What arterial ring at the base of the human brain helps connect its major blood supplies?

The Circle of Willis is the arterial ring at the base of the human brain that links its major blood supplies.

This polygon of blood vessels surrounds the optic chiasm and the infundibulum of the pituitary region. It connects branches of the internal carotid arteries with branches of the vertebrobasilar system, allowing blood to reach the brain through interconnected routes.

The circle is named after English physician Thomas Willis, whose 1664 book Cerebri Anatome described the cerebral circulation in detail. Its communicating arteries can provide collateral flow if one major route becomes narrowed, although this backup is often incomplete because the circle varies considerably among individuals.

The Circle of Willis is not a single large artery and does not guarantee protection from stroke. Aneurysms commonly occur at arterial junctions in this region, especially where the anterior communicating artery meets nearby vessels. The circle’s protective value therefore depends on its actual anatomy and the severity of any blockage.