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