Physiology of pancreas as the secretor of insulin

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 main purpose of this chapter is to discuss the physiologic roles of insulin and glucagon and the pathophysiology of diseases, especially diabetes mellitus, caused by abnormal secretion or activity of these hormones.

Physiologic Anatomy of the Pancreas. 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
millimeter in diameter and organized around small capillaries into which its cells
secrete their hormones.

The islets contain three major types of cells, alpha, beta, and
delta cells, which are distinguished from one another by their morphological and
staining characteristics.

Physiology of pancreas

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. In addition, at least one other type of cell,
the PP cell, is present in small numbers in the islets and secretes a hormone of uncer-
tain function called pancreatic polypeptide.

The close interrelations among these cell types in the islets of Langerhans allow
cell-to-cell communication and direct control of secretion of some of the hormones
by the other hormones.

For instance, insulin inhibits glucagon secretion, amylin
inhibits insulin secretion, and somatostatin inhibits the secretion of both insulin and
glucagon.

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