Digestion
Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
Exocrine pancreatic insufficiency
The exocrine pancreas is a portion of this organ that contains clusters of ducts (acini) producing bicarbonate anion, a mild alkali, as well as an array of digestive enzymes that together empty by way of the interlobular and main pancreatic ducts into the duodenum (upper small intestine). The hormones cholecystokinin and secretin secreted by the stomach and duodenum in response to distension and the presence of food in turn stimulate the production of digestive enzymes by the exocrine pancreas. The alkalization of the duodenum neutralizes the acidic chyme produced by the stomach that is passing into it; the digestive enzymes serve to catalyze the breakdown of complex foodstuffs into smaller molecules for absorption and integration into metabolic pathways. The enzymes include proteases (trypsinogen and chymotrypsinogen), hydrolytic enzymes that cleave lipids (the lipases phospholipase A2 and lysophospholipase, and cholesterol esterase), and amylase to digest starches. EPI results from progressive failure in the exocrine function of the pancreas to provide its digestive enzymes, often in response to a genetic condition or other disease state, resulting in the inability of the animal involved to properly digest food.