Pancreas – Alimentary Canal
The pancreas was discussed as an endocrine gland . It also has an exocrine function— secretion of a digestive fluid called pancreatic juice (pan′′kre-at′ik joos). The pancreas is an elongated, somewhat flattened organ posterior to the stomach and behind the parietal peritoneum. A duct joins the pancreas to the duodenum (the first section of the small intestine) and transports pancreatic digestive juice to the intestine. The dual nature of the pancreas begins in the embryo. First, ducts form whose walls harbor progenitor cells. Some of the progenitor cells divide to yield daughter cells that specialize as exocrine cells, and others divide to yield cells that differentiate into endocrine cells.
The two functions are elaborated as the gland develops further. The endocrine part of the pancreas consists of group of cells that are closely associated with blood vessels. These groups form “islands” of cells called pancreatic islets (islets of Langerhans) . The pancreatic islets include two distinct types of cells—alpha cells, which secrete the hormone glucagon, and beta cells, which secrete the hormone insulin. The pancreatic duct usually connects with the duodenum at the same place where the bile duct from the liver and gallbladder joins the duodenum, although other connections may be present. A hepatopancreatic sphincter controls the movement of pancreatic juices into the duodenum.
Pancreatic juice contains enzymes that digest carbohydrates, fats, nucleic acids, and proteins. The carbohydrate-
digesting enzyme pancreatic amylase splits molecules of starch or glycogen into disaccharides. The fat-digesting enzyme pancreatic lipase breaks triglyceride molecules into fatty acids and glycerol. Pancreatic juice also contains two nucleases, which are enzymes that break down nucleic acid molecules into nucleotides.
The protein-splitting (proteolytic) enzymes are trypsin, chymotrypsin, and carboxypeptidase (kar-bok′′se-pep′tı˘-da – s). These enzymes split the bonds between particular combinations of amino acids in proteins. No single enzyme can split all possible amino acid combinations, so several enzymes are necessary to completely digest protein molecules.
The proteolytic enzymes are stored in tiny cellular structures called zymogen granules (zi-mo′jen gran′u¯ lz). These enzymes, like gastric pepsin, are secreted in inactive forms. After the inactive forms of the proteolytic enzymes reach the small intestine, other enzymes activate them. For example, pancreatic cells release inactive trypsinogen, which is activated to trypsin when it contacts the enzyme enterokinase (en′′ter-o-ki′na – s) secreted by the mucosa of the small intestine.
Regulation of Pancreatic Secretion
The nervous and endocrine systems regulate release of pancreatic juice, as they do gastric and small intestinal secretions. For example, when parasympathetic impulses stimulate gastric juice secretion, other parasympathetic
impulses stimulate the pancreas to release digestive enzymes. Also, as acidic chyme enters the duodenum, the duodenal mucous membrane releases the peptide hormone secretin (se-kre′tin) into the bloodstream. This hormone stimulates secretion of pancreatic juice that has a high concentration of bicarbonate ions.
These ions neutralize the acid in chyme and provide a favorable environment for digestive enzymes in the intestine.
Proteins and fats in chyme in the duodenum also stimulate the intestinal wall to release cholecystokinin. Like secretin, cholecystokinin travels via the bloodstream to the pancreas. Pancreatic juice secreted in response to cholecystokinin has a high concentration of digestive enzymes.