| Circle of Willis | |
|---|---|
 Diagram of the arterial circulation at the base of the brain (inferior view), the circle of Willis is drawn in the upper half. 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. Thetemporal pole of thecerebrum and thecerebellar hemisphere have been removed on the right side. Inferior aspect (viewed from below). | |
| Details | |
| Identifiers | |
| Latin | circulus arteriosus cerebri circulus Willisii |
| MeSH | D002941 |
| TA98 | A12.2.07.080 |
| TA2 | 4516 |
| FMA | 50454 |
| Anatomical terminology | |
Thecircle of Willis (also calledWillis' circle,loop of Willis,cerebral arterial circle, andWillis polygon) is acirculatory anastomosis that suppliesblood to thebrain and surrounding structures in reptiles, birds and mammals, including humans.[1] It is named afterThomas Willis (1621–1675), anEnglish physician.[2]
The circle of Willis is a part of thecerebral circulation and is composed of the following arteries:[3]
Themiddle cerebral arteries, supplying the brain, are also considered part of the Circle of Willis[4]
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 leftposterior cerebral arteries arise from thebasilar artery, which is formed by the left and rightvertebral arteries. Thevertebral arteries arise from thesubclavian arteries.
Theanterior 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 and do not directly affect the circle of Willis.
Considerableanatomic variation exists in the circle of Willis, with classic anatomy seen only in about one-third of people.[5]In one common variation the proximal part of theposterior cerebral artery is narrow and its ipsilateralposterior communicating artery is large, so theinternal carotid artery supplies the posterior cerebrum; this is known as a fetal posterior communicating cerebral artery. In another variation the anterior communicating artery is a large vessel, such that a single internal carotid supplies both anterior cerebral arteries; this is known as an azygos anterior cerebral artery.[6]
The arrangement of the brain's arteries into the circle of Willis is believed to create redundancy (analogous toengineered redundancy) forcollateral circulation in thecerebral 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 otherblood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms ofischemia.[7]
However, considering that the circle of Willis is present in many non-human species (reptiles, birds and mammals), and that arterial narrowing is mostly associated with old age and the human lifestyle, more generally applicable explanations of its functions have been suggested, such as dampening ofpulse pressure waves within the brain[8] and involvement in forebrain sensing ofwater loss.[1]
The adaptive flow that the circle of Willis introduces can also lead to reduced cerebral perfusion.[9][10] Insubclavian steal syndrome, blood is "stolen" from the vertebral artery on the affected side to preserve blood flow to theupper limb. Subclavian steal syndrome results from aproximalstenosis (narrowing) of thesubclavian artery, one of arteries originating off of theaortic arch. Subclavian steal syndrome has potential to affect flow in the circle of Willis.
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