Cerebral Arteries and Circle of Willis

The cerebral arteries deliver oxygenated blood to the brain through two paired systems: the internal carotid arteries forming the anterior circulation and the vertebral arteries, joining as the basilar artery, forming the posterior circulation. At the base of the brain these systems are linked by the circle of Willis, an anastomotic ring of arteries that can redistribute flow between territories when one vessel is narrowed or occluded.

Definition

The cerebral arteries are the arteries supplying the brain, derived from the internal carotid and vertebrobasilar systems; the circle of Willis is the anastomotic arterial ring at the base of the brain that interconnects these systems through the anterior and posterior communicating arteries.

Scope

This entry covers the major cerebral arteries (anterior, middle, and posterior cerebral arteries and their parent vessels), the components and connections of the circle of Willis, the territories each artery supplies, and the common anatomical variants of the circle. It is a reference description of normal arterial anatomy and its variations, not clinical guidance on stroke or vascular disease.

Core questions

• What are the components of the anterior and posterior cerebral circulations?
• Which arteries form the circle of Willis and how do they connect the two circulations?
• What region of the brain does each major cerebral artery supply?
• How often is the circle of Willis complete, and what are its common anatomical variants?

Key concepts

• Internal carotid artery (anterior circulation)
• Vertebrobasilar system (posterior circulation)
• Anterior, middle, and posterior cerebral arteries
• Anterior and posterior communicating arteries
• Arterial territories
• Collateral flow and anastomosis
• Circle-of-Willis variants (hypoplasia, fetal-type posterior cerebral artery, absent communicating arteries)

Mechanisms

The internal carotid artery enters the cranium and divides into the anterior cerebral artery (supplying medial frontal and parietal cortex) and the middle cerebral artery (supplying the lateral hemisphere and deep structures via lenticulostriate branches). The vertebral arteries unite to form the basilar artery, which gives off cerebellar and pontine branches before dividing into the two posterior cerebral arteries (supplying the occipital lobe and inferomedial temporal lobe). The circle of Willis closes the ring: the anterior communicating artery links the two anterior cerebral arteries, and the paired posterior communicating arteries connect each internal carotid to the posterior cerebral artery, joining the anterior and posterior circulations. Each artery perfuses a defined territory, mapped in detail for the hemispheres and for the brainstem and cerebellum (tatu-1998, tatu-1996). When a vessel is compromised, the communicating arteries can provide collateral flow, but only when they are of adequate caliber (standring-2020).

Clinical relevance

Because each cerebral artery has a characteristic territory, the pattern of a deficit reflects which vessel is involved, and the completeness of the circle of Willis influences how well collateral flow can compensate. This anatomy underlies the interpretation of vascular imaging and stroke localization; the content here is educational reference material describing structure and variation, not advice for diagnosing or managing any individual.

Epidemiology

A fully complete, symmetric circle of Willis is present in only a minority of people; imaging studies report frequent variants such as hypoplasia or absence of one or both communicating arteries and fetal-type origin of the posterior cerebral artery (kizilgoz-2022, barboriak-1997). These variants are anatomical findings rather than diseases, though they affect the capacity for collateral circulation.

Evidence & guidelines

Territorial maps and the standard description of the arteries are drawn from anatomical and radiological studies and reference texts (tatu-1998, tatu-1996, standring-2020); the prevalence of circle-of-Willis variants is documented in MR-angiography series (kizilgoz-2022, barboriak-1997). This is descriptive anatomy and not a clinical practice guideline.

History

Thomas Willis described the basal arterial ring in Cerebri Anatome (1664), and it has carried his name since. Detailed mapping of the territories supplied by each cerebral artery was refined through twentieth-century anatomical and imaging work, notably the territory atlases of Tatu and colleagues, while the advent of magnetic resonance angiography allowed systematic study of how often the circle is complete and how its variants are distributed (tatu-1998, tatu-1996, kizilgoz-2022).

Key figures

• Thomas Willis
• Henri Duvernoy
• Laurent Tatu
• Julien Bogousslavsky

Seminal works

• tatu-1998
• tatu-1996
• standring-2020

Frequently asked questions

Which arteries make up the circle of Willis?

The two anterior cerebral arteries joined by the anterior communicating artery, the two internal carotid arteries, the two posterior communicating arteries, and the two posterior cerebral arteries arising from the basilar artery.

Is the circle of Willis always complete?

No. A fully complete, symmetric ring is found in only a minority of people; hypoplastic or absent communicating arteries and a fetal-type posterior cerebral artery are common anatomical variants documented on MR angiography.