Definition of Circle of Willis

Circle of Willis: An arterial circle at the base of the brain that is of critical importance. The circle of Willis receives all the blood that is pumped up the two internal carotid arteries that come up the front of the neck and that is pumped from the basilar artery formed by the union of the two vertebral arteries that come up the back of the neck. All the principal arteries that supply cerebral hemispheres of the brain branch off from the circle of Willis.

The circle of Willis is often not complete. Maximally, only a third of people enjoy a complete circle of Willis. This is of importance in the event that one of the major arteries (an internal carotid or vertebral artery) supplying the circle of Willis is occluded. The presence of a complete circle of Willis permits a continuing supply of blood to the entire brain and helps avert a stroke.