The circle of Willis is a ring-shaped network of arteries at the base of the brain that acts as a backup system for blood flow. If one artery feeding the brain becomes narrowed or blocked, this structure can reroute blood through alternative pathways to keep brain tissue alive. It sits just outside the brain tissue itself, in the fluid-filled space surrounding the brain, wrapped around the stalk of the pituitary gland.
How the Circle Is Built
The circle of Willis connects two separate blood supply systems that feed the brain. The internal carotid arteries, which run up either side of the neck, supply the front and middle portions of the brain. The vertebral arteries, which travel up through the spine, merge into a single vessel and supply the back of the brain. These two systems operate somewhat independently until they meet at the circle of Willis, where small connecting arteries bridge them together.
The connecting vessels are what make the circle work. The anterior communicating artery links the left and right sides at the front, while a pair of posterior communicating arteries bridge the front and back supply systems on each side. Together, these connections form the pentagon or ring shape that gives the structure its name. The whole arrangement functions like a traffic roundabout: if one road in is blocked, blood can still reach every exit through alternative routes around the circle.
Why It Matters for Brain Blood Flow
The brain consumes roughly 20% of the body’s oxygen despite making up only about 2% of body weight. Even a few minutes without blood flow can cause permanent damage. The circle of Willis exists as a safeguard against exactly this scenario.
When a carotid artery on one side becomes severely narrowed or blocked, the connecting arteries allow blood from the opposite carotid, or from the vertebral system, to flow across and compensate. The system works on a principle similar to an electrical circuit called a Wheatstone bridge: pressure differences across the network automatically drive blood toward the areas that need it most. If a blockage occurs on one side, the communicating vessels on the front of the circle carry substantially more flow to make up the difference. The posterior connections help maintain pressure to the middle portions of the brain, particularly when blockages affect opposite sides of the circle.
For these connecting arteries to actually reroute blood effectively, they need to be at least 0.4 to 0.6 millimeters in diameter. The posterior communicating arteries, being longer, need to be slightly wider than the anterior one to overcome the greater resistance that comes with a longer vessel.
Most People Don’t Have a “Textbook” Circle
One of the most striking facts about the circle of Willis is how rarely it matches the diagram in an anatomy textbook. A study of cadaver specimens found that only 8% had a fully complete, symmetrical circle. The remaining 92% had some kind of variation, with one or more connecting arteries being unusually small, absent, or duplicated. The most common variations involve the anterior communicating artery at the front of the circle.
These variations are not diseases. Most people live their entire lives without knowing their circle is incomplete, because the brain’s blood supply is adequate under normal conditions. The variations only become clinically relevant when something goes wrong, like a sudden blockage in a major artery. A person with a fully intact circle has more rerouting options available and may tolerate a blockage with less damage than someone whose connecting arteries are too small to carry meaningful flow.
Aneurysms and the Circle of Willis
The circle of Willis is the most common location for a specific type of brain aneurysm called a berry aneurysm, named for its round, berry-like shape. These tend to form at points where arteries branch apart, because the vessel wall is naturally thinner at these forks and must withstand constant pressure from blood flow pushing against it.
Three spots account for the majority of cases: the junction near the anterior communicating artery, the point where the middle cerebral artery splits into branches, and the fork where the internal carotid meets the posterior communicating artery. Aneurysms in the vertebral-basilar system at the back of the circle make up fewer than 10% of cases. About 20 to 30% of people with a berry aneurysm have more than one.
Most berry aneurysms never rupture and are often discovered incidentally during brain imaging done for other reasons. When one does rupture, it causes bleeding into the space around the brain, which is a medical emergency. The location of the aneurysm within the circle influences both the symptoms it produces and how it can be treated.
The Name Behind the Circle
The structure is named after Thomas Willis, a 17th-century English physician who published a landmark work on brain anatomy in 1664 called Cerebri Anatome. That book contained the first detailed illustration of the arterial ring at the base of the brain and introduced the word “neurology” into the medical vocabulary. Willis recognized the structure’s purpose clearly: he described it as a system by which “full circulation to all parts of the brain can be maintained even when the carotid or vertebral arteries are blocked.” That observation, made over 350 years ago, remains the core reason the circle of Willis matters in modern medicine.