The brain is highly metabolically active, yet has no effective way to store oxygen or glucose. It depends on a large and stable blood supply. The brain is approximately 2% of body weight, but uses ~15% of cardiac output and accounts for 25 % of oxygen consumption. Autoregulation of cerebral blood supply maintains a consistent flow of blood to the brain. However, local flow-rates vary, dependent upon which part of the brain is active at any given time. The major branches of the arterial blood supply to the brain are shown in figure 7.