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The Brain's "Plan B" for Stroke: The Circle of Willis

Updated on April 9, 2016
M G Del Baglivo profile image

The author holds a degree in Zoology and Physiology from Rutgers University and is a former vascular and neuro-ultrasound technologist.

Hippocrates (460 to 370 BC) first described paralysis caused by ischemia (loss of blood supply) in the brain. He termed it apoplexy (from the Greek for “struck down with violence”) and that name survived until 1927 when apoplexy became “CVA” for Cerebrovascular Accident. Since 1990, at the suggestion of the American Stroke Association, the term “brain attack” has come into use. The reason for the suggested change in terminology was to encourage the public to view the urgency and need for action at the first signs of stroke as seriously as those for heart attack.

Regardless of the name used for stroke, it is the fifth leading cause of death in the United States. Approximately 800,000 cases occur each year – one every 4 minutes – and almost 130,000 victims do not survive. Many of those who do live suffer major disability including blindness, the inability to speak, paralysis and dementia. Care for stroke patients costs the health care system 34 billion dollars a year.

There are two types of strokes, ischemic and hemorrhagic. The former results when blood supply to the brain is interrupted by a blood clot or other substance causing an “infarction” of brain tissue. The latter is a bleed into the brain from a ruptured artery starving brain tissue of blood and nutrients and increasing intracerebral pressure in the skull. Ischemic accounts for 87% of all strokes. However, the brain has a redundant system of arterial supply to deal with these occlusive strokes.

The Most Common Thrombotic or Embolic Block of an Artery in the Brain

  • Blood clot (thrombotic) that is formed in the brain (50% of all strokes).

  • Blood clot (embolic) formed in other parts of the body (often from heart valves) and carried to the brain.

  • Atherosclerotic plaques (embolic) that dislodge from the lining of an artery. These plaques are of the same composition (and cause) as those found in the cardiac arteries.

  • Fats (embolic) that leak from fractures of large bones such as a femur or the pelvis.

The plaque and fat type of emboli generally do not occlude major vessels but can affect the smaller arteries and capillary beds.

MRI Angiogram of the Circle of Willis
MRI Angiogram of the Circle of Willis | Source

The Circle of Willis (circulus arteriosus cerebri)

Sitting at the base of the mammalian brain is an arterial structure that is a wonder of nature and evolution. First described by Thomas Willis (1621–1675), an English physician and pioneer in brain anatomy, the Circle (actually a pentagon) is a connection of arteries without which total occlusion of a vessel can result in severe cerebral injury or death.

The Arteries of the Circle of Willis

  • Internal carotid artery (left and right)

  • Basilar artery

  • Anterior cerebral artery (left and right)

  • Posterior cerebral artery (left and right)

  • Anterior communicating artery

  • Posterior communicating artery (left and right)

The left and right middle cerebral arteries are the main branches of their respective internal carotids and are not considered part of the Circle. However, in the case of a total occlusion of an internal carotid below the level of the middle cerebral artery, the area of the brain served by that vessel would be perfused by blood from the Circle.

Arteries of the Circle of Willis
Arteries of the Circle of Willis | Source

The Arteries Supplying Blood to the Circle of Willis

The bilateral carotid arteries and single basal artery bring oxygenated blood from the heart to the brain. A common carotid is located on each side of the neck and the basilar is found in the posterior skull. The three vessels (the left and right carotid and the basal arteries) are the only sources of blood coming from the heart to the brain and anastomose at the Circle of Willis. The remaining vessels that form the Circle take this blood and distribute it throughout the brain.

The Carotid Arteries

There are three sets of vessels in the carotid system:

  • Common Carotid Artery (left and right)

  • Internal Carotid Artery (left and right)

  • External Carotid Artery (left and right)

The Common Carotids

Each of the common carotids arises from the heart at a different location. The right emerges from the brachiocephalic trunk (an aortic branch) and the left originates from the aortic arch. Each then travels up the neck and may be detected as a pulse under the jaw. Both the left and right common carotid arteries are subject to stenosis (narrowing). Narrowing is suspected when a stethoscope is placed over the vessel and a bruit (pronounced brew-ee) is detected. This “vascular murmur” is caused by the turbulent flow of blood through the narrowed vessel or “partial occlusion.” This is usually caused by atherosclerotic plaques and, less commonly, as the result of direct traumatic injury to the neck, aneurysm or fibromuscular dysplasia. Bruit detection requires further investigation by Doppler ultrasound of the neck. When necessary, additional diagnostic imaging tests are CT angiography, magnetic resonance angiography or cerebral angiogram.

The Internal and External Carotids

Each common carotid artery divides to form the internal and external carotid arteries. The former supplies blood to the anterior brain (cerebrum). The latter is responsible for the blood supply of the thyroid, larynx, tongue, nose, oral cavity, neck, face, ears, and lower portions of the skull, as well as the meninges, one of the layers that cover the brain. Atherosclerotic plaques can accumulate in either or both of these vessels; the bifurcation into the two vessels is especially prone to the development of plaques. The external carotid does not enter the skull chamber but divides into branches that distribute blood to the elements of the head outside the skull. Emboli arising from the external can cause infarcted areas of these structures. The internal carotid enters the skull and supplies the brain with blood via the Circle of Willis. Emboli travelling from and through the internal carotid have the potential to enter the brain and cause partial or total occlusion of large and small arteries.

