THE BRAIN
Blood supply to the brain
Nutrients and oxygen are carried to the brain by many blood vessels. These are found both on the surface of the brain and deep within it. As well as bringing in chemicals which the brain needs to function properly, the blood removes materials from the brain, either because they are unwanted or because they are required elsewhere in the body.
The brain gets its blood supply from two pairs of arteries, the internal carotid arteries and the vertebral arteries. At the base of the brain, the vertebral arteries (right and left) join together to form the basilar artery. The basilar artery joins up with the internal carotid arteries to form a ring known as the Circle of Willis (named after Thomas Willis, the great 17th century neuroanatomist), from which the cerebral arteries (anterior, middle and posterior) branch off into the brain. The Circle of Willis acts as a protective mechanism - even if one of the arteries gets blocked, blood will continue to flow around the circle and maintain the supply.
Although the brain accounts for only about 2% of our body weight, it receives 15-20% of the body's blood supply. Because the brain is so vital to our survival, and brain cells die so quickly if deprived of oxygen, the body automatically gives the brain top priority when it comes to blood. Even when other organs are crying out for blood, the body will always try to maintain the brain's supply.