Pancreas as an Endocrine Gland

The pancreas is an elongated gland lying behind the stomach, in close association with the duodenum, and is unusual among the body's glands in being heterocrine — that is, in possessing both an exocrine and an endocrine portion within the same organ, each with an entirely different secretion and mode of release. The much larger exocrine portion, made up of clusters of acinar cells, secretes pancreatic juice, rich in digestive enzymes, into the duodenum via the pancreatic duct; this exocrine role in digestion lies outside the scope of the present chapter. Scattered throughout the exocrine tissue, forming only a small fraction of the pancreas's total mass, are numerous small clusters of endocrine cells called the islets of Langerhans, which secrete their hormones directly into the blood rather than into a duct, and it is this endocrine portion that concerns the chapter here.

Each islet of Langerhans contains at least four distinct types of hormone-secreting cells. Alpha (α) cells secrete glucagon. Beta (β) cells, the most numerous cell type within the islets, secrete insulin. Delta (δ) cells secrete somatostatin. PP cells (also called F cells) secrete pancreatic polypeptide. Of these four hormones, insulin and glucagon are by far the most physiologically important for the regulation of blood glucose level, and the two act as a directly antagonistic pair, exactly as PTH and calcitonin do for blood calcium.

Insulin is released by the beta cells in direct response to a rise in blood glucose level, as occurs after a meal, and its overall effect is to lower blood glucose back toward normal. It does this chiefly by promoting the uptake of glucose from the blood into cells throughout the body, particularly muscle and adipose (fat) tissue, and by promoting glycogenesis — the storage of glucose as glycogen — principally in the liver and skeletal muscle; insulin also promotes fat synthesis and protein synthesis more broadly, reflecting its overall role as the body's chief 'fed state' or storage-promoting hormone. Glucagon, released by the alpha cells in response to a fall in blood glucose level, as occurs between meals or during fasting, has the opposite overall effect, raising blood glucose back toward normal. It does this chiefly by stimulating glycogenolysis — the breakdown of stored glycogen back into glucose — in the liver, and by stimulating gluconeogenesis, the synthesis of new glucose from non-carbohydrate precursors such as amino acids, again principally in the liver; glucagon can be thought of as the body's chief 'fasting state' hormone, mobilising stored energy reserves when dietary glucose is not currently available. …