At rest, the adult human brain receives approximately 15 percent of the body’s blood supply.
The brain is only a small fraction of total body mass, yet it requires a continuous delivery of oxygen and glucose. Blood reaches it mainly through the internal carotid and vertebral arteries, which connect through the arterial circle at the base of the brain. Cerebral blood flow is tightly regulated so that neural tissue receives a relatively stable supply.
The often-quoted figure of 15 percent refers to blood flow, not the brain’s share of body weight. The brain also uses roughly one-fifth of the body’s oxygen and energy under typical resting conditions, which is a related but different measurement.
A sudden interruption of cerebral blood flow can injure brain tissue rapidly, as in an ischemic stroke. The brain’s blood vessels therefore have important protective and regulatory roles beyond simply delivering nutrients.