Each tissue of the body has a specialized function and depends on every other tissue so their actions must be coordinated for the body to function normally (Integration). The liver plays a central role in distributing the products of metabolism to other tissues via the bloodstream. Amino acids, sugars and some triglycerides and other nutrients are sent to the liver after absorption from the digestive tract. The liver processes these nutrients and sends them as fuels and their precursors to the cells of the body.
The brain normally uses only glucose as its fuel (Integration). The liver cellÆs glucose transporter system keeps the concentration of glucose in the hepatocyte (liver cell) equal to that in the bloodstream, from which the brain derives its sugar. The liver converts glucose, fructose, mannose and galactose to glucose-6-phosphate, which can be converted to glucose when the level of sugar in the bloodstream falls; can be stored in the liver as glycogen; can be oxidized through glycolysis and the TCA cycle for energy production; can be oxidized to acetyl-CoA for the synthesis of lipids, fatty acids and cholesterol, or for use by the brain under starvation conditions; or converted into ribose-5-phosphate and NADPH via the pentose phosphate pathway. Amino acids can also be degraded to acetyl-CoA by the liver for use by the brain during starvation.
Low blood sugar levels which result from starvation can cause irreversible brain damage (Andersen; Integration). When starvation occurs, the brain has to use ketone bodies as a source of acetyl-CoA. It uses its energy to maintain an electrical potential in the neurons so they continue to function. In starvation, neuronal signals from the brain induce the release of epinephrine from the adrenal medulla, the normal response to stress. This results in glycogen phosphorylate activation and inactivation of glycogen synthase, i.e. glucose storage is inhibited and its relea...