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Digestive System Dr. Dinithi Peiris Dept. of Zoology University of Sri Jayewardenepura 1 Definitions A. Ingestion = to bring food into the digestive system B. Digestion = to break food down into smaller molecules that can be absorbed by the body a) 1. Mechanical – physical, makes use of chewing, grinding, etc. b) 2. Chemical - makes use of enzymes C. Egestion = elimination of non-digestible materials 2 Four basic process of GI Digestion l Absorption l Motility l secretion l 3 1 Digestive Organs TRUE l food passes through l Alimentary Canal • one way passage for food/nutrients/waste l mouth---> anus ACCESSORY l no food passes through them l secrete enzymes that aid in digestion ex. Liver, salivary glands 4 Digestive Function: Integrated Steps Digestion: the preparation of food by the animal for absorption It refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium 5 Digestive Function: Integrated Steps Mechanical processing is physical manipulation and distortion that makes materials easier to swallow and increase the surface area for enzymatic attack. Ingestion occurs when materials enter the digestive tract via the mouth 6 2 Digestive Function: Integrated Steps Physical • small particles produced from large ones • food -> food particles • change in particle's physical state ex. Chewing Chemical • food is digested into components • food -> nutrients ex. Starch Δ monosaccharides 7 Digestive Function: Integrated Steps Secretion is the release of water, acids, enzymes, buffers and salts by the epithelium of the digestive tract and by glandular organs Absorption is the movement of organic substrates, electrolytes, vitamins and water across the digestive epithelium and into the interstitial fluid of the digestive tract 8 Digestive Function: Integrated Steps Excretion is the elimination of waste products from the body. Secreted into the lumen of the tract Most will leave the body through defecation 9 3 Mouth--chewing Path of Food Pharynx--conscious swallowing Esophagus--transport to stomach Stomach--mechanical and chemical breakdown Small Intestines-- chemical digestion and absorption Large Intestines-- reabsorb water, form feces Rectum---collect and expel feces 10 Major Components: Accessory Organs: Human elementary MOUTH(ORAL CAVITY) Canal PHARYNX ESOPHAGUS STOMACH LIVER GALLBLADDER SMALL INTESTINE PANCREAS LARGE INTESTINE (COLON) RECTUM ANUS 11 Fig.37.6, p. 616 Digestion 12 4 Mouth or the oral cavity • It is the site for mechanical & chemical digestion • Mechanical digestion begins as a person bites with grinding and chewing with teeth. • Mammalian teeth are specialized for specific functiono small fragments 13 Mouth or the oral cavity • During this process food is broke down into small fragments • This will increase the surface area for enzymatic action 14 Jaw, Cheek & Teeth l Used for storage- rodents and squirrels l Modified placoid scales- sharks l Polyhyodont- permanent replacement of teeth l Diphyodont- two sets of teeth l Monophyodont- one set of teeth 15 5 Mammalian Teeth l Incisors l Canines • For cutting • Ex: elephant tusks • For piercing • Ex: walrus tusks Figure: Mammalian teeth l Diastema- space without teeth; specializations. l Premolars & Molars • To matriculate food e.g., no canines 16 Salivary glands l l Produce saliva • Prevents bacterial infection • Lubrication • Contains salivary amylase Three pairs • Parotid: Largest • (25%) • Submandibular • (70%) • Sublingual: Smallest (5%) 17 Composition of saliva Hypo-osmotic solution ( 97-99.5% water) l pH 6.8 – 7.4 l proteins: amylase, lysozyme, albumins, globulins, lactase l Na, K, phosphate, bicarbonate, chloride l Mucin – a glyco protein for lubrication l 18 6 Composition of saliva Hormones : estrogen, progesterone, prolactin l Metabolic wastes: urea, uric acid l l Resident bacteria reduce nitrate (NO3-) into nitrite (NO2-), converted into NO (nitric oxide), esp. in acidic environment (around gums, acids produced by bacteria), NO bactericical 19 Functions of saliva Lysozymes, " Proxidases" Urea," Uric acid" Thiocyanate " Absorb " Latent heat" Carbonic anhydrases" ." Cleansing" Buffering " & protec-" Amylases," tion" Lactase" Excretory" Digestion" Lipase" products" Thermo" regulation" Water " intake" Dryness " Lubricat-" Help in" Ion & " speech" swallowing" by keeping the" Mouth & tongue" moist" Mucins" 