Energy is required for the normal functioning of the organs in the body. Many tissues can also use fat or protein as an energy source but others, such as the brain and red blood cells, can only use glucose.
Glucose is stored in the body as glycogen. The liver is an important storage site for glycogen. Glycogen is mobilized and converted to glucose by gluconeogenesis when the blood glucose concentration is low. Glucose may also be produced from non-carbohydrate precursors, such as pyruvate, amino acids and glycerol, by gluconeogenesis. It is gluconeogenesis that maintains blood glucose concentrations, for example during starvation and intense exercise.
The pancreas has both endocrine and exocrine functions. The endocrine tissue is grouped together in the islets of Langerhans and consists of four different cell types each with its own function. Alpha cells produce glucagon. Beta cells produce proinsulin. Proinsulin is the inactive form of insulin that is converted to insulin in the circulation. Delta cells produce somatostatin. F or PP cells produce pancreatic polypeptide.
Insulin secretion is increased by elevated blood glucose concentrations, gastrointestinal hormones and Beta adrenergic stimulation. Insulin secretion is inhibited by catecholamines and somatostatin.
Insulin and glucagon work synergistically to keep blood glucose concentrations normal.
Insulin: An elevated blood glucose concentration results in the secretion of insulin: glucose is transported into body cells. The uptake of glucose by liver, kidney and brain cells is by diffusion and does not require insulin. Click on the thumbnail for details of the effect of insulin:
Glucagon: The effects of glucagon are opposite to those of insulin. Click on the thumbnail for details on the effect of glucagon:
The most important cellular energy source is glucose.