When the blood glucose level begins to fall, the liver releases glucose back into the circulating blood. Thus the liver removes excess glucose from the blood after a meal. Also, it returns it to the blood when it is needed between meals. Under normal circumstances, about 60% of the glucose in a meal is stored in the liver as glycogen and released later.<br>If the muscles are not exercised after a meal, much of the glucose transported into the muscle cells by insulin is stored as muscle glycogen. Muscle glycogen differs from liver glycogen. It cannot be reconverted into glucose and released into the circulation. The stored glycogen must be used by the muscle for energy.<br>The brain is quite different from other body tissues with regard to glucose uptake. Insulin has little or no effect on the uptake or use of glucose by the brain; the cells of the brain do not have adequate storage capacity. Also, because the brain normally uses only glucose for energy, it cannot depend on stored supplies of glycogen. Therefore, it is essential that the serum glucose be maintained at a level that provides adequate energy to these tissues. When the serum glucose level falls too low, signs and symptoms of hypoglycemia can develop quickly. These include progressive irritability, altered mental status, fainting, convulsions, and even coma.
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