L35 Physiology of Pancrease Liver and Gallbladder (Albarwani Fall 2024) PDF

L 35: Exocrine Pancreas, Liver and Gall Bladder  Objectives: L35: Exocrine Pancreas, Liver and Gall Bladder 1. Outline the composition of the pancreatic juice. 2. Understand how pancreatic secretion is regulated. 3. Explain the function of bile. 4. Describe the role of cholesy...

L 35: Exocrine Pancreas, Liver and Gall Bladder  Objectives: L35: Exocrine Pancreas, Liver and Gall Bladder 1. Outline the composition of the pancreatic juice. 2. Understand how pancreatic secretion is regulated. 3. Explain the function of bile. 4. Describe the role of cholesystokinin on the gall bladder function and the sphincter of Oddi. 5. Describe the mechanisms by which chyme from the stomach is neutralized in the duodenum. 6. Explain the digestive functions of the liver 7. Define enterohepatic circulation Pancreas, liver and gallbladder  The pancreas is a mixture of exocrine and endocrine tissue. The endocrine pancreas (islets of Langerhans 1%) comprises cells that secrete glucagon, insulin, somatostatin, and pancreatic polypeptide). 1. Acinar cells (grouped into lobules): granules containing the digestive enzymes (zymogen granules) are The exocrine pancreas formed within the cell and (99%) emptys its secretions discharged by exocytosis into into the small intestine: 1. digestive enzymes the small pancreatic ducts. (acinar cells) 2. alkaline fluid (ductal 2.Ductular and ductal cells). epithelial cells: secrete an alkaline fluid (NaHCO3-).  Pancreatic juice ▪ About 1200-1500ml daily with pH 7.5-8.5 of clear, colorless liquid consisting: 1. Mostly water. 2. Enzymes for digestion of all types of food 3. Sodium bicarbonate (buffers stomach acidic chyme).  The pancreatic juice What prevents the pancreas from digesting itself? 1. Inactive enzymes (zymogens) 2. Trypsin inhibitor: the same cells that secrete proteolytic enzymes into the acini of the pancreas also secrete trypsin inhibitor. Trypsin Inhibitor prevents activation of trypsin both inside the secretory cells and in the acini and ducts of the pancreas. And, because it is trypsin that activates the other pancreatic proteolytic enzymes, trypsin inhibitor prevents activation of the Brush border enzymes (not part of juice) others as well. (enteropeptidase) activate trypsinogen to trypsin then trypsin activates all other proteolytic enzymes Regulation of pancreatic secretion  Hormonal control pancreatic exocrine secretion Secretin CCK. Vagal stimulation stimulates secretion of enzymes Predict what happens to pancreatic secretion during the cephalic, gastric, and intestinal phases. Which phase is the most potent? What are the mediators? The liver and the gallbladder 9  Blood supply to the liver Receives blood from: 1a: Hepatic artery (arterial blood) 1b: Hepatic portal vein (venous blood from the digestive organs) b. Hepatic vein: carries venous blood from the liver  Histology of the liver ▪ Repeating functional units called liver lobules made of hepatocytes arranged around a central vein ▪ Hepatocytes are the major functional cells of the liver, the body’s “chemical factories.” ▪ Blood supply is from the hepatic artery and hepatic portal vein, and blood is left by the hepatic vein. ▪ Hepatic sinusoids (very leaky capillaries). ▪ Kupffer’s cells (stellate reticuloendothelial cells) – fixed macrophages. ▪ Branches of the hepatic portal vein, hepatic artery, and bile duct form the portal triad.  Functions of the liver related to digestion The liver has many important functions. However, the secretion of bile salts, which aid fat digestion and absorption, is the only liver function directly related to digestion.  Bile ▪ Bile is secreted continuously from the liver. ▪ Hepatocytes secrete 500–1000 ml of bile/day. ▪ It is a yellow, brownish, or olive-green liquid with a pH of 7.6–8.6. ▪ Bile flowing out of the liver is stored and concentrated in the gallbladder until it is released in response to the ingestion of a meal. Composition of bile 1. Bile acids (synthesized from cholesterol) 2. Mostly of water 3. Cholesterol. 4. Lecithin (phospholipid). 5. Bile pigments 6. Inorganic ions (mainly HCO3- and Cl-) The path of bile from the liver to the duodenum ▪ Between meals, the sphincter of Oddi is closed. Bile is diverted to the gallbladder, where it is stored and concentrated 5-20 times. The gallbladder musculature relaxes to receive the bile ▪ During digestion, the sphincter of Oddi is open, and the gallbladder contracts so bile enters the duodenum. ▪ The functions of the gallbladder are the storage, concentration, and discharge of bile into the intestine ▪ What contracts the gallbladder and opens the sphincter? Liver secretion and gallbladder emptying Vagus CCK Secretin  Function of bile salts (emulsification) in digestion Overall, emulsification greatly increases the Fat emulsification total surface area of the fatty substance, and Is the process by which fat globules the resulting droplets mix with water. Lipases are broken into smaller pieces (reduce (and co-lipase) can then digest the fat surface tension) by the detergent molecules more effectively action. Bile salts are emulsifiers. They have water-soluble and lipid-soluble portions (amphipathic).  Functions of bile salts in fat digestion and absorption ▪ Lipase digests TG and releases free fatty acids and mono glycerol. ▪ Bile salts form small cylindrical globules called micelles ▪ A micelle is 3 to 10 nm thick, while emulsified fat droplets are 200-5000 nm. micelles : water soluble Bile acid, cholesterol, mono glycerol, fatty acids, fat-soluble vitamins Bile salts enhance the absorption of fatty acids and cholesterol by forming complexes (micelles) that are very soluble in chyme Enterohepatic circulation of bile salts ▪ Bile salts are synthesized from cholesterol ▪ Most bile salts are recycled between the liver and small intestine through the enterohepatic circulation What stimulates bile salt secretion? ▪ The most potent choleretic is bile salts themselves ▪ During a meal, as more bile salts are returned by the enterohepatic circulation to the liver, they act as potent choleretics to stimulate further bile secretion. 5% enter the colon reabsorbed by Na- bile salt co- transporter