The Islets of Langerhans secrete insulin and glucagon directly into the blood. Inadequate secretion of insulin, inadequate structure or function of insulin or its receptors results in impaired metabolism of glucose, carbohydrates, proteins and fats, characterized by hyperglycemia and glycosuria. Hyperglycemia is the most frequently observed sign of diabetes and is considered the etiologic source of diabetic complications both in the body and in the eye. Glucagon is a hormone that opposes the action of insulin. It is secreted when blood glucose levels fall.
Glucagon increases blood glucose concentration partly by breaking down glycogen in the liver.
Following a meal, glucose is absorbed into the blood. In response to increased blood glucose levels, insulin is secreted causing rapid uptake, storage, or use of glucose by the tissues of the body. Unused glucose is stored as glycogen in the liver. Between meals, when blood glucose is at minimal levels, tissues continue to require an energy source to function properly. Stored glycogen, via glucagon, is converted to glucose by a pathway known as glycogenolysis.