HOME

TheInfoList



OR:

The circle of Willis (also called Willis' circle, loop of Willis, cerebral arterial circle, and Willis polygon) is a
circulatory anastomosis A circulatory anastomosis is a connection (an anastomosis) between two blood vessels, such as between arteries (arterio-arterial anastomosis), between veins (veno-venous anastomosis) or between an artery and a vein (arterio-venous anastomosis). ...
that supplies blood to the brain and surrounding structures in reptiles, birds and mammals, including humans. It is named after Thomas Willis (1621–1675), an English physician.


Structure

The circle of Willis is a part of the
cerebral circulation Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arteri ...
and is composed of the following arteries: * Anterior cerebral artery (left and right) * Anterior communicating artery *
Internal carotid artery The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these ...
(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 of Willis.


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 are major arteries of the neck. Typically, the vertebral arteries originate from the subclavian arteries. Each vessel courses superiorly along each side of the neck, merging within the skull to form the single, midline b ...
. The
vertebral arteries The vertebral arteries are major arteries of the neck. Typically, the vertebral arteries originate from the subclavian arteries. Each vessel courses superiorly along each side of the neck, merging within the skull to form the single, midline b ...
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 An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with morphological features different from those that are typically described in the majority of individuals. Anatomical variations are categ ...
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 The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these ...
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.


Function

The arrangement of the brain's arteries into the circle of Willis is believed to create redundancy (analogous to engineered redundancy) for collateral circulation in the
cerebral circulation Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arteri ...
. 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. 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 of
pulse pressure Pulse pressure is the difference between systolic and diastolic blood pressure. It is measured in millimeters of mercury (mmHg). It represents the force that the heart generates each time it contracts. Resting blood pressure is normally approx ...
waves within the brain and involvement in forebrain sensing of water loss.


Clinical significance


Aneurysms


Subclavian steal syndrome

The adaptive flow that the circle of Willis introduces can also lead to reduced cerebral perfusion. In
subclavian steal syndrome In general, Subclavian means beneath the clavicle, and it may refer to: * Subclavian vein * Subclavian artery In human anatomy, the subclavian arteries are paired major arteries of the upper thorax, below the clavicle. They receive blood from th ...
, blood is "stolen" from the vertebral artery on the affected side to preserve blood flow to the upper limb. Subclavian steal syndrome results from a proximal stenosis (narrowing) of the subclavian artery, one of arteries originating off of the
aortic arch The aortic arch, arch of the aorta, or transverse aortic arch () is the part of the aorta between the ascending and descending aorta. The arch travels backward, so that it ultimately runs to the left of the trachea. Structure The aorta begins a ...
. Subclavian steal syndrome has potential to affect flow in the circle of Willis.


Additional images

Image:Willis US 124320875.jpg, Fetal ultrasound image at the level of circle of Willis, showing PCA, MCA and ACA Image:Cerebral angiography, arteria vertebralis sinister injection.JPG, Cerebral
angiogram Angiography or arteriography is a medical imaging technique used to visualize the inside, or lumen, of blood vessels and organs of the body, with particular interest in the arteries, veins, and the heart chambers. Modern angiography is perform ...
showing an anterior/posterior projection of the vertebro basilar and posterior cerebral circulation, the posterior aspect of the circle of Willis, and one of its feeding vessels Image:CerebralCirculationAnteriorView.svg, An anterior view of major cerebral and cerebellar arteries. Image:Arteries_beneath_brain_Gray_closer.jpg Image:Circle of Willis 4.jpg, Circle of Willis Image:Circle of Willis 5.jpg, Circle of Willis


See also

*
Cerebral circulation Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arteri ...
* Leptomeningeal collateral circulation


References


External links

* {{Authority control Arteries of the head and neck