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The Circle of Willis

The Circle of Willis is a grouping of arteries at the base of the brain
It is named after an English physician named Thomas Willis, he discovered it and then published the findings in 1664
It is the joining area of several arteries at the inferior (bottom) side of the brain.The internal carotid arteries branch into smaller arteries at the Circle of Willis and supply oxygenated blood over 80% of the cerebrum

Formation of  Circle of Willis
The brain receives its blood supply from four main arteries:
2 internal carotid arteries 
2 vertebral arteries 
The vertebral arteries on each side  unite to form Basilar artery 
The basilar artery and the carotids together  form the circle of Willis below the hypothalamus
The circle of Willis is the origin of six large vessels that supply the cerebral cortex
The clinical presentation of vascular disease in the cerebral circulation is depended upon the  vessels or combinations of vessels that are involved.

Peculiarities of cerebral blood flow
The arteries and arterioles that supply blood to the brain are highly specialized,they include both vascular smooth muscle and endothelial cells and unlike vascular cells from the peripheral circulation or other vascular beds. 
The vascular smooth muscle is highly responsive to changes in pressure, a process called myogenic activity,which contributes to autoregulation of cerebral blood flow. 
The endothelial cells in the cerebral circulation are also highly specialized and they provide a barrier to fluid movement called the blood-brain barrier.