Circle of Willis

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Is a partial ring-main system in children, but falls short of full effectiveness in adults


The arterial system was named after Thomas Willis by his student Richard Lower. Willis was by no means the first to describe the cerebral blood vessels, but like many eponyms, his name has become firmly associated with it.
Arteries Supplying the Brain

The circle of Willis or cerebral arterial circle is the loop of arteries at the base of the brain, which is formed from the vertebral arteries and the internal carotid arteries and gives rise to the arteries which supply the brain, as well as some other structures in the head.

The main branches are sometimes subdivided into segments.[1]


How to learn the Circle of Willis

Rupert Millard learnt the Circle of Willis by retaining the details necessary to complete a diagram of it.

Vessels in:

Vessels out:

File:CircleofWillis1.png To make this diagram into a ring shape, add an anterior communicating artery and a pair of posterior communicating arteries.

File:CircleofWillis2.png It is now necessary to fill in the details on the basilar artery, which is formed by the union of the left and right vertebral arteries. It is easiest to add the branches to the diagram in the anterior to posterior direction, even though the blood is flowing in the opposite direction. The basilar artery gives rise to, going from anterior to posterior:

The vetebral arteries give rise to:


The posterior spinal artery is a branch of either the vertebral arteries or the posterior inferior cerebellar arteries.

Anatomical variations

A normal cerebral circulation based on magnetic resonance angiogram

The sizes of the communicating vessels vary with only about a half of people having a 'full' circle of Willis.

Anterior Circulation

  • It is also possible for there to be two anterior communicating arteries.
  • The anterior cerebral artery is divided into segments with the horizonal segment known as A1. In about 10% of people, this segment is missing or hypoplastic

Posterior Circulation

  • The posterior communicating arteries are absent in about a third of people.
  • A fetal-type posterior circulation, also known as a fetal origin of the posterior cerebral artery, is an anatomical variant present in about 10-30% of people[2][3] of people where the posterior cerebral artery branches from the internal carotid artery rather than the basilar artery.[4] The posterior communicating artery is therefore larger than the first (horizontal) part of the posterior cerebral artery (known as the P1 segment).

Clinical consequences

In children, the circle structure does provide redundancy - if one of the four major arteries is occluded, the circulation to the brain is maintained. However, in adults the communicating arteries are generally small compared to the major arteries supplying blood to the circle and so a sudden occlusion is not compensated for and a stroke results. The circle is better able to compensate for partial occlusions that are gradual in onset.

Internal carotid artery disease in patients with a fetal-type posterior circulation may present with ischaemic symptoms in both the anterior and posterior territories.

It is a common location for a berry aneurysm.