The Circle of Willis

The Arterial Circle of WillisThere is a grouping of arteries near the base of the brain which is called the Arterial Circle of Willis. It is named after a very influential English physician named Thomas Willis, who discovered it and then published his findings in his 1664 work, a seminal peace on the inner workings of the brain entitled, Cerebri anatomi (from the Latin for Anatomy of the Brain).Purpose of the Circle

The Circle of Willis is truly one of the most important, yet overlooked areas in the brain, and one of the most efficiently designed systems in the human body.

While it does not specifically help one to think or react, per se, the more glamorous portions of the brain could not possibly function without the Circle.

The entire supply of blood to the brain is efficiently supplied by these arteries, and if one thinks that it's not important to have a good supply of blood to the brain, a victim of a stroke will surely be able to explain otherwise (a stroke occurs when the blood supply to the brain is temporarily interrupted).

Physiological Details

The key element which makes the Circle of Willis so spectacularly effective in doing its job and so very remarkable in its design is the fact that the arteries contained therein (the Anterior Cerebral and the Posterior Cerebral Arteries being two of the main ones) are arranged in a looping pattern which allows for a complex system of redundancy built into the flow of the blood.

This is important because if for some reason one of the arteries is having problems, the others can do their best to pick up the slack and maintain cerebral profusion, wherein the pressure of blood flowing through the various parts of the brain is kept at normal levels.

While the ingenious design of the Circle helps significantly to prevent certain conditions from developing within the brain related to blood flow, it is not entirely perfect. There is always the possibility of several conditions occurring, such as arterial blockage (usually due to a blood clot) or cerebral hemorrhage, either of which can cause devastating strokes and other cerebral abnormalities.

Rather than being frightening, however, this really should go to show how important blood flow is when discussing something as delicate as the brain, and just how thankful one should be to have the Circle of Willis in there doing its best to look out for the cerebral functions.

What Does the Circle Look Like?

If one was to look in an anatomy book, they will most likely see a chart displaying a common shape of the various arteries in the Circle of Willis, but the truth is that this textbook example, while a decent representation, actually varies quite widely in the human brain, as far as the specific shape is concerned (but not in terms of function).

The textbook variation of the Circle only occurs, in fact, roughly 34.5% of the time, though they tend to function similarly no matter how they are set up.

That, then, is the Arterial Circle of Willis. One of the greatest part of the human brain, bar none.


The Circle of Willis. The Internet Stroke Center.

Circle of Willis. Medline Plus: Medical Encyclopedia.