Circle of Willis: How It Protects Brain Blood Flow

The circle of Willis is a ring-shaped network of arteries at the base of the brain that connects the two major blood supplies feeding your head. It sits just above your pituitary gland, roughly behind your eyes, and its job is to act as a roundabout for blood: if one feeder artery gets blocked or narrowed, blood can reroute through the ring to keep all parts of the brain supplied. What makes this structure especially interesting is that the “textbook” complete circle is actually uncommon. Imaging studies consistently find that fewer than half of people have a fully intact version, which has real consequences for stroke risk, migraine, surgery, and long-term brain health.

What the Circle Looks Like and What It Connects

Your brain gets blood from two systems: a pair of internal carotid arteries feeding the front, and a pair of vertebral arteries (which merge into the basilar artery) feeding the back. The circle of Willis is the junction where these two systems meet. It is made up of short connecting segments. The anterior communicating artery bridges the left and right sides at the front. A pair of posterior communicating arteries bridge the front and back circulations on each side. Together with segments of the larger cerebral arteries, these connectors form a closed loop, at least in theory.

The naming goes back to 1664, when Thomas Willis published De Cerebri Anatome, a landmark anatomy text that included detailed drawings by Christopher Wren (the same person who later designed St Paul’s Cathedral in London). Willis recognized the ring as a compensatory system: if one vessel failed, the others could pick up the slack.¹ That insight has held up for nearly four centuries, though modern research has added layers of nuance.

Most People Do Not Have a Complete Circle

One of the most consistent findings in brain vascular imaging is how variable the circle of Willis is from person to person. In a well-known MR angiography study of 150 subjects, only about 42% had an entirely complete circle. The front half was intact in roughly three-quarters of people, but the back half was complete in only about half.² A meta-analysis looking specifically at the posterior communicating arteries found that over half of the general population has some form of variation in this region, with roughly one in five people missing a functioning posterior communicating artery on one side and a similar proportion missing both.³

These are not rare anomalies. They are normal human variation, and the most common pattern in the posterior circulation is actually the “variant” form where one or both posterior communicating arteries are too small to carry meaningful flow. A large imaging series from a single hospital found that the most common posterior configuration was hypoplasia or absence of both posterior communicating arteries, meaning the front and back halves of the circle were effectively disconnected.⁴ Women appear to have these variants slightly more often than men, though the difference is modest.

In a stroke-focused study, about 34% of subjects had an incomplete circle overall.⁵ The spread across studies, from about a third to over half showing some incompleteness, largely comes down to how strictly “complete” is defined and what imaging technique is used. Smaller connecting arteries can be too thin to show up on standard MR scans even when they exist, which inflates the apparent incompleteness rate.

Why the Variations Exist Before Birth

The circle of Willis starts forming remarkably early in fetal development. The internal carotid arteries are the first to take shape, starting around embryonic day 24, because the developing brain needs blood long before the posterior circulation matures. Initially, the back of the brain gets its supply through temporary connections from the carotid system. As the embryo grows, the basilar and vertebral arteries become independent, and the connecting segments that will form the circle begin to appear.⁶

A study tracking human embryos at different Carnegie stages found that many embryos still had an incomplete circle at the latest stages examined. Out of 20 embryos studied, 13 had an unclosed ring, and the researchers noted it was difficult to predict from the embryonic anatomy whether the circle would eventually close, since the same variations seen in embryos are commonly found in adults.⁷ In other words, many of the “incomplete” circles seen on adult brain scans were never complete to begin with. They are not the result of disease or aging; they are the way those arteries developed from the start.

How the Circle Reroutes Blood When a Vessel Is Blocked

The classic explanation of the circle of Willis, unchanged since Thomas Willis himself proposed it, is that it is a backup system. If one carotid artery becomes severely narrowed, blood can flow across the communicating arteries from the opposite side or from the posterior circulation to maintain supply to the affected hemisphere. Computational modeling has helped quantify this. Simulations of different circle configurations with varying degrees of carotid narrowing found that the pressure drop to the middle cerebral artery on the affected side only becomes significant when the blockage reduces the artery’s opening by more than about 86%. When the circle has all its segments intact, the alternate pathways can raise pressure on the affected side substantially without robbing flow from other regions.⁸

That said, the compensatory theory is not quite as straightforward as it sounds. Some researchers have argued that calling the circle a “backup” oversimplifies what is going on, pointing out that the connecting arteries may serve other hemodynamic roles beyond simple rerouting during a blockage.⁹ Still, the clinical evidence that an intact circle helps during acute blockages is strong enough that surgeons rely on it daily.

Aneurysms and the Geometry of Blood Flow

Brain aneurysms, those balloon-like weak spots in artery walls, disproportionately form at or near the circle of Willis. This is not coincidence. The circle is where arteries branch and change direction, and at those branch points blood hits the vessel wall at higher speeds and sharper angles. The resulting mechanical stress, called wall shear stress, weakens the artery wall over time. Computational fluid dynamics studies have confirmed that the locations where shear stress is highest correspond closely to the sites where aneurysms most commonly appear. Differences in vessel size and asymmetric branching angles amplify this effect.¹⁰

Once an aneurysm forms, the same physics governs whether it will rupture. Modeling of patient-specific aneurysms within the circle has shown that wall stress concentrates at the neck of the aneurysm, the narrow point where it connects to the parent artery, rather than at the dome. This is one reason why the shape and size of an aneurysm’s neck matter so much in deciding whether and how to treat it.¹¹ People whose circle of Willis is asymmetric or incomplete may face altered flow patterns that increase shear stress at specific branch points, which could partly explain why aneurysms are more common in people with certain anatomical variants.

