The pancreas is a small, pennant-shaped gland located posterior to the pyloric part of the stomach. It is connected by a duct to the duodenum, approximately 10 cm distal to the pyloric sphincter. The pancreas has both endocrine and exocrine functions. The majority of the cells within the pancreas secrete pancreatic juice, which is the digestive (exocrine) function of the pancreas. Pancreatic juice is collected by tiny ducts that merge to form large ducts, which enter the pancreatic duct. The pancreatic duct extends the length of the pancreas and usually forms a smaller accessory pancreatic duct. The pancreatic duct joins with the bile duct where they both empty their secretions into the duodenum. Their common opening is controlled by the hepatopancreatic sphincter, which dilates to allow pancreatic juice and bile to enter the duodenum. The accessory pancreatic duct allows pancreatic juice to enter the duodenum independently of bile.
Control of Pancreatic Secretion
Pancreatic secretion, like gastric secretion, is controlled by both neural and hormonal mechanisms. Neural control is via parasympathetic axons. When parasympathetic nerve impulses activate the stomach mucosa, they also stimulate the pancreas to secrete pancreatic juice.
Hormonal control of pancreatic secretion results from two hormones that stimulate different types of pancreatic cells. Acid chyme entering the duodenum stimulates the intestinal mucosa to release the hormone secretin. Secretin is carried by blood to the pancreas, where it stimulates
secretion of pancreatic juice that is rich in bicarbonate ions. Bicarbonate ions neutralize the acidity of the chyme entering the small intestine. Lipid-rich chyme stimulates production of cholecystokinin by the intestinal mucosa. CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes.
Major Hormones Regulating Digestive Secretions
|Gastrin||Stimulates gastric juice secretion||Gastric mucosa; parasympathetic nerve impulses, stomach stretching, and food in the stomach stimulate release of gastrin|
|Cholecystokinin||Reduces gastric juice secretion; stimulates secretion of pancreatic juice that is rich in digestive enzymes; stimulates contraction of gallbladder and relaxes hepatopancreatic sphincter causing release of bile||Intestinal mucosa; lipid-rich chyme stimulates the release of cholecystokinin|
|Secretin||Stimulates secretion of pancreatic juice that is rich in bicarbonate ions; inhibits gastric secretion||Intestinal mucosa; acid chyme stimulates the release of secretin|
Pancreatic juice contains enzymes that act on each of the major classes of energy foods: carbohydrates, fats, and proteins. Their digestive actions occur within the small intestine.
Pancreatic amylase, like salivary amylase, acts on starch and glycogen, splitting these polysaccharides into maltose, a disaccharide. Maltose, however, is too big to be absorbed by the body.
Pancreatic lipase acts on fats (triglycerides) and splits them into monoglycerides and fatty acids that are absorbable.
Trypsin is the major pancreatic enzyme in pancreatic juice. Trypsin splits proteins into peptides. Like pepsin in the stomach, it is secreted in an inactive form and is activated only when mixed with intestinal secretions within the small intestine. This mode of secretion prevents the pancreatic cells from being digested by their own enzymatic secretions.