What is the main factor in the control cerebral blood flow?

• A. PO2
• B. pH
• C. PCO2
• D. Hematocrit

Answer: (C)

The most powerful driver of cerebral blood flow is the arterial PCO2 level. CO2 easily crosses the blood-brain barrier and, once in the brain, forms carbonic acid and lowers the pH of the brain interstitial and CSF compartments. This drop in pH causes cerebral arterioles to dilate, increasing blood flow to wash out CO2 and meet metabolic needs. Conversely, when PCO2 falls, the vessels constrict and CBF decreases. This CO2-driven mechanism dominates over changes in PO2 within the normal physiological range, because cerebral autoregulation maintains flow across a wide PO2 span and only becomes a major signal when oxygen is severely depleted. Hematocrit affects blood viscosity and flow resistance but does not acutely regulate cerebral vessel diameter. Clinically, adjusting ventilation to alter PCO2 is a direct way to modulate cerebral blood flow.