role of pancreas

• Exocrine portion of pancreas


  Secretes pancreatic juice, which is released into the duodenum through the pancreatic duct

  
• Endocrine portion of pancreas

  Consists of clusters of cells called the pancreatic islets (islets of Langerhans) that secrete hormones into the blood

  
• Pancreatic glands


  1. Acinar cells secrete digestive enzymes

  2. Duct cells secrete bicarbonates

  
• Pancreatic enzymes


  • Digest carbohydrates, fats, proteins, and nucleic acids
  • Bicarbonates neutralize hydrochloric acid entering small intestine from the stomach

  
• Primary pancreatic secretion


  • Has ionic composition similar to blood plasma, and is isotonic
  • In the ducts, the pancreatic juice is enriched with bicarbonates and remains isotonic

  
• Bicarbonate secretion in the pancreatic ductal cell

  1. Cells take in CO2 from the blood, which is used to generate carbonic acid

  2. Carbonic acid dissociates to bicarbonate and H+

  3. Bicarbonate is secreted into the lumen of the duct by HCO3-/Cl- exchanger

  4. Cl- delivered into the duct cell then passively leaks back through the CFTR chloride channel

  5. H+ is removed from the cell by the Na+/H+ exchanger

  
• With increasing secretion rate

  • The pancreatic juice contains more bicarbonate, and hence becomes more alkaline
  • Cl- concentration in the pancreatic juice is reduced, because bicarbonate is secreted into the lumen in exchange for delivery of Cl- into the ductal cells
  • Na+ and K+ concentrations in the pancreatic juice do not change

  
• Pancreatic juice


  • Produced at about 1 liter per day
  • Has an alkaline pH of 7.1-8.2
  • Buffers gastric acid in chyme, stops the action of pepsin, and creates appropriate pH for digestive enzymes

  
• Pancreatic enzymes


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

  
• Activation of proteases of the pancreatic juice

  1. Trypsinogen is converted to trypsin in the duodenum by enterokinase

  2. Trypsin then activates chymotripsinogen, procarboxypeptidase, proaminopeptidase, and proelastase

  
• Endopeptidases


  Trypsin, chymotrypsin, elastase

  
• Exopeptidases


  Carboxypeptidase and aminopeptidase

  
• Pancreatic amylase


  Splits starch into oligosaccharides, trisaccharides, and disaccharides

  
• Pancreatic fat-digesting enzymes


  • Lipase
  • Cholesterol ester hydrolase
  • Phospholipase A2

  
• Bile assists the action of pancreatic lipase


  By inducing fat emulsification
• Colipase


  Acts as a cofactor for lipase by binding to both bile acids and lipase, bringing lipase back to the surface of the lipid droplet
• Pancreatic nucleases


  Ribonuclease and deoxyribonuclease split RNA and DNA into mononucleotides

  
• Summary of pancreatic enzymes

  • Protein-digesting enzymes: Trypsin, chymotrypsin, carboxypeptidase, aminopeptidase, elastase
  • Carbohydrate-digesting enzyme: Amylase
  • Triglyceride-digesting enzymes: 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

  1. Secretin and CCK stimulate secretion of bicarbonates and enzymes respectively

  2. Gastrin stimulates secretion of enzymes

  
• Neural regulation of pancreatic secretion

  1. Sympathetic fibers inhibit pancreatic secretion

  2. Parasympathetic fibers stimulate pancreatic secretion