The Anatomy of the Circle of Willis

The Group of Arteries That Supply Blood to the Brain

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The circle of Willis is a group of blood vessels in the brain that connect with each other, forming a continuous structure that resembles a circle. These nine arteries supply blood to a large portion of the brain. Most of the time, blood can flow through the vessels of the circle of Willis without any interruption. Aneurysms, which are defective outpouchings in blood vessels, are the most common problems involving this structure.


The circle of Willis is a small structure, with an average circumference of just a few centimeters. Each of the blood vessels in the circle of Willis has a diameter of about 1.5 to 2.5 millimeters. 


The circle of Willis is a rounded structure with uneven sides that resembles a circle, but it isn't a perfect circle. It's symmetrical on the left and right sides and approximately parallel to the top of your head.

The circle of Willis is composed of: 

  • One anterior communicating artery (ACOM): The ACOM is short and makes up the front of the circle of Willis. 
  • The left and right anterior cerebral arteries (ACAs): These vessels run along the sides of the circle of Willis. 
  • The left and right internal carotid arteries (ICAs): The ICAs travel in the front of the neck, through the carotid canal, to enter into the brain. This large blood vessel divides into the ACA and the middle cerebral artery (MCA). The MCA is not part of the circle of Willis. 
  • The left and right posterior cerebral arteries (PCAs): The PCAs are branches of the single basilar artery, which is formed by merging of the vertebral arteries in the back of the neck. The left and right PCAs are the longest parts of the circle of Willis and run along its sides, with a corresponding ICA in between the PCA and the ACA on each side.
  • The left and right posterior communicating arteries (PCOMs): The PCOMs are found in the back portion of the circle of Willis (nearer to the back of the head) and are relatively short.

Although it has nine sides, the circle of Willis is shaped more like a hexagon because the ICAs are very short and the two PCAs are almost straight.

Radiologist looking at brain scan
Phil Boorman / Getty Images 


The circle of Willis is located deep in the center of the brain, near two other important structures—the pituitary gland and the optic chiasm. It's often described as being located at the base of the brain because it lies in the inferior (lower) surface of the brain. The location is also described as the interpeduncular fossa, a cave-like structure underneath the brain. 

Anatomic Variation

There can be variations in the shape and vascular structure of the circle of Willis. One or more of the arteries can be absent or may be smaller or larger than average. This variability tends to be more common with older age and often doesn’t have any clinical consequences at all. 


Several of the arteries of the circle of Willis branch into smaller vessels that directly provide blood to the brain. 

Arteries are blood vessels that deliver oxygen and nutrient-rich blood to the cells of the body. Veins take blood from cells and back to the heart and then to the lungs to be replenished with oxygen. All of the blood vessels that make up the circle of Willis are arteries, and none of them are veins.

The ACAs provide blood to the anterior (front) region of the brain. This area of the brain is involved with decision-making, self-control, thinking, planning, emotions, and physical movements of the body. 

The PCAs provide blood to areas in the back of the brain, including the occipital lobe (which integrates vision), the brainstem (which controls eye and face movement and breathing) and the cerebellum (which controls coordination). 

Redundant Blood Supply

One of the unique features of the circle of Willis is that its continuous structure creates a redundant blood supply in the brain. What this means is that the ACOM and PCAs, which do not directly send blood to the brain, connect the ACAs and the ICAs—arteries that directly send blood to the brain.

If one ICA is blocked, blood can still reach the corresponding ACA through the other segments of the circle of Willis—this is possible because of the circle of Willis' redundant blood supply.

Associated Conditions

There are several conditions that can affect the circle of Willis, including structural changes and illnesses. These conditions can affect brain function if inadequate blood supply to any region of the brain occurs. 


An aneurysm is a defective out-pouching in the wall of a blood vessel. The out-pouching can physically press on nearby regions of the brain, causing these areas to develop an altered function.

An aneurysm in the circle of Willis can impinge on the optic chiasm, which may impair vision in one or more visual fields. It can also place pressure on the pituitary stalk (a part of the pituitary gland), disturbing its function.

