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Circle of Willis

Recommended video: Arteries of the brain II [13:10]
Arteries of the brain seen from the inferior view of the brain.
Cerebral arterial circle (Circulus arteriosus cerebri)

The circle of Willis (cerebral arterial circle or circulus arteriosus) is an anastomotic ring of arteries located at the base of the brain. This arterial anastomotic circle connects the two major arterial systems to the brain, the internal carotid arteries and the vertebrobasilar (vertebral and basilar arteries) systems. It is formed by four paired vessels and a single unpaired vessel with numerous branches that supply the brain. 

The main function of the circle of Willis is to provide a collateral blood flow between the anterior and posterior arterial systems of the brain. Additionally, it offers the alternate blood flow pathways between the right and left cerebral hemispheres. This way the circle protects the brain from ischemia and stroke in cases of vascular obstruction or damage.

This article will discuss the anatomy and function of the circle of Willis.

Key facts about the circle of Willis
Structure Polygonal-shaped anastomosis of arteries of varying calibre
Location Interpeduncular fossa at the base of the brain
Component arteries From anterior to posterior:
Anterior communicating artery (from the anterior cerebral artery)
Anterior cerebral arteries (from the internal carotid artery)
Internal carotid arteries (from the common carotid artery)
Posterior communicating arteries (from the posterior cerebral artery)
Posterior cerebral arteries (branch of the basilar artery)
Cerebral circulation Anterior circulation: internal carotid arteries, anterior cerebral arteries, anterior communicating artery and middle cerebral arteries
Posterior circulation: vertebral arteries, basilar artery, posterior cerebral arteries and posterior communicating arteries
Functions Forms anastomotic connections between anterior and posterior cerebral circulations, provides arterial blood supply to the brain via several branches 
  1. Structure
    1. Anterior arc of the circle of Willis
    2. Posterior arc of the circle of Willis
  2. Function
  3. Sources
+ Show all


The circle of Willis is located on the inferior surface of the brain within the interpeduncular cistern of the subarachnoid space. It encircles various structures within the interpeduncular fossa (depression at the base of the brain) including the optic chiasm and infundibulum of the pituitary gland.

Although significant anatomic variations exist, the circle of Willis is typically composed of three cerebral and two communicating arteries that link the internal carotid arteries and the vertebrobasilar system. The internal carotid arteries supply most of the forebrain. The vertebrobasilar system is composed of two vertebral arteries and one basilar artery and supplies the occipital lobe, brainstem and cerebellum.

Anterior arc of the circle of Willis

Anteriorly, the short anterior communicating artery (AComm) forms an anastomotic channel between the right and left anterior cerebral arteries (ACA), which are terminal branches of the internal carotid artery within the cranial cavity. The anterior cerebral arteries travel toward the anteromedial aspects of the interhemispheric fissure of the brain, to supply the midline regions of the frontal, parietal and cingulate cortices as well as the corpus callosum.

At the point of connection between the anterior cerebral and internal carotid arteries, the internal carotid gives off its lateral terminal branch known as the middle cerebral artery (MCA). Together with the AComm and the middle cerebral arteries, the anterior cerebral arteries form the anterior cerebral circulation.

Posterior arc of the circle of Willis

The posterior arc of the circle of Willis is formed by the posterior cerebral arteries (PCA), on each side, and the posterior communicating arteries (PComm), which connect the posterior cerebral arteries to their ipsilateral internal carotid arteries. 
The right and left vertebral arteries unite at the pontomedullary junction to form the basilar artery. The basilar artery then bifurcates into the paired posterior cerebral arteries at the superior border of the pons

The vertebral arteries, basilar artery, posterior cerebral arteries, together with the PComm form the posterior cerebral circulation. The posterior cerebral arteries through their central and cortical branches supply the occipital lobe of the brain, the inferior aspect of the temporal lobes, midbrain, thalamus and choroid plexus of the third and lateral ventricles.

Test your knowledge on the brain arteries with the following quiz!


The primary role of the circle of Willis is to form anastomoses between the internal carotid arteries and the vertebrobasilar system of arteries on the ventral aspect of the brain. These connections provide channels that allow blood flow between the anterior and posterior cerebral circulations. These channels are especially important if a vessel becomes obstructed, as they serve as collateral routes. It is important to note that although the anatomical connections of the circle of Willis are usually complete, parts of the circle may be functionally restricted. This is due to significant variations in the size of component arteries that limit the normal flow of blood through these regions. 

Several small perforating (central) arteries emerge from the circle of Willis, many of which pass into the brain through the anterior and posterior perforated substances, which are areas of grey matter located at the basal forebrain and the interpeduncular fossa respectively. The perforating arteries supply surrounding structures on the ventral surface of the brain such as the optic chiasm, pituitary gland, mammillary bodies and pineal gland.  Additionally, they provide blood supply for the deep structures of the brain including the internal capsule, basal ganglia and thalamus.

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