Cerebral blood flow is regulated by which mechanism?

• A. Intrinsic autoregulatory mechanisms
• B. Extrinsic neural control
• C. Hormonal control
• D. Gravity and body position

Answer: (A)

Cerebral blood flow is kept constant mainly by intrinsic autoregulatory mechanisms that adjust the brain’s own vessel tone to match its metabolic needs and perfusion pressure. The vessels in the brain can constrict or dilate in response to changes in arterial pressure (the myogenic response) so that flow remains steady despite fluctuations in blood pressure. Metabolic factors tighten or loosen these vessels according to activity: higher CO2 or lower pH in brain tissue signals need for more blood, causing vasodilation, while low CO2 or higher pH causes constriction. Endothelial factors like nitric oxide also help tune this response.

This autoregulatory system operates over a wide range of mean arterial pressures, so normal changes in systemic pressure don’t dramatically change cerebral blood flow. Extrinsic neural control and hormones can modulate flow, but they don’t drive the steady-state regulation in the way intrinsic autoregulation does. Gravity and body position can influence the actual pressure difference reaching the brain, but the brain’s autoregulatory mechanisms adjust diameter to keep flow relatively constant despite those changes.