The Henderson-Hasselbalch equation predicts that for every 1 mEq/L rise in HCO3-, the pH change in the blood is:
- A 0.015 pH units, assuming PaCO2 does not change
- B 0.05 pH units, regardless of the starting [HCO3-]
- C Variable and dependent on the buffering capacity at that particular pH and [HCO3-] ✓
- D Exactly 0.10 because of the 10:1 buffer relationship
Explanation
The Henderson-Hasselbalch equation gives pH = pKa + log([HCO3-]/[H2CO3]). A change in HCO3- by 1 mEq/L at a constant PaCO2 does not produce a fixed pH change — it depends on the logarithmic relationship and the prevailing [HCO3-]. At normal values (HCO3- ~24, PaCO2 ~40), a 1 mEq/L HCO3- change approximates ~0.015 pH units, but this differs at other HCO3- values. The key concept is that the pH-HCO3- relationship is logarithmic and non-linear. Option A gives the correct approximate value at normal range, but Option C correctly identifies the variable/dependent nature as the 'best' physiological answer.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.