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The pancreas is an organ that serves two functions:
- exocrine - it produces pancreatic juice containing digestive enzymes.
- endocrine - it produces several important hormones, including insulin.
The pancreas is an organ located posterior to the stomach and in close association with the duodenum.
In humans, the pancreas is a small elongated organ in the abdomen. It is described as having a head, body and tail. The pancreatic head abuts the second part of the duodenum while the tail extends towards the spleen. The pancreatic duct runs the length of the pancreas and empties into the second part of the duodenum at the ampulla of Vater. The common bile duct commonly joins the pancreatic duct at or near this point.
It is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the superior mesenteric artery. Venous drainage is via the pancreaticoduodenal veins which end up in the portal vein. The splenic vein passes posterior to the pancreas but is said to not drain the pancreas itself. The portal vein is formed by the union of the superior mesenteric vein and splenic vein posterior to the body of the pancreas. In some people (some books say 40% of people), the inferior mesenteric vein also joins with the splenic vein behind the pancreas (in others it simply joins with the superior mesenteric vein instead).
The pancreas produces enzymes that break down all categories of digestible foods (exocrine pancreas) and secretes hormones that affect carbohydrate metabolism (endocrine pancreas).
The pancreas is covered in a tissue capsule that partitions the gland into lobules. The bulk of the pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters called acini (singular acinus). The cells are filled with secretory granules containing the pre-cursor digestive enzymes (mainly trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase) that are secreted into the lumen of the acinus. These granules are termed zymogen granules (zymogen referring to the inactive precursor enzymes).
Zymogen granules are localized to the subapical area of pancreatic acinar cells. After fusion with the apical membrane, they are flushed into the duodenum, where enterokinases (bound to enterocytes but facing the lumen of the duodenum) catalyze the activation of trypsinogen into trypsin. Trypsin, an endopeptidase, cleaves amino acids from chymotrypsinogen to produce an active endopeptidase, chymotrypsin. These in turn can 'chop up' polypeptides, released from stomach, into absorbable units. They also activate the other enzymes released. It is important to synthesize inactive enzymes in the pancreas to avoid autodegradation, which can lead to pancreatitis.
The pancreas is the main source of enzymes for digesting fats (lipids) and proteins - the intestinal walls have enzymes that will digest polysaccharides. Pancreatic secretions from ductal cells contain bicarbonate ions and are alkaline in order to neutralize the acidic chyme that the stomach churns out. Control of the exocrine function of the pancreas are via the hormones gastrin, cholecystokinin and secretin, which are hormones secreted by cells in the stomach and duodenum, in response to distension and/or food and which cause secretion of pancreatic juices.
The two major proteases the pancreas secretes are trypsinogen and chymotrypsinogen. These zymogens are inactivated forms of trypsin and chymotrypsin. Once released in the intestine, the enzyme enterokinase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin. The free trypsin then cleaves the rest of the trypsinogen and chymotrypsinogen to their active forms.
Pancreatic secretions accumulate in intralobular ducts that drain to the main pancreatic duct, which drains directly into the duodenum.
Due to the potency of its enzyme contents, it is a very dangerous organ to injure and a puncture of the pancreas tends to require careful medical intervention.
Embedded throughout the exocrine tissue are small clusters of cells called the Islets of Langerhans, which are the endocrine cells of the pancreas and secrete insulin, glucagon, and several other hormones. The islets contain three major types of cells — alpha cells, beta cells, and delta cells. The largest number of cells are, by far, the beta cells which produce insulin. The alpha cells produce glucagon and the delta cells produce somatostatin, which lead to both decreased glucagon and insulin levels. There are also the PP cells and the D1 cells, about which little is known.
Pancreas comes from the Greek pankreas (a combination of pan and kreas) which means 'all meat'. Kreas in Homeric literature meant edible animal flesh. An example of one such food that can be made from the pancreas of a calf, lamb or pig is sweetbread.
Diseases of the pancreas
- Benign tumours
- Carcinoma of pancreas
- Cystic fibrosis
- Exocrine pancreatic insufficiency
- Pancreatic pseudocyst
http://www.randomhouse.com/wotd/index.pperl?date=20010222 Review 2005-03-10
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