Local Variation

Overall cerebral blood flow is constant, but can vary locally

Although we've discussed anatomic variations of the cerebral arteries, our description up to now suggests that they are a system of rigid branching pipes. In reality, however, the cerebral vessels continuously adjust their own caliber by contraction and relaxation of smooth muscle in their walls so that: 

• Constant cerebral blood flow is normally maintained over mean arterial pressures that range between approximately 60-150 mmHg. Thus, as intra-arterial pressure goes up, cerebral vessels normally constrict and conversely when pressure drops, cerebral vessels dilate. At extremely low or high arterial pressure levels this response fails, and in those situations cerebral blood flow follows blood pressure more directly. The concept that cerebral vessels themselves have a role in controlling cerebral blood flow is often referred to as autoregulation.
• Local blood flow can change in response to local neuronal metabolism. For example, increases in arterial (and hence extracellular) carbon dioxide tension in a specific brain region will lead to dilation of its arterioles and an immediate increase in local blood flow.

These responses protect the brain from ischemia by increasing cerebral blood flow (delivery of oxygen and glucose, and removal of carbon dioxide and other metabolites) in the face of decreased arterial pressures. They also support regional variation in brain activity by providing rapid local adjustments of blood flow.