Lecture 15 - Physiology of the Small Intestine and Accessory Organs 1 PDF

Physiology of the small intestine and accessory organs 1 Dr Susan Currie HW424 [email protected] Learning outcomes To know the physiology of the accessory organs to the upper GI tract To understand how each of these organs is under neuronal and hormonal control To understand t...

Physiology of the small intestine and accessory organs 1 Dr Susan Currie HW424 [email protected] Learning outcomes To know the physiology of the accessory organs to the upper GI tract To understand how each of these organs is under neuronal and hormonal control To understand the role of each of these organs in digestion Accessory organs and digestion Pancreas – Structure – Function – Regulation of secretions Liver – Structure – Metabolic function – Bile Gallbladder  Falciform ligament Left hepatic duct LIVER Right hepatic duct Cystic duct Common hepatic duct Common bile duct GALL BLADDER Accessory duct PANCREAS Tail DUODENUM Body Duodenal Pancreatic duct papilla Head Hepatopancreatic ampulla  Pancreas - functions Exocrine (99% of cells) – Accessory organ to digestion – Secretion of enzyme-rich fluid into duodenum – Break down all categories of digestible food Endocrine (1% of cells) – Release of hormones into bloodstream that affect carbohydrate metabolism Insulin (Islet -cells) Glucagon (Islet -cells) Somatostatin (Islet -cells)  Pancreas - structure Lobule Structure similar to salivary glands Duct Acinar cells secrete enzymes and fluid Main into duct system Acinus Duct Interlobular duct duct Pancreatic duct fuses with bile duct on Cholecystokinin acinar ACh entry to duodenum Enzyme cells secretion Constitution of pancreatic juice under control of local nerves and hormones duct cells Secretin HCO - rich 3  Composition of Pancreatic juice Aqueous component Enzymatic component Due to the potency of its enzyme contents, the pancreas is a very dangerous organ to injure.  Aqueous component 200-800ml secreted each day Rich in bicarbonate (pH8) Helps to neutralise acidic chyme as it enters the duodenum Secretion stimulated by secretin Enzymatic component Proteolytic enzymes (cleave peptides) – trypsin, chymotrypsins, carboxypeptidases – Secreted in inactive form, activated in duodenum by Enterokinase. – Autodegradation of the pancreas may occur if these enzymes were secreted in active form Pancreatic amylase (breaks down starch) – Majority of starch digestion – secreted in active form Lipolytic enzymes (fat digestion) – Lipase – secreted in active form – colipase, cholesterol esterase, phospholipase A2 - activated by trypsin in duodenum  Regulation of secretions Parasympathetic CHYME containing partially digested impulses proteins, fats etc secretion of pancreatic enzymes secretion of aqueous secretion of pancreatic pancreatic juice juice rich in digestive enzymes SECRETIN CHOLECYSTOKININ Control of pancreatic secretions Under nervous and hormonal control Nervous reflex involves medulla and vagal innervation (cephalic phase) Gastrin released in response to stomach distension (gastric phase) Secretin and CCK secreted by mucosa in response to presence of chyme in duodenum (intestinal phase) – secretin – bicarbonate-rich aqueous juice – CCK - enzymatic juice  What is pancreatitis? Pancreatic enzymes are activated within the pancreas, causing them to attack the organ itself. Two forms – acute and chronic. Acute pancreatitis becomes chronic when pancreatic tissue is destroyed and scarring develops. Causes – gallstones, alcohol abuse, unknown cause (idiopathic) Treatment – pain and infection management, electrolyte therapy. In severe cases – surgical intervention.  The Liver Lies in abdomen under diaphragm – Largest organ in the body (1.8kg in men, 1.3kg in women) – Holds ~13% of the total blood supply at any given moment. – Has over 500 (!!) estimated functions.  The Liver – some functions Processing digested food from intestine Manufacture of bile Storage – Converts excess monosaccharides to glycogen – Storing iron, vitamins and other essential chemicals Metabolism – Breaks down stored glycogen, fat or protein to glucose (hormonal control) – Metabolises drugs and breaks down poisons – Bactericidal activity (Kupffer cells)  Liver structure Hepatic (liver) cells arranged in radial pattern around central vein Hepatocytes make up functional units called lobules Lobules form lobes of liver (two main lobes)  Liver structure Central Sinusoids vein Liver cells Kupffer cells Space of Disse Bile canaliculi Terminal lymphatics Lymphatic vessel Portal vein Portal Hepatic triad artery Bile duct  Hepatic blood supply Receives double blood supply – oxygenated blood from hepatic artery – de-oxygenated, nutrient rich blood from portal vein Hepatic cells – extract oxygen and most nutrients – detoxify or store poisons and drugs – secrete products (except bile) into hepatic vein Blood supply to the liver the enterohepatic circulation Aorta Inferior vena cava Hepatic artery 30% of blood flow to liver Hepatic vein 70% of blood flow to liver Portal vein  Bile Two characteristics – excretory product of liver metabolism – a digestive secretion Bile salts emulsify fat into small droplets Bile cholesterol made soluble by bile salts Bile pigments (bilirubin) which are yellow/brown give faeces and urine their colour. Absorbed from blood. (salts, cholesterol and pigments secreted by liver cells (hepatocytes)) Bicarbonate ions (neutralise acid chyme) secreted by epithelial cells lining bile ducts. Secretion from the liver increased by vagal stimulation and secretin Circulation of bile Enterohepatic circulation  Gallbladder Stores and concentrates bile by extracting water and ions. Can lead to increased insoluble cholesterol levels. Bile enters gallbladder by cystic duct when small intestine is empty Ejection of bile into duodenum occurs when – protein or fat-rich chyme enters duodenum causes CCK release – CCK causes gallbladder to contract and sphincter of Oddi to relax thus allowing bile to enter the duodenum.  What are gallstones? Crystalline deposits that accumulate when there is too much cholesterol and not enough bile salts. Can be dissolved or if severe, can have gall bladder removed (cholecystectomy)  Why is jaundice a symptom of gallstones? Stones can block the common bile duct Increased levels of bilirubin in blood plasma Discolouration of skin (jaundice) Reinforcement of learning points To know the physiology of the accessory organs to the GI tract – Pancreas – Liver – Gall Bladder To understand how each of these is under neuronal and hormonal control To understand the role each of these organs plays in digestion – Production of enzymes – Production and storage of bile