Using the Henderson-Hasselbalch equation, pH = pKa + log_ 10 dfrac [A^-] [HA] , the plasma bicarbonate buffer system has a pKa of 6.1 and a normal base:acid (HCO_3^-:H_2CO_3) ratio of about 20:1. What pH does this ratio produce, and why is it physiologically significant?

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MBBS QUESTION #10299

Question 1

Using the Henderson-Hasselbalch equation, $pH = pKa + \log_{10}\dfrac{[A^-]}{[HA]}$, the plasma bicarbonate buffer system has a pKa of 6.1 and a normal base:acid ($HCO_3^-$:$H_2CO_3$) ratio of about 20:1. What pH does this ratio produce, and why is it physiologically significant?

pH $\approx$ 7.4, the normal physiological pH of arterial blood✔️
pH $\approx$ 6.1, equal to the buffer's pKa
pH $\approx$ 4.7, well below normal blood pH
pH $\approx$ 20, an impossible physiological value

Correct Answer Explanation

$pH = 6.1 + \log_{10}(20) = 6.1 + 1.3 = 7.4$. This is why maintaining the 20:1 $HCO_3^-$:$H_2CO_3$ ratio — via renal bicarbonate regulation and respiratory CO2 elimination — is the central physiological target for normal acid-base balance, even though the buffer's pKa itself is far from 7.4.

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