The circle of Willis
(also called the cerebral arterial circle
or arterial circle of Willis
) is a circle of arteries
that supply blood to the brain
. It is named after Thomas Willis
), an English physician
The basilar artery and middle cerebral arteries, though they supply the brain, are not considered part of the circle.
The arrangement of the brain's arteries into the Circle of Willis creates redundancies 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
Considerable anatomic variation
exists in the circle of Willis. Based on a study of 1413 brains, the "textbook version" 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.
Subclavian steal and the Circle of Willis
The redundancies that the Circle of Willis introduce can also lead to reduced cerebral perfusion. In subclavian steal syndrome
, which results from a proximal stenosis
(narrowing) of the subclavian artery
(a vessel that is supplied by the same vessel (the Aorta
) that eventually feeds the circle of Willis via the Common carotid artery
), 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 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.