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The Digestive System
 

The Digestive System

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    The Digestive System The Digestive System Presentation Transcript

    • The Digestive System www.freelivedoctor.com
      • Motility :
        • Movement of of food through the GI tract.
          • Ingestion:
            • Taking food into the mouth.
          • Mastication:
            • Chewing the food and mixing it with saliva.
          • Deglutition:
            • Swallowing the food.
          • Peristalsis:
            • Rhythmic wave-like contractions that move food through GI tract.
      Functions of the GI Tract www.freelivedoctor.com
      • Secretion:
        • Includes both exocrine and endocrine secretions.
          • Exocrine:
            • HCl, H 2 0, HC0 3 - , bile, lipase, pepsin, amylase, trypsin, elastase, and histamine are secreted into the lumen of the GI tract.
          • Endocrine:
            • Stomach and small intestine secrete hormones to help regulate the GI system.
              • Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin.
      Functions of the GI Tract (continued) www.freelivedoctor.com
      • Digestion:
        • Breakdown of food particles into subunits (chemical structure change).
      • Absorption:
        • Process of the passage of digestion (chemical subunits) into the blood or lymph.
      • Storage and elimination:
        • Temporary storage and elimination of indigestible food.
      Functions of the GI Tract (continued) www.freelivedoctor.com
    • Digestive System (GI)
      • GI tract divided into:
        • Alimentary canal.
        • Accessory digestive organs.
      • GI tract is 30 ft long and extends from mouth to anus.
      Insert fig. 18.2 www.freelivedoctor.com
    • Layers of GI Tract
      • Composed of 4 tunics:
        • Mucosa.
        • Submucosa.
        • Muscularis.
        • Serosa.
      www.freelivedoctor.com
      • Lines the lumen of GI tract.
        • Consists of simple columnar epithelium.
      • Lamina propria:
        • Thin layer of connective tissue containing lymph nodules.
      • Muscularis mucosae:
        • Thin layer of smooth muscle responsible for the folds.
          • Folds increase surface area for absorption.
      • Goblet cells:
        • Secrete mucus.
      Mucosa www.freelivedoctor.com
      • Thick, highly vascular layer of connective tissue.
      • Absorbed molecules enter the blood and lymphatic vessels.
      • Submucosal plexus (Meissner’s plexus):
        • Provide autonomic nerve supply to the muscularis mucosae.
      Submucosa www.freelivedoctor.com
      • Responsible for segmental contractions and peristaltic movement through the GI tract.
        • Inner circular layer of smooth muscle.
        • Outer longitudinal layer of smooth muscle.
      • Contractions of these layers move food through the tract; pulverize and mix the food.
      • Myenteric plexus located between the 2 muscle layers.
        • Major nerve supply to GI tract.
          • Fibers and ganglia from both sympathetic and parasympathetic nervous systems.
      Muscularis www.freelivedoctor.com
    • Serosa
      • Binding and protective outer layer.
      • Consists of areolar connective tissue covered with simple squamous epithelium.
      www.freelivedoctor.com
      • Extrinsic innervation:
        • Parasympathetic nervous system:
          • Vagus and spinal nerves:
            • Stimulate motility and GI secretions.
        • Sympathetic nervous system:
          • Postganglionic sympathetic fibers that pass through submucosal and myenteric plexuses and innervate GI tract:
            • Reduce peristalsis and secretory activity.
      Regulation of the GI Tract www.freelivedoctor.com
      • Enteric nervous system:
        • Sites where parasympathetic fibers synapse with postganglionic neurons that innervate smooth muscle.
      • Submucosal and myenteric plexuses:
        • Local regulation of the GI tract.
      • Paracrine secretion:
        • Molecules acting locally.
      • Hormonal secretion:
        • Secreted by the mucosa.
      Regulation of the GI Tract (continued) www.freelivedoctor.com
      • Mastication (chewing):
        • Mixes food with saliva which contains salivary amylase.
          • Enzyme that can catalyze the partial digestion of starch.
      • Deglutition (swallowing):
        • Begins as a voluntary activity.
