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Gastro Intestinal System Dr. V.P.C.Rajakruna MBBS (COLOMBO) Digestion • Processing of food • Types – Mechanical (physical) • • • • • Chew Tear Grind Mash Mix – Chemical • Catabolic reactions • Enzymatic hydrolysis – Carbohydrate – Protein – Lipid • Phases – Ingestion – Movement – Digestion – Absorption – Further digestion Anatomy of GI System – Structures ( Tubal pathway) • Mouth - stratified squamous • Oral Cavity - stratified squamous • Pharynx - stratified squamous epithelium • Esophagus - simple-columnar • Stomach - simple columnar • Duedenum - simple columnar • Jejenum- simple columnar • Ileum - simple columnar • Cecum - simple columnar • Ascending colon - simple columnar • Transverse colon - simple columnar • Descending colon - simple columnar • Sigmoid colon- simple columnar • Rectum - nonkeratinized statified squamous • Anus- nonkeratinized statified squamous • Accessory structures – Not in tube path – Organs • Teeth • Tongue • Salivary glands • Liver • Gall bladder • Pancreas Layers of GI System • • • • • • Consist of 4 tunics From the inside out: Mucosa Submucosa Muscularis Serosa Mucosa • Lines the lumen • Consist of epithelium,a connective tissue layer called lamina propria and a thin layer of smooth muscle layer called muscularis mucosae • Muscularis mucosae is responsible for the folding of GI tract Submucosa • Highly vascular layer of connective tissue • Consist of nerve plexus which supply the autonomic nerves ( Meissner’s plexus) Muscularis • Also called muscularis externa • Consist of inner circular and outer longitudinal smooth muscle layers • Responsible for the segmental contractions and the peristaltic movements of GI tract • Mesenteric plexus is situated in between these two layers ,provide the major nerve supply to the GI tract Serosa • completes the wall of GI tract • Consist of areolar tissue The physiological functions of the GI system • 1. Motility – movement of food through GI tract • Ingestion – taking food in to the mouth • Mastication – chewing the food and mixing with saliva • Deglutition – swallowing food • Peristalsis – rhythmic wave like contractions that move food through the GI tract • 2. Secretion • Exocrine – digestive enzymes, HCl,HCO3• Endocrine - hormones • 3. Digestion • Break down of food molecules in to smaller sub units which can absorb • 4. Absorption • Passage of digested end products in to blood or lymph Oral Cavity • Mouth or oral cavity – Vestibule: Space between lips or cheeks and alveolar processes – Oral cavity proper • Lips (labia) and cheeks • Palate: Oral cavity roof – Hard and soft • Palatine tonsils • Tongue: Involved in speech, taste, mastication, swallowing Teeth • Two sets – Primary, deciduous, milk: Childhood – Permanent or secondary: Adult (32) • Types – Incisors, canine, premolar and molars Salivary Glands • Produce saliva – Prevents bacterial infection – Lubrication – Contains salivary amylase and lipase • Breaks down starch and lipids • Three pairs – Parotid: Largest – Submandibular – Sublingual: Smallest Pharynx Nasopharynx Oropharynx: Transmits food normally Laryngopharynx: Transmits food normally Esophagus Transports food from pharynx to stomach Muscular tube approximately 25cm in length Located posteriorly to the trachea within the mediastinum of the thorax Passes through esophageal hiatus (opening) of diaphragm and ends at stomach Sphincters • Upper • Lower Skeletal muscle Skeletal muscle + smooth muscle Smooth muscle Deglutition (Swallowing) • Three phases – Voluntary • Bolus of food moved by tongue from oral cavity to pharynx – Pharyngeal Reflex: Upper esophageal sphincter relaxes, elevated pharynx opens the esophagus, food pushed into esophagus – Esophageal • Reflex: Epiglottis is tipped posteriorly, larynx elevated to prevent food from passing into larynx • Swallowed food is pushed from one end of the esophagus to the other along the intestine by a wave like muscular contraction called peristalsis • Peristalsis is produced by a series of localized reflexes in response to the distension of the GI tract wall by the food bolus • Passing of food bolus along the GI tract happens due to muscular contractions and relaxations Stomach • Openings – Gastroesophageal: To esophagus – Pyloric: To duodenum • Regions – Cardiac – Fundus – Body – Pyloric • J shaped • Most distensible part of the GI tract • Layers – Serosa – Muscularis: Three layers • Outer longitudinal • Middle circular • Inner