PANCREAS

compound gland common to all vertebrates, functioning both as an exocrine gland discharging digestive enzymes into the gut and as an endocrine gland secreting the hormones insulin and glucagon, vital in carbohydrate (sugar) metabolism, into the bloodstream. The term pancreas also designates a gland found in many invertebrates, the primary function of which is the secretion of digestive enzymes. In humans, the pancreas is attached to the duodenum, the upper portion of the small intestine. A large main duct, the duct of Wirsung, collects pancreatic juice and empties into the duodenum. In many individuals a smaller duct (duct of Santorini) also empties into the duodenum. Enzymes active in the digestion of carbohydrate, fat, and protein flow from the pancreas through these ducts continuously and at an increased rate when food is present in the small intestine. Their flow is controlled by the vagus nerve and by the hormones secretin and pancreozymin, which are produced in the intestinal mucosa. When food enters the duodenum, secretin and pancreozymin are released into the bloodstream by the duodenum's secretory cells. When these hormones reach the pancreas, its cells are stimulated to produce and release large amounts of water, bicarbonate, and digestive enzymes, which then flow into the intestine. The cells in the pancreas that produce digestive enzymes are called acinar cells (from Latin acinus, grape), so named because the cells aggregate to form bundles that resemble a cluster of grapes. Located between the clusters of acinar cells are scattered patches of another type of secretory tissue, collectively known as the islets (or islands) of Langerhans, named for the 17th-century German pathologist Paul Langerhans. The islets are responsible for the secretion of insulin and glucagon, which control the amount of sugar stored in the body. Insulin stimulates the body's cells to remove sugar from the bloodstream and utilize it. Glucagon has the opposite effect of insulin; it releases stored sugar and increases the blood-sugar level, acting as a control mechanism whenever the body produces too much insulin. Insulin and glucagon are secreted directly into the bloodstream. The islets of Langerhans also secrete, in much smaller quantities, somatostatin, one of whose functions is to inhibit the secretion of insulin and glucagon, and pancreatic polypeptide, whose biological role is uncertain. The pancreas may be the site of acute and chronic infections, tumours, and cysts. Should it be removed surgically, life can be sustained by the administration of insulin and of potent pancreatic extracts. Approximately 80 to 90 percent of the pancreas can be removed surgically without producing an insufficiency of either endocrines (insulin and glucagon) or exocrines (water, bicarbonate, and enzymes).