        • Involves 3 phases:
          • Oral phase is voluntary.
          • Pharyngeal and esophageal phases are involuntary.
            • Cannot be stopped.
        • Larynx is raised.
        • Epiglottis covers the entrance to respiratory tract.
      From Mouth to Stomach www.freelivedoctor.com
      • Involuntary muscular contractions and relaxations in the mouth, pharynx, larynx, and esophagus are coordinated by the swallowing center in the medulla.
      • Esophagus:
        • Connects pharynx to the stomach.
          • Upper third contains skeletal muscle.
          • Middle third contains a mixture of skeletal and smooth muscle.
          • Terminal portion contains only smooth muscle.
      From Mouth to Stomach (continued) www.freelivedoctor.com
    • Esophagus
      • Peristalsis:
        • Produced by a series of localized reflexes in response to distention of wall by bolus.
      • Wave-like muscular contractions:
        • Circular smooth muscle contract behind, relaxes in front of the bolus.
        • Followed by longitudinal contraction (shortening) of smooth muscle.
          • Rate of 2-4 cm/sec.
        • After food passes into stomach, LES constricts.
      Insert 18.4a www.freelivedoctor.com
      • Most distensible part of GI tract.
        • Empties into the duodenum.
      • Functions of the stomach:
        • Stores food.
        • Initiates digestion of proteins.
        • Kills bacteria.
        • Moves food (chyme) into intestine.
      Stomach www.freelivedoctor.com
    • Stomach (continued)
      • Contractions of the stomach churn chyme.
        • Mix chyme with gastric secretions.
        • Push food into intestine.
      Insert fig. 18.5 www.freelivedoctor.com
    • Stomach (continued)
      • Gastric mucosa has gastric pits in the folds.
      • Cells that line the folds deeper in the mucosa, are gastric glands.
      Insert fig. 18.7 www.freelivedoctor.com
      • Secrete gastric juice:
        • Goblet cells: mucus.
        • Parietal cells: HCl and intrinsic factor.
        • Chief cells: pepsinogen.
        • Enterochromaffin-like cells (ECL): histamine and serotonin.
        • G cells: gastrin.
        • D cells: somatostatin.
        • Stomach: ghrelin.
      Gastric Glands www.freelivedoctor.com
    • HCl Production
      • Parietal cells secrete H + into gastric lumen by primary active transport, through H + / K + ATPase pump.
      • Parietal cell’s basolateral membrane takes in Cl - against its electrochemical gradient, by coupling its transport with HC0 3 - .
      Insert fig. 18.8 www.freelivedoctor.com
      • HCl production is stimulated:
        • Indirectly by gastrin.
        • Indirectly by ACh.
      • ACh and gastrin stimulate release of histamine.
        • Histamine:
          • Stimulates parietal cells to secrete HCl.
      HCl Production (continued) www.freelivedoctor.com
    • HCl Functions
      • Makes gastric juice very acidic.
        • Denatures ingested proteins (alter tertiary structure) so become more digestible.
      • Activates pepsinogen to pepsin.
        • Pepsin is more active at pH of 2.0.
      Insert fig. 18.9 www.freelivedoctor.com
    • Digestion and Absorption in the Stomach
      • Proteins partially digested by pepsin.
      • Carbohydrate digestion by salivary amylase is soon inactivated by acidity.
      • Alcohol and aspirin are the only commonly ingested substances absorbed.
      www.freelivedoctor.com
    • Gastric and Peptic Ulcers
      • Peptic ulcers:
        • Erosions of the mucous membranes of the stomach or duodenum produced by action of HCl.
      • Zollinger-Ellison syndrome:
        • Ulcers of the duodenum are produced by excessive gastric acid secretions.
      • Helicobacter pylori:
        • Bacterium that resides in GI tract that may produce ulcers.
      • Acute gastritis:
        • Histamine released by tissue damage and inflammation stimulate further acid secretion.
      www.freelivedoctor.com
      • Parietal and chief cells impermeable to HCl.
      • Alkaline mucus contains HC0 3 - .
      • Tight junctions between adjacent epithelial cells.
      • Rapid rate of cell division (entire epithelium replaced in 3 days).
      • Prostaglandins inhibit gastric secretions.
      Protective Mechanisms of Stomach www.freelivedoctor.com
    • Small Intestine
      • Each villus is a fold in the mucosa.
      • Covered with columnar epithelial cells interspersed with goblet cells.
      • Epithelial cells at the tips of villi are exfoliated and replaced by mitosis in crypt of Lieberkuhn.
      • Lamina propria contain lymphocytes, capillaries, and central lacteal.
      Insert fig. 18.12 www.freelivedoctor.com
      • Duodenum and jejunum:
        • Carbohydrates, amino acids, lipids, iron, and Ca 2+ .
      • Ileum:
        • Bile salts, vitamin B 12 , electrolytes, and H 2 0.
      Absorption in Small Intestine www.freelivedoctor.com
      • Microvilli contain brush border enzymes that are not secreted into the lumen.
        • Brush border enzymes remain attached to the cell membrane with their active sites exposed to the chyme.
      • Absorption requires both brush border enzymes and pancreatic enzymes.
      Intestinal Enzymes www.freelivedoctor.com
    • Intestinal Contractions and Motility
      • 2 major types of contractions occur in the small intestine:
        • Peristalsis:
          • Slow movement.
          • Pressure at the pyloric end of small intestine is greater than at the distal end.
        • Segmentation:
          • Major contractile activity of the small intestine.
          • Contraction of circular smooth muscle.
            • Mix chyme.
      Insert fig. 18.14 www.freelivedoctor.com
    • Contractions of Intestinal Smooth Muscles
      • 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.
        • Slow waves spread from 1 smooth muscle cell to another through nexuses.
      Insert fig. 18.15 www.freelivedoctor.com
      • When slow waves above threshold, it triggers APs by opening of VG Ca 2+ channels.
      • Inward flow of Ca 2+ :
        • Produces the upward depolarization phase.
        • Stimulates contraction of smooth muscle.
      • Repolarization:
        • VG K + channels open.
          • Slow waves decrease in amplitude as they are conducted.
      • May stimulate contraction in proportion to the magnitude of depolarization.
      Contractions of Intestinal Smooth Muscles www.freelivedoctor.com
    • Cells and Electrical Events in the Muscularis Insert fig. 18.16 www.freelivedoctor.com
      • Outer surface bulges outward to form haustra.
      • Little absorptive function.
        • Absorbs H 2 0, electrolytes, several vitamin B complexes, vitamin K, and folic acid.
          • Intestinal microbiota produce significant amounts of folic acid and vitamin K.
          • Bacteria ferment indigestible molecules to produce short-chain fatty acids.
            • Does not contain villi.
      • Secretes H 2 0, via active transport of NaCl into intestinal lumen.
        • Guanylin stimulates secretion of Cl - and H 2 0, and inhibits absorption of Na + (minor pathway).
            • Membrane contains Na + /K + pumps.
              • Minor pathway.
      Large Intestine www.freelivedoctor.com
      • Small intestine:
        • Most of the fluid and electrolytes are absorbed by small intestine.
          • Absorbs about 90% of the remaining volume.
        • Absorption of H 2 0 occurs passively as a result of the osmotic gradient created by active transport.
          • Aldosterone stimulates NaCl and H 2 0 absorption in the ileum.
      • Large intestine:
        • Absorbs about 90% of the remaining volume.
          • Absorption of H 2 0 occurs passively as a result of the osmotic gradient created by active transport of Na + and Cl - .
      Fluid and Electrolyte Absorption in the Intestine www.freelivedoctor.com
    • Defecation
      • Waste material passes to the rectum.
      • Occurs when rectal pressure rises and external anal sphincter relaxes.
      • Defecation reflex:
        • Longitudinal rectal muscles contract to increase rectal pressure.
          • Relaxation of internal anal sphincter.
        • Excretion is aided by contractions of abdominal and pelvic skeletal muscles.
          • Push feces from the rectum.
      www.freelivedoctor.com
    • Structure of Liver
      • Liver largest internal organ.
