The circle of Willis is a ring- or pentagon-shaped structure of arteries located at the posterior aspect of the brain. It’s where two essential blood flow circulations for your brain connect — the internal carotid artery pathway and the vertebral artery pathway. The internal carotid arteries supply fresh, oxygen-rich blood to most of the front of your brain. The vertebral arteries do the same for most of the back of your brain.
It functions as a fail-safe. If one of the two blood vessel pathways becomes damaged, such as from a ruptured brain aneurysm or stroke, the other pathway can still provide blood flow to the affected part of your brain.