In chronic respiratory acidosis, pH falls by _____ for every increase of 10 mmHg in PCO2.

• A. 0.08
• B. 0.03
• C. 0.02
• D. 0.04

Answer: (B)

In chronic respiratory acidosis, the kidneys have time to compensate by increasing bicarbonate, which buffers the extra CO2. This renal compensation makes the pH drop much less for a given rise in PCO2. The typical amount is about 0.03 pH units for every 10 mmHg increase in PCO2. This contrasts with acute respiratory acidosis, where the pH falls more steeply (about 0.08) because there hasn’t been time for bicarbonate to accumulate. So the correct number reflects the buffering effect of metabolic compensation, yielding only a small pH decline per CO2 rise.