Neurovascular anatomy is important to understand, both for neurocritical care as well as vascular. The Circle of Willis is an important anatomical structure, deep in the center of the brain. Blood flow enters the brain via the carotid arteries and then is distributed via the Circle to the rest of the brain. The Circle of Willis allows blood from the left carotid artery to supply the right side of the brain and vice versa. This means that stenosis or blockage of the carotid artery is better tolerated if the Circle is intact. In this illustration from The Radiologist Page, you can see how decreased flow in one carotid artery can be compensated if blood is able to flow through the Circle of Willis. Overall flow may be diminished, but it won’t be absent on one side.
From a neurocritical care standpoint, you can see how the various cerebral arteries, the source of most acute ischemic strokes, disseminate out from the center. The Circle of Willis is also the location of most aneurysmal subarachnoid hemorrhages.