Glucagon is an endocrine hormone which acts to raise the level of glucose in the bloodstream. Glucagon is produced by the alpha cells of the pancreas, where it is stored until hypoglycemia occurs. It has an effect opposite that of insulin, which acts to reduce blood glucose levels. In the liver, glucagon stimulates the breakdown of glycogen into glucose, which is then released into the bloodstream. A similar reaction occurs in muscle tissue, although the glucose produced there is used by the muscle and not released. Glycogenolysis can maintain blood glucose levels for several hours, until the glycogen stores are depleted.
In the longer term, glucagon also stimulates gluconeogenesis in the liver and kidneys; a process in which new glucose molecules are created from chemicals released by the breakdown of protein and fat. Thus the body can maintain blood glucose levels over a period of days to weeks at the expense of muscle and fat stores.