20 Control of salivation • 1-1.5 / day • Controlled by the parasympathetic & sympathetic nervous system • Control are of 2 types • Unconditional reflexes – present at birth food in mouth → brain stem →parasympathicus output → increased salivation esp. in response to acidic foods (fruit juices, vinegar) or hot spices 21 7 Control of salivation • Conditional reflexes – learned by previous experiences sight, smell, hearing of food preparation, thought of food This is triggered via cortical stimulation 22 Control of salivation l Effect of sympathetic nervous system • decreases overall secretion • change to thick, mucous secretion • strong sympathetic stimulation → vasoconstriction of vessels of serving glands → near cessation of salivation (dry mouth, difficulty swallowing, talking) • accumulation of food particles → enhanced bacterial growth → halitosis 23 Control of salivation l Salivary glands Infection • Mumps (myxovirus infection) spread to testes in male adults → 25% chance of sterility 24 8 Deglutition (swallowing) l l l Tongue mixes food pieces & secretions producing a bolus Pharynx initiate swallowing Swallowing occurs when the tongue moves the bolus to back of the mouth 25 Deglutition (swallowing) Swallowing occurs in three phases • Buccal phase (Voluntary) • Bolus of food moved by tongue from oral cavity to pharynx • Pharyngeal (involuntary) • Initiated when the tongue forces bolus back into pharynx 26 Deglutition (swallowing) Reflex: soft plates rises to block naso-pharynx • Upper esophageal sphincter relaxes, • Elevated pharynx opens to the esophagus, • Peristalsis movement pushes food into esophagus • Esophageal Reflex: Epiglottis is tipped posteriorly, larynx elevated to prevent food from passing into larynx 27 9 Deglutition (swallowing) Esophageal Phase • P e r is t a ls is ( a lt e r na t e c o nt r a c t io ns a nd relaxations of smooth muscles forming wave) continues to mix food with saliva • Mucus secreted but no enzyme • Cardiac sphincter opens in response to peristalsis moving food into stomach and closing to prevent food back up • No absorption or digestion in esophagus 28 Deglutition (swallowing) 29 Mechanisms of peristalsis • Peristalsis is a reflex initiated by presence bolus in esophagus of • C ircular muscle layer contracts behinds pushing bolus down & relaxes in front • Longitudinal smooth muscle layer contracts (shortening) moving wall to receive bolus • Rate 2 – 4 cm per sec 30 10 Mechanisms of peristalsis 31 Stomach Thick walled, saclike structure . l l Openings • Gastroesophageal: To esophagus • Pyloric: To duodenum Regions • Cardiac • Fundus • Body • Pyloric 32 Digestion in stomach – G a s t r i c m u c o s a i s fo l d e d i n t o invaginations (gastric pits) – At the base of gastric pits, Gastric glands are located – Highly convoluted inner surface stretches during meals – 50 ml capacity when empty; 2L – 4L capacity when full . 33 11 Gastric secretion • Stomach secretes 2L of gastric juice/day. Highly acidic due to presence of HCL • Gastric mucosa is folded into invaginations (gastric pits) At the base of gastric pits, Gastric glands are located • Mucosal epithelial cells: Divide at a rate higher than elsewhere. Entire stomach mucosa is replaced every 3 days 34 stomach 35 Gastric glands Six types of gastric glands are present 1. Parietal cells: release HCl and intrinsic factor (vitamin B 12 absorption) 2.Chief cells: release Pepsinogen, rennin, lipase, amylase 3. Mucous cells: release mucus (thick 4. Enterochromaffin – like cells: release histamine 5. G cells: release gastrin 6. D cells: secrete somatostatin 36 12 Composition of gastric juice 1. Water (99.5%) 2. HCl (40 – 60 meq / L) 3. Enzymes 4. Gastric mucin 5. Inorganic salts – Na, K, Ca & Mg 6. Intrinsic factor 37 Functions of stomach l l l l Mechanical breakdown of food – smooth muscle in wall Mixing of food – via contraction of smooth muscles Protein breakdown – pepsin secreted by epithelial lining Acidic conditions – for pepsin to work & to kill bacteria 38 Functions of stomach l Bulk storage of undigested food in the fundus l Absorption of water, ions & some drugs (e.g. aspirin, alcohol) l Preliminary digestion of proteins l Fat digestion: Fat + Gastric lipase = fatty acid and glycerol Inactive pepsinogen + HCL = Active pepsin Protein + Pepsin (pH – 2) = Peptone (smaller chains) 39 13 Summary 40 PBL A student complained a stomach ache during night, Her room-mate suggests that she should one of the new acid blockers that she has seen on TV. These drugs are known as histamine receptor blockers and bind competitively to H2 – type histamine receptors. Fortunately the roommate has just bought a packet of drugs. Two hours after taking the drug the student felt better. 