The circle of Willis is a junction of several important arteries at the bottom part of the brain. It helps blood flow from both the front and back sections of the brain.
The circle of Willis gets its name from the physician Thomas Willis, who described this part of the anatomy in 1664.
It may play a passive role in protecting a person from some health issues, such as stroke. However, it has an association with intracranial aneurysms. Keep reading to learn more about the circle of Willis, including its function and associated diseases.
The circle of Willis is an important junction of arteries at the base of the brain. The structure encircles the middle area of the brain, including the stalk of the pituitary gland and other important structures.
Two arteries, called the carotid arteries, supply blood to the brain. They run along either side of the neck and lead directly to the circle of Willis.
Each carotid artery branches into an internal and external carotid artery. The internal carotid artery then branches into the cerebral arteries. This structure allows all of the blood from the two internal carotid arteries to pass through the circle of Willis.
The structure of the circle of Willis includes:
- left and right internal carotid arteries
- left and right anterior cerebral arteries
- left and right posterior cerebral arteries
- left and right posterior communicating arteries
- basilar artery
- anterior communicating artery
The circle of Willis is critical, as it is the meeting point of many important arteries supplying blood to the brain. The internal carotid arteries branch off from here into smaller arteries, which deliver much of the brain’s blood supply.
The circle of Willis plays an important role, as it allows for proper blood flow from the arteries to both the front and back hemispheres of the brain. The arteries that stem off from the circle of Willis supply much of the blood to the brain.
The circle of Willis also serves as a sort of safety mechanism when it comes to blood flow. If a blockage or narrowing slows or prevents the blood flow in a connected artery, the change in pressure can cause blood to flow forward or backward in the circle of Willis to compensate.
This mechanism could also help blood flow from one side of the brain to the other in a situation in which the arteries on one side have reduced blood flow. In an emergency, such as a stroke, this may reduce the damage or aftereffects of the event.
Importantly, the circle of Willis does not actively carry out the function. Instead, the natural shape of the circle and the way that pressure acts in the area simply allow for bidirectional blood flow when necessary.
Structural differences in the circle of Willis are common. The classic, complete anatomy of a circle of Willis is, in fact, only apparent in a minority of cases.
It is more common to see one of a few variations in the structure of the circle of Willis. A study in PLOS ONE notes that about 70% of people may have an incomplete circle of Willis.
An incomplete circle of Willis can take a few different forms. A smaller study in the Journal of Morphological Sciences found that at least one variation was present in the circle of Willis in 54% of cases. The most common structural difference was the absence of a posterior communicating artery connecting to the circle of Willis.
Other variations are also common. For instance, a change in the anterior communicating artery may keep the circle of Willis from being complete. A few different changes are possible here, including fenestration and duplication.
Fenestration occurs when a single vessel divides into two channels and then becomes one channel again later.
Duplication means that there are two distinct arteries where there is usually one.
In rarer cases, people may have an azygos anterior cerebral artery (ACA), which occurs when the two ACA blood vessels fuse into one.
Genetics may play a role here, and certain forms of incomplete circle of Willis structures may be more common among family members.
Several diseases and conditions have an association with the circle of Willis, including:
The structure and function of the circle of Willis may protect against stroke in people who have a complete circle of Willis. The complete circle allows blood to go from one side of the brain to the other, even when blockages or thinning vessels occur.
The change in pressure from a blockage or thinning vessel could cause blood to flow backward through the circle of Willis and still reach similar areas of the brain or other important structures. This process is called collateral circulation, and it may protect a person from major events or a lack of oxygen in the brain due to impaired blood flow.
However, collateral circulation is not a guaranteed effect, and it may only occur in people with a mostly or fully complete circle of Willis.
The circle of Willis is a very common place for intracranial aneurysms to occur. An estimated 85% of all intracranial aneurysms occur here.
Aneurysms refer to arteries that bulge or balloon out. The major risk with aneurysms is rupturing, which causes bleeding in the brain. A ruptured aneurysm can cause an extremely severe headache alongside other symptoms, such as vision problems, light sensitivity, and a stiff neck.
Subclavian steal syndrome
Subclavian steal syndrome is a rare condition that may go unnoticed, as it is largely asymptomatic.
It occurs when there is not enough blood supply to an arm through the subclavian artery. When this happens, extra blood flows through the circle of Willis to make up for the lack of blood supply, but this may result in insufficient blood going to the brain.
If symptoms do occur, they may appear due to an ischemic event. Some possible symptoms of subclavian steal syndrome include:
- a numb arm or arm pain
- general fatigue
- cold skin from lack of blood supply
- dizziness when exercising
The circle of Willis is an important juncture in the body. As the carotid arteries travel up to the base of the brain, they branch into the circle of Willis. From there, other smaller arteries that stem off from the circle supply much of the blood to the brain.
The circle of Willis may also play a passive role in protecting against stroke in people who have a complete circle of Willis. However, a complete circle of Willis is less common than other variants.
Many aneurysms also occur within the circle of Willis, although the risk of rupture may be less than it would be in smaller arteries.