In contrast to its small mass, the brain receives a very large supply of blood, and the body, seemingly taking no chances that this supply is interrupted, uses three redundant arterial circuits to connect the thoracic aorta to the cerebral vasculature. In terms of function, the importance of the brain is obvious, and it seems intuitive that the body should cater extravagantly to its command center. There are metabolic reasons, however, behind these features of the cerebrovascular system.
Neurons can extract energy from only a few, critical types of molecules.
The most familiar compound, and the one preferred by the brain, is glucose. The brain cannot metabolize fat, and it cannot use protein for energy. In addition to having few energy sources, neural tissue can store neither glucose nor the two ketones. A limited reserve of glucose is, however, maintained in the liver in the form of glycogen. Whether the energy metabolites are derived from dietary sources or the liver, a reliable vascular pathway is required.