oblique – Submucosa – Mucosa Special Features • Rugae: Folds in stomach when empty • Gastric pits: Openings for gastric glands • Gastric gland containing cells • Goblet cells: Mucus • Parietal cells: Hydrochloric acid and intrinsic factor • Chief cells : Pepsinogen • Enterochromafin like cells: Histamine • G cells: Gastrin • D cells: Somatostatin • The exocrine secretions of the gastric cells together with large amount of water form highly acidic solution known as Gastric juice ( pH = 2 or less than 2 ) Functions of stomach • • • • Store food Initiate digestion of protein Kill bacteria Move the food into small intestine Pepsin and HCl secretion • The parietal cells secrete H+ , at a pH as low as 0.8, in to the gastric lumen by active transport against million to one concentration gradient • This is done by H+/K+ ATPase pumps, moving K+ in to the cell cytoplasm from the gastric lumen while secreting H+ in to the gastric juice Functions of gastric juice • Ingested proteins are denatured at low pH • Pepsinogen converts in to active pepsin • Pepsin is more active under acidic conditions • Kills bacteria Digestion and absorption in the stomach • Proteins are only partially digested • Carbohydrates and lipids not digested at all ( salivary amylase inactivated by acidic media) • Absorption??????????? certain drugs yes Mechanism of gastric secretion Smell, taste of food CNS activation and send the commands to stomach Increase volume of gastric juice and secretion of gastrin Cephalic phase – few minutes Stimulation of stretch receptors of the stomach wall Increase Pepsin production Stimulation of chemoreceptors as pH increases Increase of Acid production Initiate the digestion of proteins Stimulation of parasympathetic NS Stimulation of Gastrin production Increase motility and initiating mixing waves Gastric Phase – 3 to 4 hours Distension of duodenum Stimulation of cholesytokinin, gastric inhibitory peptide in the presence of acids, carbohydrates and lipids Inhibition of gastric acid and pepsinogen secretion Reduction of gastric mortality Control of food entering in to the duodenum Intestinal phase – take hours Small Intestine • Site of greatest amount of digestion and absorption • Extends from pyloric sphincter of the stomach to the ileocecal valve opening into the large intestine • Approximately 3m in length • Longest part of the GIT • Divisions – Duodenum – 20-30cm – Jejunum – 2/5 from the rest – Ileum – 3/5 from the rest (Peyer’s patches or lymph nodules) • Modifications – Circular folds or plicae circulares, villi, lacteal, microvilli( increase the surface area) • Cells of mucosa – Absorptive, goblet, granular, endocrine Small Intestine Secretions • Mucus – Protects against digestive enzymes and stomach acids, allow smooth transmit of food bolus • Digestive enzymes – Disaccharidases: Break down disaccharides to monosaccharides – Peptidases: Hydrolyze peptide bonds – Nucleases: Break down nucleic acids • Duodenal glands – Stimulated by vagus nerve, secretin, chemical or tactile irritation of duodenal mucosa Intestinal contractions and motility • Peristalsis- much weaker than in the stomach and esophagus • Intestinal motility ( the movement of food bolus or chyme ) is slow • Segmentation – major contractile activity • This occur by the smooth muscles of small intestine • Smooth muscle of intestine capable of spontaneous electrical activity and auto rhythmic contractions Intestinal Reflexes • Gastroileal – increased gastric activity causes increased motility of the ileum and increased motility of the ileum and increased movement of chyme through the ileocecal sphincter • Ileogastric – distension of the ileum causes a decrease in gastric motility • Intestino intestinal – overdistention of one intestinal segment causes relaxation through out the rest of the intestine Functions of Small Intestine • Mixing: Segmental contraction that occurs in small intestine • Secretion: Lubricate, liquefy, digest • Digestion: Mechanical and chemical • Absorption: Movement from tract into circulation or lymph • Elimination: Waste products removed from body What are absorbed? • Absorption of carbohydrates, lipids , amino acids, calcium and iron occurs primarily in duodenum and jejunum • Bile salts, vitamin B12, water and electrolytes are absorbed primarily in the ileum Large Intestine • Extends from ileocecal valve to anus • Consists of cecum, colon, rectum, anal canal • Movements sluggish • Cecum – Blind sac, vermiform appendix