Normal Anatomy and Physiology

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(From Applegate EJ: The Anatomy and Physiology Learning System: Textbook. Philadelphia, WB Saunders, 1995.) 599 497 599 497 80 66

The pancreas can be divided into three parts: the head, which lies within the loop of duodenum; the midportion, which is called the body; and the tail, which extends laterally and left to the hilus of the spleen (see Figure 12-1). More than 98% of the entire pancreas consists of exocrine tissue—acini, ductules, and ducts. The endocrine cells are arranged into islets of Langerhans that are scattered through the entire organ but are most prominent in the tail.

The digestive juices produced by the exocrine pancreatic cells drain through the main pancreatic duct into the duodenum. The terminal part of the main pancreatic duct is confluent with the common bile duct, with which it shares the common entry into the duodenum called the papilla of Vater. There is often an accessory duct entering the duodenum as well, which is unrelated to the bile duct. This close relationship of the head of the pancreas and the duodenum, as well as the common bile duct, is important for an understanding of the obstructive symptoms caused by tumors of the head of the pancreas. Reflux of bile into the pancreatic duct as a result of obstruction of the papilla of Vater may be important in the pathogenesis of pancreatitis, as explained later. Hormones produced by the endocrine cells are released into the blood circulation; therefore, there is no need for endocrine excretory ducts.

The exocrine pancreas is the main source of digestive enzymes, the most important of which include the following:

The pancreas in relation to the liver, duodenum, and stomach. The inset shows the acini, ducts, and islets.

Normal Anatomy and Physiology The pancreas is a gland composed of an exocrine and an endocrine part (Figure 12-1). It is located in the retroperitoneal space of the upper abdomen and is closely attached to other retroperitoneal structures, most notably, the ganglia and nerves of the celiac plexus. Because of this close relationship between the pancreas and the retroperitoneal nerves, pain radiating into the back is one of the common features of pancreatic diseases. Figure 12-1

	•	Amylase, which is essential for the digestion of starch
	•	Lipase, which is essential for the digestion of lipids
	•	Peptidases, such as trypsin and chymotrypsin, which are essential for the digestion of proteins

All these enzymes are synthesized in the acinar cells and released into the ductal system in an inactive form (i.e., like proenzymes). Pancreatic juice also contains bicarbonate and small amounts of mucin, which are released from the ductal cells.

The secretion of pancreatic juices is controlled by the vagus nerve and the polypeptide hormones cholecystokinin and secretin. These hormones are released from the duodenum in response to the entry of acidic and fat-rich food into its lumen from the stomach. Cholecystokinin stimulates the secretion of enzymes, whereas secretin stimulates the release of bicarbonate. The pancreatic juices that contain proenzymes and bicarbonates are mixed with the duodenal content. This results in activation of enzymes through the action of intestinal enteropeptidase and the alkalization of the luminal content through the action of bicarbonates. Bicarbonates act as buffers to neutralize the gastric hydrochloric acid, and by raising the pH in the intestine, they provide optimal conditions for the action of pancreatic digestive enzymes. It should be noted that cholecystokinin also stimulates contraction of the gallbladder and secretin stimulates production of bile in the liver. Because bile and pancreatic juices have a common terminal outflow tract, it is easy to see how heavy meals, especially those rich in lipids, can overburden the pancreaticobiliary ductal system and cause potentially harmful consequences.

Did You Know?

Islets of Langerhans were discovered in 1868 by a German medical student, Paul Langerhans. He also described the phagocytic cells of the epidermis, which also bear his name. Not bad for a young man who did not even have a doctoral degree at that time!

The endocrine pancreatic cells secrete several polypeptide hormones, the most important of which are insulin, glucagon, and somatostatin. The excess or deficiency of these hormones produces distinct clinical symptoms, which is discussed later.

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