What is the Willis Ring?

The ring, circle or polygon of Willis is one of the names by which the cerebral arterial circle is an anatomical structure described by the English physician Thomas Willis (1621-1675) and which consists of an arterial ring at the base of the brain that allows maintaining the blood supply in case of blockage in any of the arteries that supply blood to the brain.

Structure and description

The circle of Willis is a polygonal ring, approximately heptagonal, formed by the anastomosis (union) of several arteries at the base of the brain, specifically in the interpeduncular fossa. It is formed by the following arteries:

basilar artery Left and right posterior cerebral arteries Left and right posterior communicating arteries Left and right internal carotid arteries Left and right anterior cerebral arteries anterior communicating artery

The left and right internal carotid arteries arise from the respective left and right common carotid arteries. Upon reaching the interpeduncular fossa at the base of the brain, these arteries divide into several branches, the anterior ophthalmic and carotid arteries, which are not part of the circle of Wiilis, and the anterior cerebral and posterior communicating arteries, which are part of the brain. circle of will

The anterior cerebral arteries arising from the internal carotids are joined by the anterior communicating artery and close the circle of Willis anteriorly.

On the back, the circle is closed by the union of the posterior communicating arteries with the posterior cerebral arteries that depart from the basilar artery, which in turn is shaped by the anointing of the left and right vertebral arteries.

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From the arteries of the circle of Willis it branches off to supply the different parts of the brain in both hemispheres.

The Willis ring has a high anatomical variability, so much so that the classic description described above is seen in only about 35% of brains. In the rest, there are variations such as weak posterior communicating arteries, on one or both sides, or absence of the anterior communicating artery.


The ring of Willis distributes blood from the internal carotid arteries and basilar trunk to the left and right cerebral hemispheres. The important thing about the arrangement of the cerebral arteries in the Ring of Willis is that it creates redundant circulation pathways.

this allows both hemispheres to continue to receive a sufficient blood supply in the event of stenosis or blockage of any of the arteries that carry blood to the ring.

medical implications

The ring of Willis is one of the most common sites of aneurysms that cause subarachnoid hemorrhages, effusions due to weakness in the vessel walls in the subarachnoid space, which is the space between the pia mater (internal meninges that cover the brain) and the arachnoid ( intermediate meninges after the pia mater).

Another medical implication of the Ring of Willis is Subclavian Steal Syndrome. In this syndrome it is due to stenosis in a subclavian artery, usually due to atheromatosis. Because of this stenosis, the cerebral artery on the same side brings less blood to the circle of Willis and is neutralized by blood on the other side. This created an asymptomatic reduction in cerebral perfusion in 95% of cases and is usually treated by surgical procedures.

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Cerebral arterial circle diagram or circle of Willis Ring Willis (Real) Arteries of the base of the brain

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