Doppler ultrasound is used to detect narrowing of the internal and external carotids and their bifurcation. Imaging of the bifurcation also aids in identifying each artery. As with the common carotid, further imaging studies would be angiography or angiogram.

Common Carotid Artery and bifurcation forming the Internal and External Carotid Arteries
Common Carotid Artery and bifurcation forming the Internal and External Carotid Arteries | Source

The Vertebral-Basilar Arteries

The right and left vertebral arteries arise from the subclavian arteries located beneath the collarbone in the chest. The vertebrals then travel up the vertebral column. They enter the skull and fuse at the level of the medulla oblongata and pons to form the single basilar artery. This artery supplies blood to the posterior brain including the cerebellum and medulla oblongata. As already stated, the basilar artery is one of only three vessels supplying the brain with blood from the heart. It connects with the arteries that form the Circle of Willis at the underside of the brain at the level of the pituitary stalk.

The vertebral and basal arteries can contain atherosclerotic plaques with narrowing of the vessels. Although plaques of the vertebral arteries do not commonly rupture and travel into the brain, vertebral insufficiency due to narrowing is possible. Symptoms of insufficiency are vertigo, dizziness and fainting. Basilar artery plaques may break off and form emboli in the brain.

Imaging modalities for the vertebral-basilar system are the same as those for the carotid system.

Bilateral Vertebral Arteries. Note anterior fusion to form single Basilar Artery
Bilateral Vertebral Arteries. Note anterior fusion to form single Basilar Artery | Source

Function of the Circle of Willis and Exceptions

This thumbnail-sized structure is best thought of as a traffic circle. If a blockage occurs in one road, vehicles can enter other roads connected to the circle and, although the route may be longer, reach the appropriate destination. This redundancy of “routes” (collateral circulation) serves to protect the mammalian brain from ischemic stroke.

How then do ischemic strokes occur when the Circle of Willis should prevent them? There are several reasons:

  1. Variation in anatomy is common among mammalian species. In other words, not everyone has the same arteries in the same place. Based on one study of 1,413 brains, the classic anatomy of the Circle of Willis is found in only 34.5 % of humans. Eighty-three variations in the structure of the Circle have been described and, depending on the specific form, collateral circulation may be significantly impaired or impossible, resulting in ischemic stroke. Variations may include the absence of a major connecting vessel.

  2. In order for the Circle to provide protection to the entire brain, blood pressure through it must be at least 50% of normal. Atherosclerotic disease of the vessels that form it can reduce pressure below this critical level making the Circle ineffective. Shock may also reduce pressure below the minimum level.

  3. Collateral arteries and their distal branches must be able to dilate in response to increased flow and pressure when they are pressed into service as blood is rerouted at the Circle. Vascular disease may make this difficult or impossible and infarct occurs distal to the thrombus or embolus.

The Value of the Circle of Willis

The anatomical variations and physiological requirements of the Circle in no way lessen its importance in protecting the mammalian brain from ischemic stroke. The reaction of the brain in response to full or partial occlusion of a major arterial vessel may be silent due to the collateral circulation made possible by the Circle. Hence, the patient shows no clinical signs of ischemic infarct. If symptoms of ischemic stroke are evident, the collateral circulation provided by the Circle may allow time for antithrombotic therapy, mechanical removal of the thrombus, angioplasty of the vessel and stent placement. These measures must be performed within several hours of the onset of the thrombotic stroke. Buying time is critical to recovery and rehabilitation from brain attack.


Anatomy Atlases, Illustrated Encyclopedia of Human Anatomic Variation: Opus II. Cardiovascular System. “Eighty-three Variations in Circle of Willis.” Accessed September 25, 2015.

Centers for Disease Control and Prevention. “Stroke.” Accessed September 24, 2015.

Centers for Disease Control and Prevention. “Stroke Facts.” Accessed September 24, 2015.

Mayfield Brain and Spine. “Carotid Stenosis (Carotid Artery Disease).” Accessed September 24, 2015.

Neuroscience on the Web, California State University, Chico. “CMSD 620 Neuroanatomy of Speech, Swallowing and Language, Blood Supply.” Accessed September 25, 2015.

The Internet Stroke Center. “Blood Vessels of the Brain.” Accessed September 24, 2015.

The Internet Stroke Center. “Ischemic Stroke.” Accessed September 24, 2015.

Wikipedia. “Circle of Willis.” Accessed September 24, 2015.

Demonstration of the Anatomy of the Circle of Willis

Circle of Willis 3D Tutorial

Transcranial Doppler Ultrasonography of the Circle of Willis

© 2015 M G Del Baglivo


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