        • Hepatocytes form hepatic plates that are 1–2 cells thick.
        • Arranged into functional units called lobules.
      • Plates separated by sinusoids.
        • More permeable than other capillaries.
      • Contains phagocytic Kupffer cells.
      • Secretes bile into bile canaliculi, which are drained by bile ducts.
      www.freelivedoctor.com
    • Structure of Liver (continued) Insert fig. 18.20 www.freelivedoctor.com
      • Products of digestion that are absorbed are delivered to the liver.
      • Capillaries drain into the hepatic portal vein, which carries blood to liver.
        • ¾ blood is deoxygenated.
        • Hepatic vein drains liver.
      Hepatic Portal System www.freelivedoctor.com
    • Enterohepatic Circulation
      • Compounds that recirculate between liver and intestine.
        • Many compounds can be absorbed through small intestine and enter hepatic portal blood.
        • Variety of exogenous compounds are secreted by the liver into the bile ducts.
      • Can excrete these compounds into the intestine with the bile.
      Insert fig. 18.22 www.freelivedoctor.com
    • Major Categories of Liver Function www.freelivedoctor.com
      • The liver produces and secretes 250–1500 ml of bile/day.
      • Bile pigment (bilirubin) is produced in spleen, bone marrow, and liver.
        • Derivative of the heme groups (without iron) from hemoglobin.
      • Free bilirubin combines with glucuronic acid and forms conjugated bilirubin.
        • Secreted into bile.
      • Converted by bacteria in intestine to urobilinogen.
        • Urobilogen is absorbed by intestine and enters the hepatic vein.
          • Recycled, or filtered by kidneys and excreted in urine.
      Bile Production and Secretion www.freelivedoctor.com
    • Metabolism of Heme and Bilirubin Insert fig. 18.23 www.freelivedoctor.com
    • Bile Production and Secretion (continued)
      • Bile acids are derivatives of cholesterol.
        • Major pathway of cholesterol breakdown in the body.
      • Principal bile acids are:
        • Cholic acid.
        • Chenodeoxycholic acid.
          • Combine with glycine or taurine to form bile salts.
            • Bile salts aggregate as micelles.
      • 95% of bile acids are absorbed by ileum.
      Insert fig. 18.25 www.freelivedoctor.com
    • Detoxification of the Blood
      • Liver can remove hormones, drugs, and other biologically active molecules from the blood by:
        • Excretion into the bile.
        • Phagocytosis by Kupffer cells.
        • Chemical alteration of the molecules.
          • Ammonia is produced by deamination of amino acids in the liver.
          • Liver converts it into urea.
            • Excreted in urine.
      www.freelivedoctor.com
    • Detoxification of the Blood (continued)
      • Inactivation of steroid hormones and drugs.
        • Conjugation of steroid hormones and xenobiotics make them anionic.
          • Can be transported into bile by multispecific organic anion transport carriers.
        • Steroid and xenobiotic receptors stimulate production of cytochrome P450 enzymes.
      www.freelivedoctor.com
    • Secretion of Glucose, Triglycerides and Ketones
      • Liver helps regulate blood glucose concentration by:
        • Glycogenesis and lipogenesis.
        • Glycogenolysis and gluconeogenesis.
      • Contains enzymes required to convert free fatty acids into ketone bodies.
      www.freelivedoctor.com
    • Production of Plasma Proteins
      • Albumin and most of the plasma globulins (except immunoglobulins) are produced by the liver.
      • Albumin:
        • Constitutes 70% of the total plasma protein.
          • Contributes most to the colloid osmotic pressure in the blood.
      • Globulins:
        • Transport cholesterol and hormones.
        • Inhibit trypsin.
        • Produce blood clotting factors I, II, III, V, VII, IX, XI.
      www.freelivedoctor.com
      • Sac-like organ attached to the inferior surface of the liver.
      • Stores and concentrates bile.
      • When gallbladder fills with bile, it expands.
        • Contraction of the muscularis layer of the gallbladder, ejects bile into the common bile duct into duodenum.
