What is the Circle of Willis?

Christine Hudson

The Circle of Willis is a ring or circle of arteries located at the base of the brain. It is also known by several other names, most commonly the cerebral arterial circle and arterial Circle of Willis. This part of the brain was first discovered by the English doctor Thomas Willis in the 17th century. The circle is important to itself and the brain, because the way the arteries are laid out provides a form of backup in case any of them is obstructed. If one artery is constricted or blocked, the flow of blood from the other arteries in the circle will most often be enough to preserve the necessary flow to the brain and prevent damage.

The Circle of Willis is a ring or circle of arteries located at the base of the brain.
The Circle of Willis is a ring or circle of arteries located at the base of the brain.

Once the Circle of Willis was discovered, years of research were conducted into its purpose and functions. It has been determined that the circle provides the necessary blood and fuel for all thought processes and even physical processes in the brain. This very efficient cluster of arteries essentially provides all of the blood that comes and goes from the brain, which it needs to survive and operate.

An artery transports oxygen-rich blood to various parts of the body.
An artery transports oxygen-rich blood to various parts of the body.

While it has been discovered that the looping pattern in which the arteries lay creates reinforcement for blood flow and greatly reduces the risk of blood pressure changes in the brain, it is not completely infallible. Some blockages caused by debris or blood clots or even hemorrhages are still able to cause devastating effects such as strokes. Sometimes even death can occur if these blockages happen to be large enough or occur in just the wrong spot.

Many assume that this structure can be represented correctly by the diagrams shown in books, which is much more complex in shape than a simple circle. The truth, however, is that about two-thirds of the human population does not have a complete “circle" — most have one or more connections missing. No concrete evidence supports that a person with an incomplete Circle of Willis is at greater risk for strokes.

While the Circle of Willis may not be studied as closely as higher functions of the brain and thought, it is generally accepted as the most important part of the brain. Without this structure, blood would not flow properly to the brain and its functions would be impossible. It is often said that life itself would not even be possible without this vital structure to keep blood flowing regularly.

The arteries in the Circle of Willis may be enough to preserve blood flow to the brain even if one artery is constricted or blocked.
The arteries in the Circle of Willis may be enough to preserve blood flow to the brain even if one artery is constricted or blocked.

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Discussion Comments


My husband has been having a lot of issues neurologically because he is missing both sides of his Circle of Willis. He gets ice pick headaches then he has slurred speech, has a gait walk and other symptoms because the blood doesn't flow right. They just did a study in July how people who are missing one or more components have migraine headaches and when you have a stroke it could be fatal.


I have an 80 percent blockage in my innominate artery, and have no symptoms (I am called asymptomatic). I have no or little blood pressure in my right arm, however, can use it quite well; they can prove now that I have not lost anything in my Circle of Willis and in fact, my left vertebral has taken over for my right vertebral.

The body does wonders and can repair itself as I have never had high blood pressure and they have my cholesterol normal or below now. No operation is recommended, but they will check me every six months. Also, it is very important to exercise and eat healthy.


I've been recently told that I have near 100 percent blockage in all of the arteries in my circle of willis. Both left and right carotid arteries completely block (internal and external) the two arteries at the back of the brain is 100 percent blocked and the other is 80 percent blocked. I've had a series of small strokes yet I'm still alive.

Yes, I have short term memory loss and my prognosis is maybe three to six months before the remaining 20 percent flow is completely reduced. Thus I've been given a death sentence and there is no cure or surgery that can fix it. Surgery would be too risky with no significant improvements for me. I'm hoping for a miracle.


@anamur-- I'm not an expert either but if a person is born with an incomplete Circle of Willis and has functioned just fine without it for years, I don't think there would be any problems because of it. If there was, it would have happened from the very beginning.

The only way there would be a problem is if there's some sort of a blockage in one or more of these arteries.

I have a relative that lost his eye sight because of a Circle of Willis aneurysm. So I'm guessing that there must be something else going on with your friend.


I completely agree with the statement that life would not be possible without the Circle of Willis functioning. If the Circle of Willis didn't provide and regulate blood flow to the brain, the brain would not receive the oxygen necessary to keep functioning. Brain cells would die and so would the person.

I have seen this personally when my uncle had a heart attack and his heart stopped for fifteen minutes. They were able to revive his heart, but since his brain did not receive blood flow and oxygen for fifteen minutes, most of his brain cells had already died. The doctors kept him on a machine to keep him alive for a couple of days until he passed away.

Without blood flow to the brain, we really can't live for long.


Even if it is pretty common for people to have an incomplete Circle of Willis, doesn't it lead to some issues over a lifetime?

I actually have a friend who is missing a posterior communicating artery in his Circle of Willis. He's in his late fifties and has been very healthy until now. Recently though, he's been experiencing some memory problems.

I wonder if this is because of his Circle of Willis anatomy? He's due for an MRI next month and I'm trying to support him through the process. I am not very knowledgeable about brain anatomy, but if someone is missing some arteries in the Circle of Willis, I think that would put them under a bigger risk of cognitive and functional issues in the brain because they have less arteries to rely on for blood supply.

What do you think?

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