The Circle of Willis is a ring of vessels that unites the anterior and posterior circulation at the base of the brain. The vertebral and basilar arteries supply the cerebellum , brain stem and spinal cord. Blood also supplies glucose principal energy source of neurons. Stores of Oxygen and glycogen in the brain enough for only about 2 mins. Cerebral veins drain into large vascular channels - sinuses.
Venous blood returns to the heart via the jugular veins.
Normally blood flow to brain remains fairly constant over a range of mean arterial pressure between 50 -160mmHg. Achieved by compensatory dilation or constriction of cerebral blood vessels in response to changes in systemic blood pressure. At either end of this range autoregulation is lost and cerebral blood flow is dependent upon systemic blood pressure. There is a coupling relationship between cerebral blood flow and metabolism cerebral blood flow rapidly responds to the needs of cerebral metabolism. In pathological conditions the relationship between cerebral blood flow and metabolism becomes uncoupled.