Pancreas

The pancreas is a composite gland: it is both exocrine and endocrine. Its endocrine part is the Islets of Langerhans.

A normal human pancreas has about 1 to 2 million islets; together they are only 1 to 2 per cent of its tissue.

The two main cell types in an islet are α-cells, which secrete glucagon, and β-cells, which secrete insulin.

Glucagon is a peptide hormone that acts mainly on liver cells (hepatocytes). It stimulates glycogenolysis and gluconeogenesis, both raising blood sugar (hyperglycemia), and it reduces cellular uptake and use of glucose. Glucagon is a hyperglycemic hormone.

Insulin is a peptide hormone central to glucose homeostasis. Its main targets are hepatocytes and adipocytes (fat cells of adipose tissue), whose uptake and use of glucose it increases, so glucose moves rapidly out of the blood and blood glucose falls (hypoglycemia).

Insulin also stimulates the conversion of glucose to glycogen (glycogenesis) in its target cells.

Blood glucose is thus kept steady by insulin and glucagon acting together.

Prolonged hyperglycemia leads to diabetes mellitus, a complex disorder in which glucose is lost in the urine and harmful ketone bodies form. It results from insulin deficiency and/or insulin resistance, and patients are successfully treated with insulin therapy.