41 HCl acid production 42 14 Phases of Gastric Secretion • food in mouth & in stomach G cells Gastrin Direct & Indirect 43 Phases of Gastric Secretion Gastrin Direct Parietal cells Indirect Histamine (ECL) & Combine with H2- R on parietal cells HCl secretion 44 Phases of Gastric Secretion • HCl production is stimulated: Indirectly by gastrin. Indirectly by ACh. • ACh and gastrin stimulate release of histamine. • Histamine: Bind to histamine receptors & stimulates parietal cells to secrete HCl. 45 15 Control of gastric secretion • Cephalic phase: Excitatory. Smell, taste, thought etc. initiates gastric secretion before food reaches stomach (30%) • Gastric phase: Excitatory. Presence of food increases gastric secretion. Mixing waves begin (70%) • Intestinal phase: Inhibitory. Presence of food in duodenum decreases gastric secretion 46 Phases of Gastric Secretion 47 Movements in Stomach Gastric motility – • Filling phase • Mixing phase • Emptying phase 48 16 Gastric Movements • Pyloric sphincter opens and closes in response to nervous and endocrine control • Nervous: Via stretch receptors. When duodenum is distended, sphincter is closed • Endocrine: Secretin and CCK released by SI inhibit opening. Hormone release controlled by - Acidic chyme - High fat content 49 Protective mechanisms of stomach • Parietal and chief cells impermeable to HCl • Alkaline mucus contains HC03-. • Tight junctions between adjacent epithelial cells. • Rapid rate of cell division (entire epithelium replaced in 3 days). • Prostaglandins inhibit gastric secretions 50 Gastritis, Gastric Ulcers • breaching of the mucosal barrier → inflammation → persistent gastritis → erosion of underlying tissue → ulcer • pain, risk of hemorrhage • triggered by hyper secretion of HCl, hypo secretion of mucus, 90% of affected patients carry Helicobacter pylori 51 17 Gastritis, Gastric Ulcers l l l l Bacteria release cytotoxins and chemotactic proteins → immune response (suspected of increasing risk for stomach cancer) Bacterium also found in >40% of healthy people Treatment: Antibiotics, H2-antihistamines, cimetidine, ranitidine (histamine R blocker) Antacids neutralize HCl - (Al(OH)3, Mg(OH)2 52 Digestion in small intestine • Three subdivisions: Duodenum, Jejunum, Ileum • Has villi and microvilli to increase absorptive surface • Food movement is slow through small intestine (3-5 hrs) • Chemical digestion is completed in small intestine. Most of the absorption is also done here 53 Intestinal juice Accessory organ secretion into small intestine 1. Pancrease 54 18 Pancreas l l Secretions Anatomy Composition of • Regions: Head, body, tail • pancreatic juice • Endocrine (exocrine) • Pancreatic islets produce insulin and • pH: 8.00 – 9.00 glucagon • Water (98%) • Exocrine • Alkaline slats • Acini produce digestive • Enzymes enzymes • Mucus 55 Pancreas l Digestive enzymes • Trypsinogen, Chymotrypsinogen • Pro carboxypeptidase • Pancreatic amylase • Pancreatic lipases • Enzymes that reduce DNA and ribonucleic acid • Cholesterol esterase • Phospholilpases 56 Bicarbonate Ion Production 57 19 Regulation of Pancreatic Juice l HCO3- & enzymes are secreted by different types of cells Acidity in the chyme HCO3Neural Hormonal Parasympathetic nerves CCK - Cholecytokinin (Ach) Act on duct cell via R Release of HCO ions 58 Regulation of pancreatic juice Distention, food in the upper intestine Neural Hormonal Entero-pancreatic reflex CCK - cholecytokinin (Ach) Act on aciner cells via R Release enzyme rich secretion (effect by the diet) 59 Regulation of pancreatic juice Somatostatin, pancreatic polypeptides IB secretion 3 phases: Cephalic, Gastric, Intestinal • Cephalic : vagus – direct secretin releasing peptides enzymes secretin enzymes 60 20 Regulation of pancreatic juice Gastric : gastrin distention enzymes gastro pancreatic reflex Intestinal : H+ + fats AA s, fats vago-vagal Secretin CCK enzymes HCO3 fluid enzyme secretion enzyme secretion This is the major phase 61 Pancreatic Secretion Control 62 2. Liver l Bile • Produced by liver • Stored in gallbladder • Bile: breaks down fats = emulsification 63 21 Bile production and secretion • The liver produces and secretes 250 – 1500 ml of bile/day. • Made in liver cells from cholesterol cholesterol oxidized in to bile acids (chenodeoxycholic acid and cholic acid) conjugated to glycine or taurine (amino acids) conjugation of bile acids with amino acids improves its ability to form micelles 64 Bile production and secretion • Bile pigment (bilirubin) is produced in spleen, bone marrow, and liver. • Free bilirubin combines with glucuronic acid and forms conjugated bilirubin. • Secreted into bile. • Converted by bacteria in intestine to urobilinogen. 65 Bile production and secretion • Urobilogen is absorbed by intestine and enters the hepatic vein. • Recycled, or filtered by kidneys and excreted in urine. • Stones: high cholesterol & low bile salts or lecithin ppt of cholesterol 66 22 Other functions of liver 1. Storage: l l l Excess glucose is converted to insoluble glycogen (insulin needed) and stored. E.g: After a heavy meal, blood glucose level rises/after exercise. When blood glucose level drops, induces liver to convert glycogen back to glucose which enters blood. In addition to glycogen, fat vitamins A,D,E and Cu & ion are stored 67 Quote for the Day Smile It Releases Endorphins!!! 68 Other functions of Liver 2. Deamination of amino acids: Excess amino acids are brought to liver and converted to urea. 3. Detoxification: harmful substances absorbed into the blood are converted to harmless substances by liver cells. Alcohol is broken down in the liver Phagocytosis: Kupffer cells phagocytize worn-out and dying red and white blood cells, some bacteria Synthesis: Albumins, fibrinogen, globulins, heparin, clotting factors, plasma protein, lipo proteins & cholesterol 4. 5. 69 23 Bile storage l l l Bile stored in the gall bladder Muscular sac Between right + quadrate liver lobes 70 Bile storage Gallbladder • Concentrates bile by removing 90% of water • Stores bile • Bile release is stimulated by CCK • CCK secreted in response to amino acids and lipids • Bile is secreted into duodenum 71 • pH 8 – 8.6 Composition • Water (86%) of bile • Mineral salts – Cl, HCO3, P04-, Na+, K+ • Bile salts – Na taurocholate, Na glycocholate *Emulsifies fat by turning into droplets • Bile pigments – Waste products ; bilirubin (golden yellow) & biliverdin (green) • Mucus • Cholesterol & lecithin; phospholipids -excretory products 72 24 Regulation of bile secretion l l Bile salts itself Hormonal Acid chyme mucus cells secretin + CCK Secretin: Acts on duct cells (as in pancrease) Bicarbonate secretion 73 Regulation of bile secretion CCK: Contraction of GB smooth muscles & relaxation of sphincter Bile salts l Neural control - insignificant 74 Emulsification of Fat by Bile 75 25 Emulsification of Fat by Bile 76 Enterohepatic circulation 77 The Wall of the Small Intestine 78 Figure 14.9 Slide 14.10A 26 Intestinal juice • Secreted about 1.5 L/day • Help buffer acids from stomach by alkaline mucus • Contain secretions from pancreas, liver and mucus from goblet cells in intestinal wall • Brush-border: Contains membrane bound enzymes that complete protein and fat digestion. 79 Digestion in small intestine l l Brush border enzymes include peptidases, disaccharidases and a protease called enteropeptidase = enterokinase Brush border enteropeptidase converts inactive trypsinogen & chymotripsinogen into active trypsin 80 l Intestinal Contractions & Motility 2 major types of contractions occur in the small intestine: • Peristalsis: Insert fig. 18.14 • Slow movement. • Highly coordinated to propel content down the intestine. • Pressure at the pyloric end of small intestine is greater than at the distal end. 81 27 Intestinal Contractions & Motility • Segmentation: • Major contractile activity of the small intestine. • Section of SM contracts in isolation in both oral& caudal directions • When smooth muscle relaxes, content flows back to the segment. • Back & forth movement of SM mixes chyme • No forward movement. 82 Contractions of Intestinal Smooth Muscles l l Occur automatically in response to endogenous pacemaker activity. Rhythm of contractions is paced by graded depolarizations called slow waves. • Slow waves produced by interstitial cells of Cajal. Insert fig. 18.15 • Slow waves spread from 1 smooth muscle cell to another through nexuses. 