Small brain aneurysms, which are often described as berry aneurysms, can cause headaches, fatigue, dizziness, and trouble concentrating. 

A brain aneurysm can also bleed slowly or rupture, leaking blood and damaging nearby areas of the brain. A ruptured circle of Willis brain aneurysm impairs blood supply to areas supplied by the circle of Willis’ arteries, producing symptoms that can include a stiff neck, severe headache, vomiting, numbness, weakness, abnormal pupils, or loss of consciousness.

If detected early, however, a brain aneurysm can often be treated before it causes harm.


A stroke is an interruption of blood flow in an artery. It prevents blood from reaching its destination in the brain. This causes the affected region to lose function. 

Despite the redundant blood supply made possible by the connecting arteries in the circle of Willis, a stroke is possible. If blood flow through one of the ACAs or one the PCAs is blocked after it leaves the circle of Willis, then a stroke can occur. 

  • An embolic stroke may occur if a small blood clot travels within the circle of Willis to lodge in an artery that supplies the brain.
  • A thrombotic stroke may occur if one of these branches becomes blocked due to disease and narrowing within the artery itself.
  • A hemorrhagic stroke can occur if one of the blood vessels bleeds, leaking blood and compromising the blood supply to its corresponding region of the brain. Roughly 15 percent of all strokes stem from hemorrhagic bleeding.


In general, infections are not common in the blood vessels of the brain. This is due to the effect of the blood-brain barrier, a protective feature of the lining of the blood vessels of the brain. However, while uncommon, an infection can spread within the circle of Willis.

The redundancy of the circulation makes an infection more likely to spread throughout the structure’s blood vessels. An infection can produce fevers, decreased alertness, loss of consciousness, and may trigger a stroke.

Traumatic Injury

Severe head trauma can stretch or tear any artery in the circle of Willis. If the arteries bleed, the built-in redundancy is not particularly effective because blood in the brain causes irritation and damage. Loss of blood from a damaged artery is likely to affect blood flow in other arteries in the circle of Willis as well.

Subclavian Steal Syndrome

When one of the arteries in the arm, the subclavian artery, becomes very narrowed, the basilar or vertebral arteries (which normally send blood to the PCAs) actually reverse their direction of blood flow in a direction described as retrograde (backward) flow.

With subclavian steal syndrome, the subclavian artery receives blood from the circle of Willis, which is described as “stealing”—hence the name subclavian steal syndrome. This problem may result in decreased blood supply to some areas of the brain that are supplied by arteries of the circle of Willis and may result in dizziness, vision changes, hearing changes, difficulty walking (ataxia), and difficulty speaking (dysarthria). 

Treatment of subclavian steal syndrome varies and ranges from conservative observation (if there are minimal symptoms or if surgery poses a health risk) to an interventional procedure aimed at repairing the stenosis (narrowing) of the subclavian artery.


If you have a defect in one or more of the blood vessels that comprise the circle of Willis, the decision process regarding your treatment can be quite complex.

For example, if you have an aneurysm, your medical team may decide to watch it over time rather than putting you through the risk of surgery. On the other hand, your medical team may decide that you need to have a reparative procedure if your aneurysm has a high risk of rupturing. This decision depends on the size and exact location of the aneurysm, which are major factors in determining its prognosis. 

Surgical Intervention

There are several types of procedures that can repair a defect in the circle of Willis. Because the structure is located so deep in the brain, it is not easily accessible.

Sometimes, minimally invasive procedures, such as endovascular techniques are used. In that case, your healthcare providers will not make an incision directly in the brain or access the brain through the skull, but instead, reach the blood vessels by threading a wire or a catheter (tube) through another blood vessel in a more accessible region.

Keep in mind that even minimally invasive procedures can result in serious complications, which may require an emergency invasive technique.


Recovery after any illness affecting the circle of Willis often takes time. These conditions can cause substantial brain damage, necessitating therapy to help you regain lost neurological function.

For example, if you have had a stroke of the ACA, you may need a great deal of cognitive and/or motor rehabilitation. If you had a stroke affecting the PCA, you may need to work on regaining motor function and/or coordination.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.