GI PHYSIOLOGY 3: Liver and Pancreas - PDF

G I PHYS I O LO GY- 3 PA N C R E A S A N D L I V E R P H YS I O LO GY DR LISA HOLBROOK 12/11/24 Code: L E A R N I N G O B J E C T I V E S - PA N C R E A S Define the endocrine and exocrine functions of the pancreas Describe the composition of pan...

G I PHYS I O LO GY- 3 PA N C R E A S A N D L I V E R P H YS I O LO GY DR LISA HOLBROOK 12/11/24 Code: L E A R N I N G O B J E C T I V E S - PA N C R E A S Define the endocrine and exocrine functions of the pancreas Describe the composition of pancreatic juice and the function of each component Describe how pancreatic juice secretion is regulated during the cephalic, gastric and intestinal phases of digestion Define the roles of secretin and CCK in regulation of pancreatic juice secretion Describe differences in ruminant, equine and canine pancreatic juice composition and secretion T H E PA N C R E A S - OV E R V I E W ENDOCRINE VS EXOCRINE FUNCTION Endocrine function Produces hormones, eg insulin and glucagon Hormones secreted directly into the bloodstream Exocrine function Pancreatic juice Site of action is the small intestine Duct mediated secretion similar to salivary secretion  STRUCTURE Microscopic structure similar to salivary glands - secretory Secretions move through intercalated ducts into one large duct Secretes into the duodenum  SPECIES DIFFERENCES In most species the pancreatic duct secretes directly into the duodenum In goats and sheep the pancreatic duct joins the common bile duct before the entrance into the duodenum Mixture of bile and pancreatic juice enters the duodenum PA N C R E AT I C J U I C E C O M P O N E N T S HCO3- and Cl- (concentration depends on secretion rate, faster = lower concentration) Enzymes: Lipase (breaks down fat  glycerol + FAs) Amylase (breaks down starch  glucose and maltose) Proteases (breaks down peptides  amino acids) A C T I VAT I O N O F PA N C R E AT I C E N Z Y M E S - All enzymes are secreted in their inactive form (inactive precursors) - Trypsin = activates all enzymes - Secreted as trypsinogen - Trypsin is formed in the duodenum by enteropeptidase - Autocatalysis = the process where the reaction product is also the catalyst for the reaction - Gland cells secrete trypsin inhibitor which can limit innapropriately activated trypsins F U N C T I O N O F PA N C R E AT I C J U I C E Alkaline (due to HCO3-) Neutralises ingesta/chyme entering the duodenum from the stomach Preventing injury to mucosa Providing optimal pH for enzymatic activity - Produced in large volumes 3 PHASES OF JUICE SECRETION Cephalic phase Changes that occur before food enters the stomach – prehension, mastication and deglutition Gastric phase Changes in digestive tract motility initiated in the stomach – peristalsis and churning of food as a trigger Enzyme production increases with feeding in most species (increase in secretion of pancreatic juice) In pigs and horses there are also increases in water and ion secretion Intestinal phase Chyme entering the duodenum causes increases in pancreatic secretion This is regulated by CCK and secretin SECRETIN Released from duodenum upon increased acidity Neuroendocrine peptide hormone Action on many organ systems Secreted by the S cells of the duodenum Stimulates HCO3- secretion - Neutralisation function Once acid in duodenum decreases, secretin production is reduced  negative feedback  CCK peptide hormone, secreted by the duodenum released when FAs and peptides/amino acids increase in duodenum stimulates pancreas to increase enzyme secretion Increases bile secretion increased enzymes = increased ingesta degradation = increased products of digestion  positive feedback SPECIES DIFFERENCES Ruminants constant rate of secretion and chyme transport Usually 4-5l of pancreatic juice produced in 24hours Horses increase in rate of secretion within a few minutes of feeding starting No change in bicarbonate concentration, but continuous secretion ensures stable conditions for microbial degradation low level of enzymes due to microbial degradation Usually 30-35l of pancreatic juice produced in 24hours Cats and Dogs great increase in the amount of juice produced after feeding little production when small intestine is empty 20kg dog will produce 2-3ml/min W H AT W E ’ V E C O V E R E D - PA N C R E A S  