Exocrine Component of Pancreas

The exocrine pancreas has ducts that are arranged in clusters called acini (singular acinus). Pancreatic secretions are secreted into the lumen of the acinus, and then accumulate in intralobular ducts that drain to the main pancreatic duct, which drains directly into the duodenum.

Control of the exocrine function of the pancreas is 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.

There are two main classes of exocrine pancreatic secretions:

Pancreatic secretions from ductal cells contain bicarbonate ions and are alkaline in order to neutralize the acidic chyme that the stomach churns out.

The pancreas is also the main source of enzymes for digesting fats (lipids) and proteins. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.)

The cells are filled with secretory granules containing the precursor digestive enzymes. The major proteases which the pancreas secretes are trypsinogen and chymotrypsinogen. Secreted to a lesser degree are pancreatic lipase and pancreatic amylase. The pancreas also secretes phospholipase A2, lysophospholipase, and cholesterol esterase.

The precursor enzymes (termed zymogens or proenzymes) are inactive variants of the enzymes; thus autodegradation, which can lead to pancreatitis, is avoided. Once released in the intestine, the enzyme enteropeptidase (formerly, and incorrectly, called enterokinase) present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin. The free trypsin then cleaves the rest of the trypsinogen, as well as chymotrypsinogen to its active form chymotrypsin.