Circle of Willis
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|Artery: 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 brain and arteries at base of the brain. Circle of Willis is formed near center. The temporal pole of the cerebrum and a portion of the cerebellar hemisphere have been removed on the right side. Inferior aspect (viewed from below).|
|Latin||circulus arteriosus cerebri|
|Gray's||subject #147 574|
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.
- 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)
Physiologic significance 
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.
Anatomic variation 
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. 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.
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 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.
Subclavian steal syndrome 
The redundancies that the Circle of Willis introduce can also lead to reduced cerebral perfusion. 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 common carotid artery.
Additional images 
- A.D.A.M. Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine (7/29/2009) (6 June 2010). "Circle of Willis". Retrieved 16 July 2010.
- Uston, Cagatay (February 20, 2004). "Dr. Thomas Willis' Famous Eponym: The Circle of Willis". Turkish Journal of Medical Sciences 34: 271–274. Retrieved 16 July 2010.
- 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. ISBN 978-0-87893-697-7.
- Moore KL, Dalley AR. Clinically Oriented Anatomy, 5th Ed., Lippincott Williams & Wilkins, Toronto. Copyright 2007
- 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 carbon dioxide reactivity: Correlation with cerebral collaterals in internal carotid artery occlusive disease". J Neurol 253 (10): 1285–1291. doi:10.1007/s00415-006-0192-1. PMID 17063318.
- Bergman RA, Afifi AK, Miyauchi R, Circle of Willis. Illustrated Encyclopedia of Human Anatomic Variation, URL: http://www.anatomyatlases.org/AnatomicVariants/Cardiovascular/Text/Arteries/CircleofWillis.shtml. Accessed on November 6, 2005.
- 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. doi:10.1161/01.STR.19.8.1036. PMID 3041649.
- Lord, RS; Adar, R; Stein, RL (1969). "Contribution of the circle of Willis to the subclavian steal syndrome". Circulation 40 (6): 871–8. PMID 5377222.
See also 
|Wikimedia Commons has media related to: Circle of Willis|