Circle of Willis

The circle of Willis (also called the cerebral arterial circle, arterial circle of Willis, or circulus arteriosus cerebri) is a circle of arteries that supply the brain. It is named after Thomas Willis (1621-1673), an English physician.

Components

 * Anterior cerebral artery (left and right)
 * Anterior communicating artery
 * Internal carotid artery (left and right)
 * Posterior cerebral artery (left and right)
 * Posterior communicating artery (left and right)

The basilar artery and middle cerebral arteries, though they supply the brain, are not considered part of the circle.

Physiologic significance
The arrangement of the brain's arteries into the Circle of Willis creates redundancies in the cerebral circulation. If any one of the arteries in the circle become blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can usually maintain cerebral perfusion.

Anatomic variation
Considerable anatomic variation exists in the circle of Willis. The "textbook version" of the circle, based on a series of 1413 brains, is only seen in 34.5% of cases.

Subclavian steal and the circle of Willis
The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion. In subclavian steal syndrome, which results from a proximal stenosis (narrowing) of the subclavian artery (a vessel that supplies a vessel that feeds the circle of Willis), blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb.

Origin of arteries
The left and right internal carotid arteries arise from the right and left common carotid arteries.

The anterior cerebral arteries and posterior communicating arteries arise from the internal carotid arteries' trifurcations.

The right and left posterior cerebral arteries arise from the basilar artery, which is formed by the left and right vertebral arteries. The vertebral arteries arise from the subclavian arteries.

The anterior communicating artery connects the two anterior cerebral arteries and could be said to arise from either the left or right side.