GIT Secretions & Absorption PDF

GIT- secretions and absorption Dr Sovan Bagchi Professor December 8, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE  Learning Objectives List the parts of digestive sy...

GIT- secretions and absorption Dr Sovan Bagchi Professor December 8, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE  Learning Objectives List the parts of digestive system List the functions of digestive system Describe nerve supply to the gastro intestinal tract Describe the composition of salivary secretion Describe the functions of salivary secretion and its regulation Describe functions of stomach and its secretions Explain the roles of bile and of pancreatic juice in digestion Describe the function and regulation of intestinal secretions  Digestive system Chief goal is to break down macromolecules (proteins, fats and carbohydrate) into smaller molecules (amino acids, fatty acids and glucose).  Functions of Digestive System 1. Ingestion of food substances into mouth 2. Secretion of necessary enzymes for digestion 3. Digestion of the food particles 4. Absorption of digested products 5. Removal of unwanted substances from the body.  Enteric Nervous System 1.Myenteric (Auerbach’s) Plexus: Regulates intestinal motility - Increased tonic contraction and rate of the rhythm of contraction 2.Submucosal (Meissner’s) Plexus: Regulates secretory activity - Control local intestinal secretion and local absorption.  Autonomic control of the GIT Parasympathetic Nerve Fibers Increase the secretory activities of the glands in the GI tract and increase the motility of GI tract. Sympathetic Nerve Fibers Inhibit the movements of GI tract and decrease the secretions of GI tract  Swallowing (Deglutition) Swallowing is coordinated activity of the tongue, soft palate, pharynx, esophagus and muscles. This has 3 stages. 1. Voluntary stage (Buccal stage) 2. Pharyngeal stage, which is involuntary and food through the pharynx into the esophagus 3. Esophageal stage – involuntary - food from the pharynx to the stomach.  Salivary Glands 1. Parotid is in front of the ear 2. Submandibular is on the posterior part of the mouth 3. Sublingual is anterior part of the floor of the mouth, under the tongue and small Buccal Gland  Composition of Saliva 0.8 to 1.5 liter of saliva is secreted in a day. It has pH between 6 to 7 Hypotonic 99.5% water 0.2% inorganic constituents 0.3% organic constituents  Inorganic Constituents NaCl & KCl as coenzyme for salivary amylase. H2CO3: NaHCO3 & NaHPO4 - Na2HPO4 as buffer systems. CaHCO3 & HPO4 activate salivary amylase. Thiocyanate (from metabolism of proteins) bactericidal action.  Organic constituents Ptyalin ( α-amylase), lysozyme & lingual lipase. Albumin, globulins, mucin, traces of urea, uric acid & lactic acid.  FUNCTIONS OF SALIVA Digestion - starch by salivary α amylase, lingual lipase – fat digestion Oral hygiene - Anti-bacterial (lysozyme) Antibody (IgA) Lubrication - Moistening of food to facilitate swallowing Articulation - Facilitation of speech Lactoferrin – Iron binding protein- iron absorption  FUNCTIONS OF SALIVA Solvent - Facilitate taste sensation - Chemical stimulation of taste buds Buffering action - Protection against any irritating substances and Neutralisation of excessive cold or hot food. Buffering - Protects oral cavity from excessive acids or alkalis.  Functional anatomy Esophagus Leaser curvature Cardiac sphincter Fundus Greater curvature Body Antrum Pylorus Pyloric sphincter Duodenum  Functions of stomach 1. Storage function 2. Mixing and grinding function 3. Gastric emptying 4. Secretion of Hydrochloric acid 5. HCl kills the bacteria 6. HCl activates Pepsinogen to Pepsin 7. Secretion of intrinsic factor  Composition of Gastric Juice 1 to 1.5 L /d Gastric juice pH 0.8 to 4 Water 99.5% Solid 0.5% Organic In organic Pepsin, Rennin, lipase (tributyrase), gelatinase, HCL, Na, K, HCO3-, urase, amylase. Corbonic Cl- , PO4 , SO4 & anhydrase, Mucin and IF Mg++  Stomach glands Tubular structure – ‘3’ types 1.Cardiac glands - cardiac region 2.Oxyntic or Fundic or Gastric glands - body and fundus of the stomach 3. Pyloric glands - pyloric region In Oxyntic mucosa In pyloric gland area Surface epithelial cells -secretes mucous Gastric pit Mucosa cells -secretes mucous Gastric G cells gland Chief cells -secrete -secretes pepsinogen gastrin Parietal cells -secretes HCl and IF Enterochomaffin- D cells -secretes like (ECL) cells somatostatin -secretes histamine  PEPSINOGEN - PEPSIN Pepsinogen (MW 42500) is synthesized from amino acids in chief cells - Type I & type II pepsinogen. Pepsinogen is activated into pepsin (MW 3500) by HCl. It convert the protein into peptone, proteases and polypeptides It is active only pH between 1.8 - 4.0 (acidic pH) - above 4.0 pH pepsin is inactivated. Sphincter of oddi Composition of Pancreatic Juice Volume: 500 to 1000 ml/day Reaction: Highly alkaline with pH of 8 to 8.3 Pancreatic juice contains 99.5% of water and 0.5% of solids The bicarbonate content is 110 to 150 mEqu/ L. Enzyme Activator Substrate End products Proteoses and 1. Trypsin Enterokinase Proteins Polypeptides 2. Chymotrypsin Trypsin Proteins Polypeptides Trypsin Polypeptides Amino acids 3.Carboxypeptidases 4. Nucleases Trypsin RNA and DNA Mononucleotides 5. Elastase Trypsin Elastin Amino acids 6. Collagenase Trypsin Collagen Amino acids  Monoglyceride 7. Pancreatic Alkaline Triglycerides s and fatty lipase medium acids Cholesterol 8. Cholesterol Alkaline Cholesterol ester and ester hydrolase medium fatty acids 9. Lysophospholip Trypsin Phospholipids PhospholipaseA ids 10. Phosphoryl Trypsin Lysophospholipids PhospholipaseB choline & FFA Facilitates action 11. Colipase Trypsin - - - of trypsin 12. Pancreatic Dextrin and - - - Starch amylase maltose  Regulation of Pancreas secretion 1. Acetylcholine, parasympathetic - vagus – Acinar cell - causes pancreatic enzymes. 2. Cholecystokinin (CCK) - Acinar cell - causes pancreatic enzymes. 3. Secretin - pancreatic ductal epithelium – water and NaHCO3.  Functions of Liver 1. Secretion of bile 2. Synthetic function - plasma proteins 3. Metabolic function – CHO, Fat, Protein & Vit 4. Excretory function - cholesterol, bile pigments, heavy metals, bacteria and virus 5. Hemopoietic function - In fetus 6. Defensive and detoxification function 7. Inactivation of hormones and drugs - GH, parathormone, cortisol, insulin, glucagon and estrogen  Bile Volume : 600 to 1000ml / day pH: 7.8 alkaline Constituents: Water, Cholesterol, Fatty acids, mucin, electrolytes, lecithin, bile salts & bile pigments. Concentrated in gall bladder about 5 fold to 20 fold - gallbladder can hold 30 to 60 ml Bile - important role in fat digestion and absorption – no digestive enzyme present  Function of the bile 1. Digestive and Absorptive functions - Fat 2. Excretory Functions - Heavy metals, toxins, Cholesterol & typhoid bacteria 3. 4. Laxative Action 4. 5. Antiseptic Action- inhibits bacterial growth 5. 6. Choleretic Action – secretion of bile 6. 7. Maintenance of pH in GIT - neutralize acid chyme 7. Prevention of Gallstone formation 8. Lubrication function 9. 10. Cholagogue Action - increases the release of bile from gallbladder  Formation of bile salt The bile salts are Na and K salts of bile acids and bile salts conjugated with glycine or taurine in liver. The bile acids are synthesized from cholesterol. The 2 principal bile acids formed in the liver are cholic acid and chenodeoxycholic acid. In the colon, bacteria convert cholic acid to deoxycholic acid (secondary) and chenodeoxycholic acid to lithocholic acid (secondary).  Jaundice (Icterus) Definition - The yellow pigmentation of the skin & mucus membrane. Normal serum bilirubin = 0.2 - 1 mg/dl Types: 1. Haemolytic jaundice (Prehepatic) 2. Hepatocellular jaundice (Hepatic) 3. Obstructive jaundice (posthepatic)  Intestinal digestive juices Volume: 1800 ml/day pH: alkaline 7.5 to 8.0. Composition: water (99.5%) and solids (0.5%). Enzyme Substrate End products 1. Peptidases Peptides Amino acids Fructose and 2. Sucrase Sucrose glucose 3. Maltase Maltose and maltriose Glucose Galactose and 4. Lactase Lactose glucose Dextrin, maltose and 5. Dextrinase Glucose maltriose 6. Trehalase Trehalose Glucose 7. Intestinal lipase Triglycerides Fatty acids  Regulation of Small Intestine Secretion Nerve: local enteric nervous reflexes - initiated by tactile or irritative stimuli from the chyme. Parasympathetic nerves increases the secretion. Sympathetic nerves decreases secretion. Hormone - Secretin and Cholecystokin  Learning Resources Text Book: Marieb EN. Human Anatomy and Physiology, 9th Edition, Pearson International Edition; 2014. ISBN-13: 978-1-2920-2649-7. Chapter 23, pp. 940-952 Power-point presentation in the moodle December 8, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE