The pancreas, in addition to its digestive functions, secretes two important hormones, insulin and glucagon, that are crucial for normal regulation of glucose, lipid, and protein metabolism. Although the pancreas secretes other hormones, such as amylin, somatostatin, and pancreatic polypeptide, their functions are not as well established.
The pancreas is composed of two major types of tissues: the acini, which secrete digestive juices into the duodenum, and the islets of Langerhans, which secret insulin and glucagon directly into the blood. The human pancreas has 1 to 2 million islets of Langerhans, each only about 0.3 millimeters in diameter and organized around small capillaries into which its cells secrete their hormones.
The beta cells, constituting about 60 per cent of all the cells of the islets, lie mainly in the middle of each islet and secrete insulin and amylin, a hormone that is often secreted in parallel with insulin, although its function is unclear. The alpha cells, about 25 per cent of the total, secrete glucagon. And the delta cells, about 10 per cent of the total, secrete somatostatin.