The Migraine Connection

An incomplete circle of Willis has been linked to migraine with aura, the type of migraine preceded by visual disturbances, tingling, or other sensory warning signs. In a prospective study comparing people with migraine to healthy controls, an incomplete circle was found in 73% of people with migraine with aura compared to 51% of controls. The difference was statistically significant and held for both the front and back portions of the circle.¹² Migraine without aura showed a similar trend (67% incomplete) but the difference from controls did not quite reach statistical significance.

A meta-analysis pooling available studies found an even more striking relationship. Migraine with aura was associated with roughly 3.5 times higher odds of an incomplete posterior circle and about 2.4 times higher odds of an incomplete anterior circle.¹³ The distinction between migraine with and without aura has been a consistent finding, with studies repeatedly showing the stronger association in the aura group.¹⁴

The proposed mechanism is that an incomplete circle reduces the brain’s ability to buffer fluctuations in blood flow. When a temporary drop in supply occurs, perhaps from a vasospasm or a shift in blood pressure, a complete circle can redistribute flow to compensate. An incomplete circle cannot do this as effectively, and the resulting brief, localized drop in blood flow could trigger the cortical spreading depression that produces the aura phenomenon. This is still a hypothesis, but it fits the observation that the aura type of migraine is the one most consistently associated with circle variants.

What It Means for Carotid Surgery

During carotid endarterectomy, a common operation to clear plaque from the carotid artery in the neck, the surgeon temporarily clamps the artery to work on it. For several minutes, the brain hemisphere on that side relies entirely on alternative blood routes, and the circle of Willis is the main one. Whether a patient’s circle can handle this temporary loss of flow has a direct impact on how the surgery is planned.¹⁵

Preoperative imaging to assess circle completeness can help stratify risk. One approach assigns a score based on how many collateral pathways are available. Patients with poor collateral capacity may be better candidates for general anesthesia with a temporary shunt (a tube that maintains blood flow during clamping), while those with a robust circle can more safely undergo clamping without one.¹⁶ Knowing the anatomy ahead of time does not eliminate risk, but it changes how the surgical team prepares.

Circle of Willis and Long-Term Brain Health

Beyond acute events like strokes and surgery, the circle of Willis may influence how your brain ages. White matter hyperintensities, the bright spots that show up on brain MRI scans in older adults and are associated with vascular damage, have been linked to circle variants. In one study, having an incomplete posterior circulation was associated with roughly twice the odds of more severe deep white matter changes.¹⁷ The idea is that a less connected circle leaves certain brain regions with marginal blood supply over decades, and the cumulative effect shows up as small-vessel damage.

That said, the evidence is not unanimous. A longitudinal study following community-dwelling older adults found no significant association between an incomplete circle and the progression of white matter damage over time.¹⁸ The difference may come down to what is being measured: having an incomplete circle could set up worse baseline white matter health without necessarily accelerating the rate of decline from that baseline. The research here is still evolving, and it would be premature to draw firm conclusions about whether an incomplete circle predicts dementia or cognitive decline.

What is well established is that the arteries of the circle develop atherosclerotic plaques with age, just like arteries elsewhere. A study examining over 1,200 arterial segments from the circle found that about 15% had advanced plaques, mainly in the larger vessels like the internal carotid, middle cerebral, basilar, and vertebral arteries. Only about 1% had the most dangerous complicated plaques, and calcification was found in just 3% of segments, mostly in the vertebral artery.¹⁹ The takeaway is that the circle’s arteries are not immune to the same plaque buildup that affects the rest of the cardiovascular system, and this buildup can compromise their ability to serve as collateral channels.

Children and the Developing Circle

Because many circle of Willis variants are present from birth, they affect children too. If any of the bridging segments are hypoplastic or absent, the capacity for collateral flow during a large vessel blockage is reduced, and this matters even in pediatric stroke, which is rarer than adult stroke but still occurs.²⁰ In a study of infants who had neonatal strokes in the middle cerebral artery territory, about 62% showed arterial changes on imaging, and abnormal findings on MR angiography were associated with roughly three times the odds of developing cerebral palsy. Complete arterial occlusion predicted a worse outcome than partial blockage.²¹

Some pediatric vascular diseases directly involve the circle. Moyamoya disease, a condition in which the internal carotid arteries gradually narrow, follows a pattern that mirrors the embryological development of the carotid system. The narrowing begins near the same branch point where the primitive carotid artery originally split during fetal development and spreads outward from there, suggesting a strong genetic component tied to the original formation of these vessels.²²

How the Circle Compares Across Species

The circle of Willis is not unique to humans. Most mammals have some version of it, though the details vary considerably. A comparative anatomy study found that in rhesus monkeys, dogs, and rabbits, the overall layout is close to the human version, with one key difference: the two anterior cerebral arteries merge into a single midline vessel rather than remaining paired as they do in humans. In dogs, rabbits, goats, and sheep, the branching pattern of the posterior cerebral and cerebellar arteries also differs, and in goats and sheep, a much longer segment of the internal carotid contributes to the ring itself.²³

These differences matter for research, since many stroke and aneurysm models are tested in animals. A dog’s circle of Willis functions similarly enough to a human’s that it can be used to study collateral flow, but the single anterior cerebral artery means the front-side crossover capacity is different. Meanwhile, some species, including cats and certain rodents, have circles with proportions that differ enough from humans that findings do not translate cleanly. The evolutionary persistence of the ring structure across mammals, despite wide variation in brain size and metabolic demands, speaks to how fundamental the backup function is. Any organ that consumes roughly a fifth of the body’s oxygen supply at rest, as the human brain does, benefits enormously from having more than one way for blood to reach it.