Endocrine System

The pancreas is a large endocrine and exocrine gland situated below and behind the stomach in the lower abdomen. The pancreas is horizontally placed such that its larger end falls to the right and its narrower end to the left. Clusters of exocrine pancreatic cells called acini secrete digestive enzymes into the stomach; while endocrine cells secrete hormones responsible for maintaining blood glucose levels.

The endocrine cells of the pancreas are contained in the islets of Langerhans which are themselves embedded in a rich network of blood and lymph vessels. About one million islet cells make up about 2% of the total pancreas: the islet cells are quite small. The three major types of endocrine cells within the islets are alpha cells, beta cells, and delta cells. Beta cells make up about 70% of islet cells and secrete insulin. Alpha cells which secrete glucagon comprise roughly 20% of islet cells. Delta cells, which comprise up to 8% of islet cells, secrete somatostatin. Another pancreatic hormone, called pancreatic polypeptide, is secreted by F cells in the islets and has just recently been characterized.

Insulin is secreted in response to high plasma glucose levels. Insulin facilitates glucose uptake into blood cells thus reducing plasma glucose levels. Glucagon has the opposite effect; low plasma glucose triggers the breakdown of stored glucogen in the liver and glucose release into the blood. By balancing these two hormones, the islets continually regulate circulating glucose levels. Both hormones are contained within secretory vesicles in the cells which release them. And monitoring of blood glucose concentrations are evaluated directly at the pancreas, as opposed to being mediated by another gland such as the pituitary. In addition, nerve endings at the islets contribute in regulating insulin and glucagon secretion. The hormone somatotostatin is also released under conditions of increased blood glucose, glucagon, and amino acids. Somatostatin inhibits additional insulin release.

The overall effect of insulin is to store fuel in muscle and adipose tissue. It also promotes protein synthesis. Hence, insulin is said to be anabolic which means that it works to build up instead of break down energy-storing molecules. Glucagon, however, is metabolic, since it promotes glycogen breakdown. However, each of the pancreatic hormones is also considered to be paracrine, since they also modulate other pancreatic cells.

Diabetes mellitus (DM) is a very serious endocrine disorder caused by insulin dysfunctions. In type I DM, the beta cells are defective and can not produce enough insulin. Type II DM is caused by a lack of target cell-receptor responsiveness to available insulin. While type I requires regular insulin injections, type II can be controlled with diet.