The brain has the highest metabolic rate of any organ structure in the human body. Although the human brain contributes only 2% to the total body weight, it uses approximately 16% of the body's cardiac output and accounts for nearly 25% of the body's oxygen consumption (1). Brain-associated endothelium includes all endothelial cells (ECs) found within the brain, including ECs of the blood–brain barrier (BBB) and circumventricular organs (CVOs) (Figure 124.1). One of the major functions of the endothelium within the brain is to direct the flow of oxygenated blood. It is estimated that approximately 600 to 700 mL of blood flows through the carotid arteries and their branches every minute to distribute oxygenated blood to the anterior portion of the brain. The capillaries, the principal vasculature within the brain, are very important components of the brain's supply system. Because of their small diameter and large number per volume of brain tissue, these capillaries provide a large surface area for delivering adequate supplies of glucose and oxygen.

BLOOD SUPPLY TO THE BRAIN

The circulatory system supplying the human brain is detailed in several references (2–4). Briefly, the brain receives its arterial supply from the carotid and vertebral arteries, which begin extracranially and run through the neck and base of the skull to reach the cranial cavity. At the base of the ear, the carotid artery bifurcates into internal and external branches.