What is the function of the circle of Willis?

To understand the function of the circle of Willis, it helps to think of this structure as a traffic roundabout.

Imagine that each of the arteries that make up the CoW is a different road. The two main “highways” that provide the most traffic (blood flow) are the internal carotid artery pathway and the vertebral artery pathway. The other, smaller roads in the CoW form a roundabout for these two highways to connect and exchange traffic.

The CoW helps ensure that if there’s a roadblock or “accident” in one of the main highways (one of your brain’s main arteries), the other highway can help move traffic (blood) to the affected part of your brain. In other words, a complete circle of Willis offers a detour route in the case of a blockage or issue.

If you have a complete circle of Willis, it may prevent you from experiencing severe effects from disease or damage to the arteries of the CoW. These conditions may include:

If you have an incomplete CoW, this roundabout doesn’t work as a fail-safe. It may increase your risk for more severe effects of these cerebrovascular conditions, especially stroke.