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Artery: Circle of Willis
Circle of Willis, arteries of the brain and brain stem.
Latin circulus arteriosus cerebri
Gray's subject #147 574
MeSH A07.
Dorlands/Elsevier {{{DorlandsPre}}}/{{{DorlandsSuf}}}

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.[1]


The basilar artery and middle cerebral arteries, though they supply the brain, are not considered part of the circle.[2]

Physiologic significance

It is traditionally believed that 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.

This traditional view is, however, not in accordance with principles of natural selection. The circle of Willis is also present in many non-human species (reptiles, birds and mammals), and arterial narrowing is usually associated with aging and the human lifestyle. Therefore, evolutionarily correct and more generally applicable explanations of its functions have been suggested, such as dampening of pulse pressure waves within the brain[3] and involvement in forebrain sensing of water loss by sensory circumventricular organs.[4]

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.[5]

Subclavian steal and the circle of Willis

The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion.[6][7] 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.

See also


  1. Uston C. Dr. Thomas Willis' famous eponym: the circle of Willis. J Hist Neurosci. 2005 Mar;14(1):16-21. PMID 15804755. Free Full Text.
  2. Moore KL, Dalley AR. Clinically Oriented Anatomy, 4th Ed., Lippincott Williams & Wilkins, Toronto. Copyright 1999. ISBN 0-683-06141-0.
  3. Vrselja Z, Brkic H, Mrdenovic S, Radic R, Curic G. (2014). Function of Circle of Willis. Journal of Cerebral Blood Flow & Metabolism 34(4), pp. 578-584. DOI: 10.1038/jcbfm.2014.7
  4. Fenrich M, Habjanovic K, Kajan J, Heffer M. (2020). The circle of Willis revisited: Forebrain dehydration sensing facilitated by the anterior communicating artery. BioEssays, e2000115. DOI: 10.1002/bies.202000115
  5. Bergman RA, Afifi AK, Miyauchi R, Circle of Willis. Illustrated Encyclopedia of Human Anatomic Variation, URL: Accessed on November 6, 2005.
  6. Klingelhofer J, Conrad B, Benecke R, Frank B. Transcranial Doppler ultrasonography of carotid-basilar collateral circulation in subclavian steal. Stroke. 1988 Aug;19(8):1036-42. PMID 3041649.
  7. Lord RS, Adar R, Stein RL. Contribution of the circle of Willis to the subclavian steal syndrome. Circulation. 1969 Dec;40(6):871-8. PMID 5377222.

External links


de:Circulus arteriosus cerebri fr:Polygone de Willis

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