20. So the typical bicarbonate to carbonic acid ratio is about 20:1.

In practical terms, with arterial CO2 around 40 mmHg, [CO2] ≈ 0.03 × 40 = 1.2 mM, and with [HCO3-] about 24 mEq/L, the ratio is 24/1.2 ≈ 20, matching the calculation. The other ratios would correspond to different pH values, so 20:1 is the normal value at pH 7.40.

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