role of pancreas

• The pancreas has an exocrine portion that secretes pancreatic juice into the duodenum through the pancreatic duct
• The pancreatic duct unites with the common bile duct from the liver and gallbladder to form the ampulla of Vater, which opens into the duodenum
• The passage of pancreatic juice and bile into the duodenum is regulated by the sphincter of Oddi, made of smooth muscle
• The endocrine portion of the pancreas consists of clusters of cells called pancreatic islets that secrete hormones like insulin, glucagon, somatostatin, and pancreatic polypeptide into the blood
• Pancreatic glands secrete pancreatic juice, which includes acinar cells that secrete digestive enzymes and duct cells that secrete bicarbonates
• Pancreatic enzymes digest carbohydrates, fats, proteins, and nucleic acids
• The mechanism of bicarbonate secretion in the pancreatic duct involves the uptake of CO2 from the blood, generation of carbonic acid, and secretion of bicarbonate into the duct lumen
• Pancreatic juice has a higher amount of bicarbonate and lower amounts of chloride compared to blood plasma
• With increasing secretion rate, the pancreatic juice becomes more alkaline and contains more bicarbonate
• Pancreatic juice, produced at about 1 liter per day, has an alkaline pH that buffers gastric acid in the duodenum and creates an appropriate pH for digestive enzymes in the small intestine
• Pancreatic enzymes include protein-digesting enzymes like trypsin, chymotrypsin, and carboxypeptidase, carbohydrate-digesting enzyme amylase, fat-digesting enzyme lipase, and nucleic acid-digesting enzymes like ribonuclease and deoxyribonuclease
• Proteases of the pancreatic juice are initially secreted in inactive form and are activated in the intestinal lumen
• Trypsinogen is converted to trypsin in the duodenum by enterokinase, which then activates other proteolytic enzymes
• Pancreatic protein-digesting enzymes include endopeptidases like trypsin and chymotrypsin, and exopeptidases like carboxypeptidase and aminopeptidase
• Pancreatic amylase digests starch into oligosaccharides, trisaccharides, and disaccharides
• Pancreatic lipase converts lipids to monoglycerides and free fatty acids
• Pancreatic fat-digesting enzymes:

  • Lipase: converts lipids to monoglycerides and free fatty acids
  • Cholesterol ester hydrolase: converts cholesterol ester to free cholesterol and fatty acids
  • Phospholipase A2: converts phospholipids to lysolecithin and fatty acids
• Bile assists the action of pancreatic lipase by inducing fat emulsification
• Colipase:

  • Prevents inactivation of lipase by bile salts
  • Binds to both bile acids and lipase, bringing lipase back to the surface of the lipid droplet for digestion
• Pancreatic nucleases:

  • Ribonuclease
  • Deoxyribonuclease
  • These enzymes split RNA and DNA into mononucleotides
• Summary of Pancreatic enzymes:

  • Protein-digesting enzymes: Trypsin, chymotrypsin, carboxypeptidase, aminopeptidase, elastase
  • Carbohydrate-digesting enzyme: Amylase
  • Triglyceride-digesting enzyme: Lipase, phospholipase A2, cholesterol ester hydrolase
  • Nucleic acid-digesting enzymes: Ribonuclease, deoxyribonuclease
• Regulation of pancreatic secretion:

  • Hormonal regulation: secretin, CCK, and gastrin
  • Neural regulation: sympathetic and parasympathetic fibers of the ANS
• Hormonal regulation of pancreatic secretion:

  • Pancreatic juice contains bicarbonates and enzymes
  • Secretion is controlled by different mechanisms
• Control of pancreatic secretion by secretin and CCK:

  • Gastrin stimulates secretion of enzymes in pancreatic acinar cells
  • Gastrin and CCK act via the same receptor
• Neural regulation of pancreatic secretion:

  • Pancreatic glands are innervated by sympathetic and parasympathetic fibers of the ANS
  • Sympathetic fibers inhibit pancreatic secretion, parasympathetic fibers stimulate it
• Phases of pancreatic secretion:

  • Cephalic phase: stimulated by thought, sight, smell, and taste of food
  • Gastric phase: stimulated by food entering the stomach
  • Intestinal phase: stimulated by food entering the duodenum
• After this lecture, you can:

  • Explain the role of acinar and ductal cells in pancreatic juice secretion
  • Compare the ionic composition of pancreatic juice and blood plasma
  • Explain the role of pancreatic enzymes in digestion
  • Describe neural and hormonal regulation of pancreatic secretion
  • Describe the three phases of pancreatic secretion: cephalic, gastric, and intestinal