Pancreas

(a)

■ Figure 18.27 Gallstones. (a) A radiograph of a gallbladder that contains gallstones (biliary calculi). (b) A posterior view of a gallbladder that has been surgically removed (cholecystectomy) and cut open to reveal its gallstones. (Note their size relative to that of a dime.)

The pancreas is a soft, glandular organ that has both exocrine and endocrine functions (fig. 18.28). The endocrine function is performed by clusters of cells called the pancreatic islets, or islets of Langerhans (fig. 18.28a), that secrete the hormones insulin and glucagon into the blood (see chapter 19). As an exocrine gland, the pancreas secretes pancreatic juice through the pancreatic duct into the duodenum. Within the lobules of the pancreas are the exocrine secretory units, called acini (fig. 18.28b). Each acinus consists of a single layer of epithelial cells surrounding a lumen, into which the constituents of pancreatic juice are secreted.

Endocrine portion

Pancreatic islet (of Langerhans)

Exocrine portion

Pancreatic acini / \

Pancreatic islet (of Langerhans)

Pancreatic acini / \

To pancreatic duct and duodenum

Duct

Acinus

To pancreatic duct and duodenum

Duct

Acinus

■ Figure 18.28 The pancreas is both an exocrine and an endocrine gland. (a) A photomicrograph of the endocrine and exocrine portions of the pancreas. (b) An illustration depicting the exocrine pancreatic acini, where the acinar cells produce inactive enzymes stored in zymogen granules. The inactive enzymes are secreted by way of a duct system into the duodenum.

Table 18.4

Enzymes Contained in

Pancreatic Juice

Enzyme

Zymogen

Activator

Action

Trypsin

Trypsinogen

Enterokinase

Cleaves internal peptide bonds

Chymotrypsin

Chymotrypsinogen

Trypsin

Cleaves internal peptide bonds

Elastase

Proelastase

Trypsin

Cleaves internal peptide bonds

Carboxypeptidase

Procarboxypeptidase

Trypsin

Cleaves last amino acid from carboxyl-terminal end of polypeptide

Phospholipase

Prophospholipase

Trypsin

Cleaves fatty acids from phospholipids such as lecithin

Lipase

None

None

Cleaves fatty acids from glycerol

Amylase

None

None

Digests starch to maltose and short chains of glucose molecules

Cholesterolesterase

None

None

Releases cholesterol from its bonds with other molecules

Ribonuclease

None

None

Cleaves RNA to form short chains

Deoxyribonuclease

None

None

Cleaves DNA to form short chains

Pancreatic juice

Other inactive ■ enzymes (zymogens)

Trypsinogen t

Trypsin

»- Active enzymes

Trypsinogen

^fiiliillt^

Epithelial cell of duodenum

■ Figure 18.29 The activation of pancreatic juice enzymes. The pancreatic protein-digesting enzyme trypsin is secreted in an inactive form known as trypsinogen. This inactive enzyme (zymogen) is activated by a brush border enzyme, enterokinase (EN), located in the cell membrane of microvilli. Active trypsin in turn activates other zymogens in pancreatic juice.

Pancreatitis (inflammation of the pancreas) may result when conditions such as alcoholism, gallstones, traumatic injury, infections, or toxicosis from various drugs provoke activation of digestive enzymes within the pancreas. Leakage of trypsin into the blood also occurs, but trypsin is inactive in the blood because of the inhibitory action of two plasma proteins, a ^antitrypsin and a2-macroglobulin. Pancreatic amylase may also leak into the blood, but it is not active because its substrate (starch) is not present in blood. Pancreatic amylase activity can be measured in vitro, however, and these measurements are commonly performed to assess the health of the pancreas.

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