Define the endocrine and exocrine functions of the pancreas  Describe the composition of pancreatic juice and the function of each component  Describe how pancreatic juice secretion is regulated during the cephalic, gastric and intestinal phases of digestion  Define the roles of secretin and CCK in regulation of pancreatic juice secretion  Describe differences in ruminant, equine and canine pancreatic juice composition and secretion LEARNING OBJECTIVES - LIVER Introduce the role of the liver Describe the role of bile salts Describe the production of bile salts and regulation of their secretion Describe the role of the gallbladder in storing and concentrating bile Describe the differences in ruminant, equine and canine bile secretion ROLE OF THE LIVER The liver handles nutrients absorbed from the intestine and regulates their release into the blood Inactivating substances (toxins, drugs) Conversion of lipid soluble substances to water-soluble metabolites Excretion of substances Production of plasma proteins Production of blood coagulation factors Excretion of bile pigments Production of cholesterol Bile production by hepatocytes STRUCTURES  BILE PRODUCTION - Made from cholesterol - Active transport processes - Bile salts are transported from the hepatocytes into bile canaliculi - Bile canaliculi gradually feed into larger canals and then into the common bile duct  B I L E S A LT S Produced from cholesterol (Bile acids) Conjugated bile acids (bound to amino acids) attract sodium to become bile salts Bile salts digest fat in the intestine Phospholipids and cholesterol are also secreted from hepatocytes in bile Phospholipids – aid with fat digestion Cholesterol – waste product  BILE STORAGE Gallbladder = site of storage when no digestion is occurring Extending from common bile duct is a side duct to the gall bladder Contraction controlled by CCK Sphincter of Oddi = band of smooth muscle cells between the bile duct and duodenum  B I L E R ECYC L I N G - Bile salts are not lipid soluble = not absorbed by diffusion from the SI - They function throughout the entire length of the SI before being absorbed at its end - Absorbed bile salt molecules are returned to the liver = enterohepatic circulation  R E G U L AT I O N O F B I L E S E C R E T I O N Stimulated by increased parasympathetic activity mediated via vagal nerves and secretin Most important factor determining the volume of bile is the amount of bile salts in the bile canaliculi - when the concentration of bile salts in the portal blood rises the amount of bile released from the hepatocytes increases R E G U L AT I O N O F B I L E S E C R E T I O N Between meals bile is stored in the gall bladder blood concentration of bile salts and, therefore, excretion of bile salts remains low Bile production is greatest when bile salts recirculate after a meal Secretin: Release stimulated by acidic duodenal content Causes increase of HCO3- from pancreas Causes increase of bile production R E G U L AT I O N O F B I L E S E C R E T I O N When there is no food in the SI, the sphincter of Oddi is closed and gallbladder relaxed Bile gets concentrated in the gall bladder Ions get actively transported out of the bile  water follows by osmosis After a meal the gallbladder contracts and the sphincter of Oddi opens In response to CCK Fat and peptides in the duodenum cause CCK release Ensuring pancreatic and hepatic secretions coincide When fat digestion is complete, the stimulus for CCK release is removed and bile is st in the gall bladder SPECIES DIFFERENCES Dogs and cats fat digestion is intermittent, therefore bile is not present in the intestine all the time  very useful to have a gallbladder Ruminants Gallbladder has a short retention time – poorly developed concentrating mechanism Horses No gallbladder Trickle feeders…  W H AT W E ’ V E C O V E R E D Introduce the role of the liver Describe the role of bile salts Describe the production of bile salts and regulation of their secretion Describe the role of the gallbladder in storing and concentrating bile Describe the differences in ruminant, equine and canine bile secretion LINKS AND READING Cunninghams Veterinary Physiology Pages 291-295 Chapter 32 Physiology of Domestic Animals (3rd edition) Pages 688-697 SF2 discussion forum for questions [email protected]