The pancreas is a retroperitoneal organ that serves two functions:
- exocrine - it produces pancreatic juice containing digestive enzymes
- endocrine - it produces several important hormones
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 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 digestive enzymes (mainly trypsin, chymotrypsin, pancreatic lipase , and amylase) that are secreted into the lumen of the acinus.
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 contain bicarbonate ion s 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 enzymes gastrin, cholecystokinin and secretin, which are enzymes 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 excretes 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 different types of cells — alpha cells (produce glucagon), beta cells (the most numerous, produce insulin), and delta cells (produce somatostatin). There are also the PP cells and the D1 cells, about which little is known.