      • When small intestine is empty, sphincter of Oddi closes.
        • Bile is forced up to the cystic duct to gallbladder.
      Gallbladder www.freelivedoctor.com
    • Pancreas
      • Exocrine:
        • Acini:
          • Secrete pancreatic juice.
      • Endocrine:
        • Islets of Langerhans:
          • Secrete insulin and glucagon.
      Insert fig. 18.26 www.freelivedoctor.com
      • Contains H 2 0, HC0 3 - and digestive enzymes.
      Pancreatic Juice www.freelivedoctor.com
    • Pancreatic Juice
      • Complete digestion of food requires action of both pancreatic and brush border enzymes.
        • Most pancreatic enzymes are produced as zymogens.
        • Trypsin (when activated by enterokinase) triggers the activation of other pancreatic enzymes.
      • Pancreatic trypsin inhibitor attaches to trypsin.
        • Inhibits its activity in the pancreas.
      Fig. 18.29 www.freelivedoctor.com
      • Neural and endocrine mechanisms modify the activity of the GI system.
      • GI tract is both an endocrine gland, and a target for the action of hormones.
      Neural and Endocrine Regulation www.freelivedoctor.com
      • Gastric motility and secretion are automatic.
      • Waves of contraction are initiated spontaneously by pacesetter cells.
      • Extrinsic control of gastric function is divided into 3 phases:
        • Cephalic phase.
        • Gastric phase.
        • Intestinal phase.
      Regulation of Gastric Function www.freelivedoctor.com
    • Cephalic Phase
      • Stimulated by sight, smell, and taste of food.
      • Activation of vagus:
        • Stimulates chief cells to secrete pepsinogen.
        • Directly stimulates G cells to secrete gastrin.
        • Directly stimulates ECL cells to secrete histamine.
        • Indirectly stimulates parietal cells to secrete HCl.
      • Continues into the 1 st 30 min. of a meal.
      www.freelivedoctor.com
      • Arrival of food in stomach stimulates the gastric phase.
      • Gastric secretion stimulated by:
        • Distension.
        • Chemical nature of chyme (amino acids and short polypeptides).
          • Stimulates G cells to secrete gastrin.
          • Stimulates chief cells to secrete pepsinogen.
          • Stimulates ECL cells to secrete histamine.
            • Histamine stimulates secretin of HCl.
        • Positive feedback effect.
          • As more HCl and pepsinogen are secreted, more polypeptides and amino acids are released.
      Gastric Phase www.freelivedoctor.com
    • Gastric Phase (continued)
      • Secretion of HCl is also regulated by a negative feedback effect:
        • HCl secretion decreases if pH < 2.5.
        • At pH of 1.0, gastrin secretion ceases.
          • D cells stimulate secretion of somatostatin.
            • Paracrine regulator to inhibit secretion of gastrin.
      Insert. Fig. 18.30 www.freelivedoctor.com
      • Inhibits gastric activity when chyme enters the small intestine.
      • Arrival of chyme increases osmolality and distension.
        • Activates sensory neurons of vagus and produces an inhibitory neural reflex:
          • Inhibits gastric motility and secretion.
          • In the presence of fat, enterogasterone inhibits gastric motility and secretion.
      • Hormone secretion:
        • Inhibit gastric activity:
          • Somatostatin, CCK, and GLP-1.
      Intestinal Phase www.freelivedoctor.com
      • Submucosal and myenteric plexuses contain 100 million neurons.
      • Include preganglionic parasympathetic axons, ganglion cell bodies, postganglionic sympathetic axons; and afferent intrinsic and extrinsic sensory neurons.
      Enteric Nervous System www.freelivedoctor.com
    • Enteric Nervous System (continued)
      • Peristalsis:
      • ACh and substance P stimulate smooth muscle contraction above the bolus.
      • NO, VIP, and ATP stimulate smooth muscle relaxation below the bolus.
      Insert fig. 18.31 www.freelivedoctor.com
    • Paracrine Regulators of the Intestine
      • Serotonin (5-HT):
        • Stimulates intrinsic afferents, which send impulses into intrinsic nervous system; and activates motor neurons.
