From Wikipedia
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 Englishphysician.[1]
Structure
The circle of Willis is a part of the cerebral circulation and is composed of the following
arteries:
·
Anterior cerebral
artery (left and
right)
·
Internal carotid
artery (left and
right)
·
Posterior
cerebral artery (left
and right)
·
Posterior
communicating artery (left
and right)
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.
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.[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.
Function
The arrangement of the brain's arteries
into the circle of Willis creates redundancy (analogous to engineered redundancy)
for collateral circulation 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
Aneurysms
Circle of Willis with the most common locations of ruptured aneurysms marked
Main article: Subarachnoid
haemorrhage
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.
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