Between the alveoli small groups of cells are found, which are termed, the islets of Langerhans. They are surrounded by a rich capillary of network and furnish the internal secretion of the pancreas; insulin and glucagon.
Two secretions are formed in the pancreas, The pancreatic fluid is an external secretion and is poured into the duodenum during intestinal digestion and the secretion formed by the islets of Langerhans are the internal secretions of insulin and glucagon, which are absorbed by the blood, carried to the tissues, and aid in regulating glucose metabolism.
Several types of cells are in t¥e islet group. The beta cells secrete insulin, and the alpha cells secrete glucagon. Insulin increases cell permeability to glucose. See figure below.
Insulin promotes the utilization of glucose in tissue cells and thereby decreases blood glucose concentration.
Insulin is essential for the maintenance of normal levels of blood glucose. Hypoglycemia can result from increased insulin secretion or from the injection of too much insulin. Hyperglycemia and glycosuria, a condition which the urine contains glucose, may result from insufficient secretion of insulin. Marked increased levels of blood sugar, if untreated, lead to coma and death. This condition is known as diabetes mellitus.
(figure available in print form)