A patient with chronic poorly controlled hypertension has an acute rise in arterial PCO2 (hypercapnia). Which mechanism best explains the resulting cerebral vasodilation?

• A Direct relaxant effect of CO2 on cerebral arteriolar smooth muscle
• B Central chemoreceptor activation triggering a parasympathetic cerebral vasodilator reflex
• C Decreased cerebrospinal fluid pH causing cerebral arteriolar relaxation ✓
• D Decreased plasma pH directly relaxing cerebral vascular smooth muscle

Explanation

CO2 diffuses across the blood-brain barrier into the CSF, where carbonic anhydrase generates H+, lowering CSF pH. This local acidity relaxes cerebral arteriolar smooth muscle, increasing cerebral blood flow. Option A is wrong because CO2 itself is not the direct mediator. Option B is wrong because the response is local, not a parasympathetic reflex. Option D is wrong because plasma pH changes do not cross the blood-brain barrier; CSF pH is the key determinant.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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