Blood Circulation and Imaging

Abstract

Although the brain accounts for only 2% of the body weight, it is ever-active and requires a copious blood supply. The brain continuously receives 20% of the arterial blood flow from the heart, metabolizes 20% of the available oxygen, and generates 20% of the bodily energy. The mean blood flow in the normal brain is 50 ml per 100 g of brain tissue per minute. Roughly 800 ml of blood flows through the brain each minute, with 75 ml present within the organ at any moment. Each day the brain utilizes about 400 kcal or about one-fifth of a 2,000-kcal diet. It takes about 7 s for a drop of blood to flow through the brain from the internal carotid artery to the internal jugular vein. The necessity for this continuous flow is because the brain stores only minute amounts of glucose and oxygen and derives its energy almost exclusively from the aerobic metabolism of glucose delivered by the blood. Consciousness is lost if the blood supply is cut off for 8–10 s. The demand for blood is the same whether one is resting, sleeping, thinking, or exercising. This level of energy consumption is sustained by the high and privileged rate of blood flow, compared to other organs, which conveys glucose and oxygen to the brain. High metabolic activity and high oxygen consumption are characteristics of cerebral metabolism that derives its energy largely from glucose, the basic energy substrate of the brain. Glucose is metabolized through glycolysis, Krebs’ citric acid cycle, and the respiratory (electron-transport) chain. In aerobic respiration, the combination of glucose and oxygen produces energy, CO₂, and H₂O. Mitochondria carry out aerobic respiration to generate ATP (adenosine triphosphate), which serves as the cell’s powerhouse. The chemical energy stored as high-energy phosphate bonds in ATP is a constant source of energy obtained from the conversion of ATP to ADP (adenosine diphosphate). Neurons differ from cells of other organs in their requirements for oxygen. Deprived of oxygen, neurons almost always die within a few minutes because they cannot build an oxygen debt. Neurons unlike muscle and other body cells cannot survive anaerobically. This constraint has enormous medical implications. When the oxygen supply is cut off following a heart attack or suffocation, the brain dies first. Only if oxygen is restored to the brain within a few minutes will the brain retain its functional viability.