health as its endocrine function. Two of the population of cells in the pancreatic parenchyma make up its digestive enzymes: Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to produce the hormone secretin and release to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice. Secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme. Acinar cells: Mainly responsible for production of the inactive pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin (CCK), which is a hormone/neurotransmitter produced by the intestinal cells (I cells) in the duodenum. CCK stimulates production of the pancreatic zymogens. Pancreatic juice, composed of the secretions of both ductal and acinar cells, contains the following digestive enzymes: Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin. Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, turns into chymotrypsin and breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin. Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein Several elastases that degrade the protein elastin and some other proteins Pancreatic lipase that degrades triglycerides into two fatty acids and a monoglyceride Sterol esterase Phospholipase Several nucleases that degrade nucleic acids, like DNAase and RNAase Pancreatic amylase that breaks down starch and glycogen which are alpha-linked glucose polymers. Humans lack the cellulases to digest the carbohydrate cellulose which is a beta-linked glucose polymer. Some of the preceding endogenous enzymes have pharmaceutical counterparts (pancreatic enzymes) that are administered to people with exocrine pancreatic insufficiency. The pancreas's exocrine function owes part of its notable reliability to biofeedback mechanisms controlling secretion of the juice. The following