World Library  
Flag as Inappropriate
Email this Article

Circle of Willis

Article Id: WHEBN0000244346
Reproduction Date:

Title: Circle of Willis  
Author: World Heritage Encyclopedia
Language: English
Subject: Posterior communicating artery, Anterior communicating artery, Aneurysm, Internal carotid artery, Cerebral circulation
Collection: Arteries of the Head and Neck
Publisher: World Heritage Encyclopedia

Circle of Willis

Circle of Willis
Schematic representation of the circle of Willis, arteries of the brain and brain stem. Blood flows up to the brain through the vertebral arteries and through the internal carotid arteries.
The arteries of the base of the brain. Basilar artery labeled below center. The temporal pole of the cerebrum and the cerebellar hemisphere have been removed on the right side. Inferior aspect (viewed from below).
Latin Circulus arteriosus cerebri
Circulus Willisi
MeSH A07.
Anatomical terminology

The circle of Willis (also called Willis' circle, loop of Willis, cerebral arterial circle, and Willis polygon) is a circulatory anastomosis that supplies blood to the brain and surrounding structures. It is named after Thomas Willis (1621–1675), an English physician.[1]


  • Structure 1
    • Origin of arteries 1.1
    • Variation 1.2
  • Function 2
  • Clinical significance 3
    • Aneurysms 3.1
    • Subclavian steal syndrome 3.2
  • Additional images 4
  • References 5
  • See also 6


The circle of Willis is a part of the cerebral circulation and is composed of the following arteries:[2]

The middle cerebral arteries, supplying the brain, are not considered part of the circle.

Origin of arteries

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

The posterior communicating artery is given off as a branch of the internal carotid artery just before it divides into its terminal branches - the anterior and middle cerebral arteries. The anterior cerebral artery forms the anterolateral portion of the circle of Willis, while the middle cerebral artery does not contribute to the circle.

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.

All arteries involved give off cortical and central branches. The central branches supply the interior of the circle of Willis, more specifically, the Interpeduncular fossa. The cortical branches are named for the area they supply. Since they do not directly affect the circle of Willis, they are not dealt with here.


Considerable anatomic variation exists in the circle of Willis. Based on a study of 1413 brains, the classic anatomy of the circle is only seen in 34.5% of cases.[3] In one common variation the proximal part of the posterior cerebral artery is narrow and its ipsilateral posterior communicating artery is large, so the internal carotid artery supplies the posterior cerebrum. In another variation the anterior communicating artery is a large vessel, such that a single internal carotid supplies both anterior cerebral arteries.


The arrangement of the brain's arteries into the circle of Willis creates redundancies or collaterals in the cerebral circulation. If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia.[4]

Clinical significance


Circle of Willis with the most common locations of ruptured aneurysms marked

Subclavian steal syndrome

The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion.[5][6] In subclavian steal syndrome, blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb. Subclavian steal syndrome results from a proximal stenosis (narrowing) of the subclavian artery, an artery supplied by the aorta which is also the same blood vessel that eventually feeds the circle of Willis via the vertebral artery.

Additional images


  1. ^ Uston, Cagatay (February 20, 2004). "Dr. Thomas Willis' Famous Eponym: The Circle of Willis" (PDF). Turkish Journal of Medical Sciences 34: 271–274. Retrieved 16 July 2010. 
  2. ^ Purves, Dale; George J. Augustine; David Fitzpatrick; William C. Hall; Anthony-Samuel LaMantia; James O. McNamara; Leonard E. White (2008). Neuroscience, 4th Ed. Sinauer Associates. pp. 834–5.  
  3. ^ Bergman RA, Afifi AK, Miyauchi R, Circle of Willis. Illustrated Encyclopedia of Human Anatomic Variation, URL: Accessed on November 6, 2005.
  4. ^ Boorder, Michiel J.; Van Der Grond, J; Van Dongen, AJ; Klijn, CJ; Jaap Kappelle, L; Van Rijk, PP; Hendrikse, J (2006). "Spect measurements of regional cerebral perfusion and  
  5. ^ Klingelhöfer, J; Conrad, B; Benecke, R; Frank, B (1988). "Transcranial Doppler ultrasonography of carotid-basilar collateral circulation in subclavian steal". Stroke; a journal of cerebral circulation 19 (8): 1036–42.  
  6. ^ Lord, RS; Adar, R; Stein, RL (1969). "Contribution of the circle of Willis to the subclavian steal syndrome". Circulation 40 (6): 871–8.  

See also

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.