The brain is critically dependent on a set of large blood vessels, illustrated in Figure 1, to supply the blood it needs. Two of the most important arteries—the anterior and middle cerebral arteries—branch from the main arteries of the neck, the carotid arteries (these are the arteries that you feel pulsing if you press your fingers to the side of your neck). In each hemisphere, these two cerebral arteries supply blood to about two thirds of the hemisphere (see Figure 1b). The posterior parts of each hemisphere are supplied by the posterior cerebral arteries. These arteries derive from the basilar artery, running along the bottom of the brainstem, which in turn arises from arteries that ascend in the spinal cord. A ring of connections among the cerebral arteries, called the circle of Willis, is thought to be an emergency mechanism through which blood might be supplied in the event of a blockage or rupture.

Figure 1  The Blood Supply of the Human Brain
The anterior, middle, and posterior cerebral arteries—the three principal arteries that provide blood to the cerebral hemispheres—are depicted here in basal (a), midsagittal (b), and lateral (c) views of the brain. The basilar and internal carotid arteries form a circle at the base of the brain known as the circle of Willis.