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A composite gland in most vertebrates, containing both exocrine cells—which produce and secrete enzymes involved in digestion—and endocrine cells, arranged in separate islets which elaborate at least two distinct hormones, insulin and glucagon, both of which play a role in the regulation of metabolism, and particularly of carbohydrate metabolism. See Carbohydrate metabolism
The pancreas of mammals shows large variations. The extremes are the unique, massive pancreas of humans, and the richly branched organ of the rabbit. Usually, the main duct, the duct of Wirsung, opens into the duodenum very close to the hepatic duct. In humans, the pancreas weighs about 2.5 oz (70 g). It can be divided into head, body, and tail. Accessory pancreases are frequently found anywhere along the small intestine, in the wall of the stomach, and in Meckel's diverticulum.
The exocrine portion of the pancreas shows tubuloalveolar glands. Each terminal alveolus is called an acinus. The various acini have central cavities, which open into intralobular ducts through narrow intercalated tubes. The interlobular ducts anastomose and ultimately form the main duct of Wirsung. The activity of the acini is stimulated by secretin as well as by pilocarpine.
The endocrine portion shows cellular masses called islands or islets of Langerhans, in which the cellular cords or masses are more or less isolated by irregular spaces filled with connective tissue and blood capillaries. The two main types of cells are the alpha and the beta cells.
Between the grapelike exocrine portion with its ducts and the islands of Langerhans, it is possible to observe connective tissue septa, numerous blood vessels, and nerves.
The pancreatic juice carried to the duodenum is a slightly alkaline liquid containing trypsinogen, which, when activated, causes the hydrolysis of the proteins into amino acids, amylase, and maltase, which act on the glucides, and lipase, which causes the hydrolysis of fatty substances. The intense stimulation of the pancreatic secretion after ingestion of food is considered to be the result of a nervous reflex originating in the mouth, and also of direct introduction of acids and fats into the duodenum, causing the liberation of a hormone called secretin into the bloodstream to stimulate the exocrine secretion.
F. Banting and C. Best (1922) prepared pancreatic extracts which were able to prevent the lethal effects of pancreatectomy. The same effect was obtained with extracts from pancreas in which, after ligature of the duct of Wirsung, the exocrine portion of the gland had disappeared.
The alpha cells and beta cells in the islets are the sources of two hormones, insulin from the beta, and glucagon, also known as the hyperglycemic factor, from the alpha. The former is a hormone which influences carbohydrate metabolism, enabling the organism to utilize sugar. The latter accelerates the conversion of liver glycogen into glucose. Glucagon elevates the blood sugar level, and its effects are the opposite of those of insulin, so that the two hormones together maintain the sugar metabolism of the body in balance. When the level of sugar in the blood becomes too low, the secretion of glucagon is stimulated. See Glucagon, Insulin
a large digestive gland of animals and man that has exocrine and endocrine functions and that helps digest and regulate the metabolism of carbohydrates, fats, and proteins. Among invertebrates, a separate pancreas, considered a detached part of the liver, is found only in cephalopods. In vertebrates, the pancreas is situated in the mesentery of the middle intestine, close to the stomach; in amniotes, it is in the mesentery of the duodenum. In lampreys and dipnoans, the pancreas is concealed in the intestinal wall. In hagfishes, fishes of the family Acipenseridae, and some fishes of the superorder Teleostei, the gland is found in the liver tissue, forming with the liver a single organ, the hepatopancreas.
The exocrine part of the pancreas has a complex alveolar and tubular structure. It is covered by a thin connective-tissue capsule from which bands of connective tissue branch out, separating the pancreatic parenchyma into separate lobules. Most of the lobules basically consist of terminal saclike dilatations, or acini, whose cells secrete pancreatic juice. The efferent ducts of the lobules merge into the common efferent ducts of the gland. The endocrine part of the pancreas consists of special groups of cells arranged like islets in the lobules, which are richly furnished with blood vessels and do not have excretory ducts.
In man, the pancreas is situated in the retroperitoneal space behind and beneath the stomach perpendicular to the spinal column at the level of the first and second lumbar vertebrae in the form of a flattened band stretching horizontally from the duodenum to the spleen. The gland is 15–25 cm long, 3–9 cm wide at the head, 2–3 cm thick, and weighs 70–80 g. The head of the pancreas, the thick right extremity, is situated in the curve of the duodenum, while the tail, the narrow left extremity, is close to the spleen. The body of the gland is shaped like a three-faced prism and is covered in front by a peritoneum.
The main efferent duct of the pancreas passes from the tail to the head and opens into the duodenum. The pancreas is supplied with blood through the superior and inferior pancreaticoduodenal arteries. Blood flows out into the portal vein system. The pancreas has a well-developed network of lymphatics. It is innervated by the parasympathetic and sympathetic nervous systems, the latter being presented by branches of the celiac, superior mesenteric, renal, and splenic plexus. Impulses travel from the cerebral cortex to the pancreas through the hypothalamus along parasympathetic nerve fibers to the acinar cells, islands, and smooth muscle fibers of the ducts. The sympathetic fibers proceed to the blood vessels.
Modern ideas on the physiology and functioning of the pancreas were established by I. P. Pavlov and his colleagues. In man and dogs, 1.5–2 liters and 600–800 ml of pancreatic juice, respectively, are secreted a day. The juice, a colorless and odorless fluid with an alkaline reaction, consists of inorganic (HCO-3, Cl-, Na+, Ca2+, Mg2+) and organic (mainly proteins and enzymes) substances. Three main groups of enzymes—proteases, lipases, and amylases—help digest proteins, fats, and carbohydrates. In man and dogs the largest amount of pancreatic juice is secreted for carbohydrate digestion, the next largest amount for proteins, and the smallest amount for fats. The composition of the enzymes in the juice varies with the diet. Secretion begins one to three min after the ingestion of food and continues for six to ten hr. The rate of secretion when the stomach is empty is insignificant.
The endocrine function of the pancreas is the production of several hormones, including insulin and glucagon, which enter the bloodstream directly. The pancreas’ functions are regulated by neurohormonal mechanisms. The gland is affected by hormones of the digestive tract—secretin, pancreozymin, and gastrin—and by hormones of the thyroid and parathyroid glands, the hypophysis, and the adrenal glands. There is a close functional relationship between the pancreas and the other organs of the digestive pancreatitis. Diabetes mellitus results from disturbance of the insulin-manufacturing mechanism.
REFERENCESShamal’ gauzen, I. I. Osnovy sramitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Pavlov, I. P. Poln. sobr. soch., 2nd ed., vol. 2, book 2. Moscow-Leningrad, 1951.
Brooks, F. P. “The Neurohumoral Control of Pancreatic Exocrine Secretion.” American Journal of Clinical Nutrition, 1973, vol. 26, no. 3. Pages 291–310.
V. B. TROITSKAIA AND T. V. SHAAK