attached • Colon – Ascending, transverse, descending, sigmoid • Rectum – Straight muscular tube • Anal canal – Internal anal sphincter (smooth muscle) – External anal sphincter (skeletal muscle) Secretions of Large Intestine • Mucus provides protection – Parasympathetic stimulation increases rate of goblet cell secretion • Pumps – Exchange of bicarbonate ions for chloride ions – Exchange of sodium ions for hydrogen ions • Bacterial actions produce gases called flatus Movement in Large Intestine • Mass movements – Common after meals • Local reflexes in enteric plexus – Gastrocolic: Initiated by stomach – Duodenocolic: Initiated by duodenum • Defecation reflex – Distension of the rectal wall by feces • Defecation – Usually accompanied by voluntary movements to expel feces through abdominal cavity pressure caused by inspiration Function of Large Intestine • • • • Little or no digestive function Absorb water passively Absorb electrolytes Absorb several B complex vitamins and vitamin K • Resident bacteria in large intestine also produce significant amount of Vit K and Folic acid which absorb by large intestine • Forms, stores and expels feces Defecation Liver • Immediately below the diaphragm in the abdominal cavity • Lobes – Major: Left and right – Minor: Caudate and quadrate • Ducts – Common hepatic – Cystic • From gallbladder – Common bile • Joins pancreatic duct at hepatopancreatic ampulla Structure of the Liver • Hepatocytes Hepatic plates Liver Lobules ( Functional Unit of the liver ) • Hepatic plates are separated each other by large capillary spaces called sinusoids • Sinusoids are lined by phagocytic kupffer cells • Middle of the lobule contains central vein central vein connects with branches of hepatic vein and hepatic artery via sinusoids • Bile is produced by the hepatocytes and secreted in to channels called bile canaliculi, located within each hepatic plate Bile canaliculi Bile duct Hepatic ducts ( carry bile away from the liver ) Hepatic Portal System • The products of digestion absorbed in to the blood capillaries in the intestine do not directly enters to the general circulation • These capillaries drain in to the hepatic portal vein • It carries this blood in to the capillaries in the liver • Then enters to the general circulation through the hepatic vein that drains the blood from the liver • Hepatic artery gives the oxygenated blood to the liver Enterohepatic circulation • In addition to the bile a wide variety of compounds secreted by the liver in to the bile ducts • These products enter to the intestine • Some are excreted with the feces • Some are again enter to the blood via the intestinal absorption • Via the intestinal capillaries they enter to the hepatic portal blood • This recirculation of compounds call enterohepatic circulation Functions of the Liver • Bile production – Salts emulsify fats, contain pigments as bilirubin • Storage – Glycogen, fat, vitamins, copper and iron • Nutrient transport • Detoxification – Hepatocytes excrete the unwanted compounds – Chemical alteration of compounds – phagocytosis • Phagocytosis – Kupffer cells phagocytize dying red and white blood cells, some bacteria • Synthesis – Albumins, fibrinogen, globulins, heparin, clotting factors Gallbladder • Sack like organ attached to the inferior surface of the liver • Bile is stored and concentrated • Stimulated by cholecystokinin and vegal stimulation in the arrival of chyme to the intestine Cystic • Liver Bile duct Hepatic duct duct Gall Bladder Gall Bladder Cystic Duct Common Bile Duct Duodenum • When the small intestine is empty of food, the sphincter of Oddi at the end of common bile duct closes and bile is forced up to the cystic duct and then to the gall bladder for storage. Bile • • • • • • • Bile acid Phospholipids Cholesterol Bilirubin Waste products Electrolytes Mucin Pancreas • Anatomy – Endocrine • Pancreatic islets produce insulin and glucagon – Exocrine • Acini produce digestive enzymes – Regions: Head, body, tail • Secretions – Pancreatic juice (exocrine) • Trypsin – digest protein • Chymotrypsin - digest protein • Pancreatic amylase - digest carbohydrates • Pancreatic lipases – digest lipids • Enzymes that reduce DNA and ribonucleic acid • Carboxypeptidase - digest protein • Bicarbonate ions Pancreatic juice secretion stimulated by secretin and cholecystokinin • Acidic chyme rapidly neutralized by the alkaline pancreatic juice in the small intestine • Vomiting • Diarrhea Alkalosis Acidosis