83 Contractions of Intestinal Smooth Muscles 84 28 Contractions of Intestinal Smooth Muscles l l l l When slow waves above threshold, it triggers APs by opening of VG Ca2+ channels. Inward flow of Ca2+: • Produces the upward depolarization phase. • Stimulates contraction of smooth muscle. Repolarization: • VG K+ channels open. • Slow waves decrease in amplitude as they are conducted. . 85 Contractions of Intestinal Smooth Muscles l l May stimulate contraction in proportion to the magnitude of depolarization. Parasympathetic NS, stretch and gastrin increase the amplitude of slow waves. • Stimulate APs. l Sympathetic NS decrease APs. 86 Summary of Digestion: Carbohydrates Amylase Maltase Starch ----------- Maltose ---------- 2 Glucose Glycogen --------- Maltose ---------- 2 Glucose Invertase Sucrose ------------------- Glucose + Fructose Lactase Lactose ------------------ Glucose + Galactose 87 29 Summary of Digestion: Proteins Pepsin 1. Complex Proteins ------------------- Peptones Trypsin, chymotrypsin, 2. Peptones ----------------------------Higher Peptides carboxypeptidase 3. Higher peptides --------------------Dipeptides dipeptidase 4. Peptides -------------------------- Amino acids 88 Summary of Digestion: Fat & Nucleic acids: lingual lipase Fat ------------------------------ Fatty acid + glycerol Bile in SI Fat ----------------------------emulsified fat pancreatic lipase Fat --------------------------- Fatty acid + glycerol deoxyribonuclease DNA----------------------- sugar, phosphate, bases RNA -------------------ribonuclease Sugar, phosphate, bases 89 Major Digestive Enzymes 90 Table 14.1 Slide 14.11 30 Absorption in Small Intestine Duodenum and jejunum: • Carbohydrates, amino acids, lipids, iron, and Ca2+. l Ileum: • Bile salts, vitamin B12, electrolytes, and H20. l 91 PBL Cholera toxin, produced by Vibrio cholerae causes a an indirect reduction in the activity of the Na + / K + ATPase in intestinal epithelial cells. This results in reduced uptake of small sugars and amino acids from the intestine 92 Absorption of monosaccharides Fructose; , Intestinal epithelium Glucose Na+ Galactose GLUT-2 GLUT-5 Fructose Lumen of intestine Glucose Galactose Brush border 3Na+ 2K+ Contra-luminal membrane GLUT-5 ATP ADP + Pi to capillaries 3Na+ 2K+ Rate = 1.4 g / kg body wt = facilitated diffusion = Na,K-ATPase + = Na -dependent co-transport 93 31 Absorption of proteins Intestinal epithelial Epithelial cell layer Blood Small peptides Peptides Amino acids Na+ Na+ C plasmic peptidases Di & tri peptides • Anti bodies through placenta H+ • Intestine: (cow, horse, goat) st during 1 few day via mothers milk H+ 94 Absorption of Fats Simple diff. into blood Small FA ≤ 10C cholesterol >10C 95 Lipid Transport Lipoproteins-how lipids travel Lots of lipid=less dense l More protein=more dense; would sink l l • Chylomicrons-most triglyceride & least dense • VLDL-remnants of chylomicrons repacked • LDL-loaded with cholesterol/little protein • HDL-carry cholesterol from cells back to liver/ lots of protein 96 32 Health Implications l LDL represents high blood cholesterol l HDL represents the cholesterol being picked up for breakdown and excretion l High LDL associated with increased risk of heart attack l LDL and HDL refer to the proportions of lipid and protein 97 Factors to Improve the LDL to HDL Ratio Weight control Monounsaturated or polyunsaturated, instead of saturated fat in the diet l Soluble fibers l Antioxidants l Moderate alcohol consumption l Physical activity l l 98 Functions of the large intestine l Reabsorb water and compact material into feces by mass movement (mixing wave) l Absorb vitamin K produced by colon bacteria l Store fecal matter prior to defecation 99 33 Functions of the large intestine l No food absorption or enzyme secretion in LI, only mucus If chyme moves too slowly, too much water is reabsorbed= constipation If chyme moves too fast, too little water reabsorbed = diarrhea 100 Rectum: defecation reflex Internal and external anal sphincters present 1. Stretching sends signals to CNS 2. Spinal cord sends parasympathetic impulse to open internal anal sphincter 101 Rectum: defecation reflex 3.External anal sphincter is under conscious control and may remain close. With continued rectal distension, reflex returns sooner and sooner 4. Young children lack voluntary control 102 34