      • Motilin:
        • Stimulates contraction of the duodenum and stomach antrum.
      • Guanylin:
        • Activates guanylate cyclase, stimulating the production of cGMP.
          • cGMP stimulates the intestinal cells to secrete Cl - and H 2 0.
            • Inhibits the absorption of Na + .
      • Uroguanylin:
        • May stimulate kidneys to secrete salt in urine.
      www.freelivedoctor.com
    • Intestinal Reflexes
      • Intrinsic and extrinsic regulation controlled by intrinsic and paracrine regulators.
      • Gastroileal reflex:
        • Increased gastric activity causes increased motility of ileum and movement of chyme through ileocecal sphincter.
      • Ileogastric reflex:
        • Distension of ileum, decreases gastric motility.
      • Intestino-intestinal reflex:
        • Overdistension in 1 segment, causes relaxation throughout the rest of intestine.
      www.freelivedoctor.com
      • Secretion of pancreatic juice and bile is stimulated by:
      • Secretin:
        • Occurs in response to duodenal pH < 4.5.
        • Stimulates production of HC0 3 - by pancreas.
        • Stimulates the liver to secrete HC0 3 - into the bile.
      • CCK:
        • Occurs in response to fat and protein content of chyme in duodenum.
        • Stimulates the production of pancreatic enzymes.
        • Enhances secretin.
        • Stimulates contraction of the sphincter of Oddi.
      Secretion of Pancreatic Juice www.freelivedoctor.com
    • Digestion and Absorption of Carbohydrates
      • Salivary amylase:
        • Begins starch digestion.
      • Pancreatic amylase:
        • Digests starch to oligosaccharides.
        • Oligosaccharides hydrolyzed by brush border enzymes.
      • Glucose is transported by secondary active transport with Na + into the capillaries.
      Insert fig. 18.32 www.freelivedoctor.com
      • Digestion begins in the stomach when pepsin digests proteins to form polypeptides.
      • In the duodenum and jejunum:
        • Endopeptidases cleave peptide bonds in the interior of the polypeptide:
          • Trypsin.
          • Chymotrypsin.
          • Elastase.
        • Exopeptidases cleave peptide bonds from the ends of the polypeptide:
          • Carboxypeptidase.
          • Aminopeptidase.
      Digestion and Absorption of Protein www.freelivedoctor.com
    • Digestion and Absorption of Protein (continued)
      • Free amino acids absorbed by cotransport with Na + .
      • Dipeptides and tripeptides transported by secondary active transport using a H + gradient to transport them into the cytoplasm.
      • Hydrolyzed into free amino acids and then secreted into the blood.
      Insert fig. 18.33 www.freelivedoctor.com
      • Arrival of lipids in the duodenum serves as a stimulus for secretion of bile.
      • Emulsification:
        • Bile salt micelles are secreted into duodenum to break up fat droplets.
      • Pancreatic lipase and colipase hydrolyze triglycerides to free fatty acids and monglycerides.
        • Colipase coats the emulsification droplets and anchors the lipase enzyme to them.
        • Form micelles and move to brush border.
      Digestion and Absorption of Lipids www.freelivedoctor.com
      • Free fatty acids, monoglycerides, and lysolecithin leave micelles and enter into epithelial cells.
        • Resynthesize triglycerides and phospholipids within cell.
          • Combine with a protein to form chylomicrons.
      • Secreted into central lacteals.
      Digestion and Absorption of Lipids (continued) www.freelivedoctor.com
      • In blood, lipoprotein lipase hydrolyzes triglycerides to free fatty acids and glycerol for use in cells.
      • Remnants containing cholesterol are taken to the liver.
        • Form VLDLs which take triglycerides to cells.
        • Once triglycerides are removed, VLDLs are converted to LDLs.
          • LDLs transport cholesterol to organs and blood vessels.
      • HDLs transport excess cholesterol back to liver.
      Transport of Lipids www.freelivedoctor.com
    • Absorption of Fat Insert fig. 18.36 www.freelivedoctor.com