24.10 Pancreas

OBJECTIVE

• Describe the location, anatomy, histology, and function of the pancreas.

From the stomach, chyme passes into the small intestine. Because chemical digestion in the small intestine depends on activities of the pancreas, liver, and gallbladder, we first consider the activities of these accessory digestive organs and their contributions to digestion in the small intestine.

Anatomy of the Pancreas

The pancreas (pan- ! all; -creas ! flesh), a retroperitoneal gland that is about 12–15 cm (5–6 in.) long and 2.5 cm (1 in.) thick, lies posterior to the greater curvature of the stomach. The pancreas consists of a head, a body, and a tail and is usually connected to the duodenum by two ducts. The head is the expanded portion of the organ near the curve of the duodenum; superior to and to the left of the head are the central body and the tapering tail.

Pancreatic juices are secreted by exocrine cells into small ducts that ultimately unite to form two larger ducts, the pancreatic duct and the accessory duct. These in turn convey the secretions into the small intestine. The pancreatic duct, or duct of Wirsung (VE ̄R-sung), is the larger of the two ducts. In most people, the pancreatic duct joins the common bile duct from the liver and gallbladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla (hep"-a-toˉ- pan-kre ̄-A-tik), or ampulla of Vater (FAH-ter). The ampulla opens on an elevation of the duodenal mucosa known as the major duodenal papilla, which lies about 10 cm (4 in.) inferior to the pyloric sphincter of the stomach. The passage of pancreatic juice and bile through the hepatopancreatic ampulla into the duodenum of the small intestine is regulated by a mass of smooth muscle surrounding the ampulla known as the sphincter of the hepatopancreatic ampulla, or sphincter of Oddi (OD-e ̄). The other major duct of the pancreas, the accessory duct (duct of Santorini), leads from the pancreas and empties into the duodenum about 2.5 cm (1 in.) superior to the hepatopancreatic ampulla.

Histology of the Pancreas

The pancreas is made up of small clusters of glandular epithelial cells. About 99% of the clusters, called acini (AS-i-n ̄ı), constitute the exocrine portion of the organ (see Figure 18.18b, c). The cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice. The remaining 1% of the clusters, called pancreatic islets (islets of Langerhans) (I ̄-lets) form the endocrine portion of the pancreas. These cells secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide. The functions of these hormones are discussed in Chapter 18.

Composition and Functions of Pancreatic Juice

Each day the pancreas produces 1200–1500 mL (about 1.2–1.5 qt) of pancreatic juice, a clear, colorless liquid consisting mostly of water, some salts, sodium bicarbonate, and several enzymes. The sodium bicarbonate gives pancreatic juice a slightly alkaline pH (7.1–8.2) that buffers acidic gastric juice in chyme, stops the action of pepsin from the stomach, and creates the proper pH for the action of digestive enzymes in the small intestine. The enzymes in pancreatic juice include a starch-digesting enzyme called pancreatic amylase; several enzymes that digest proteins into peptides called trypsin (TRIP-sin), chymotrypsin (k ̄ı"-moˉ-TRIP-sin), carboxypeptidase (kar-bok"-se ̄-PEP-ti-daˉs), and elastase(e ̄-LAS-taˉs);the principal triglyceride–digesting enzyme in adults, called pancreatic lipase; and nucleic acid–digesting enzymes called ribonuclease (r ̄ı"-boˉ-NOO-kle ̄-aˉs) and deoxyribonuclease (de ̄-oks-e ̄-r ̄ı"-boˉ-NOO-kle ̄-aˉs) that digest ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) into nucleotides. The protein-digesting enzymes of the pancreas are produced in an inactive form just as pepsin is produced in the stomach as pepsinogen. Because they are inactive, the enzymes do not digest cells of the pancreas itself. Trypsin is secreted in an inactive form called trypsinogen (trip-SIN-oˉ -jen). Pancreatic acinar cells also secrete a protein called trypsin inhibitor that combines with any trypsin formed accidentally in the pancreas or in pancreatic juice and blocks its enzymatic activity. When trypsinogen reaches the lumen of the small intestine, it encounters an activating brush-border enzyme called enterokinase (en"- ter-oˉ-KI ̄-naˉs), which splits off part of the trypsinogen molecule to form trypsin. In turn, trypsin acts on the inactive precursors (called chymotrypsinogen, procarboxypeptidase, and proelastase) to produce chymotrypsin, carboxypeptidase, and elastase, respectively.