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Gastrointestinal Gastrointestinal Physiology Physiology (Digestive System) (Digestive System) Dr. G. AHMAD Dr. G. AHMAD Assistant Professor Physiology Assistant Professor Physiology

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  • Gastrointestinal Physiology(Digestive System)

    Dr. G. AHMADAssistant Professor Physiology

  • Functions of the Digestive SystemIngest food

    Break down food into nutrient molecules

    Absorb molecules into the bloodstream

    Rid the body of indigestible remains

  • Main Divisions of the Digestive SystemAlimentary CanalContinuous, muscular digestive tube winding throughout the bodyDigests and absorbs food particlesContains the following organs:Mouth, Pharynx, Esophagus, Stomach, Small and Large IntestinesAccessory Digestive OrgansContains the following organs:Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Pancreas

  • Digestive System Divisions

  • Digestive ProcessesIngestionMechanical digestionPropulsionChemical digestionAbsorptionDefecation

  • Actions of Digestive (GI) TractIngestionOccurs when material enters via the mouthMechanical ProcessingCrushing / Shearing makes material easier to move through the tractDigestionChemical breakdown of food into small organic compounds for absorption

    SecretionRelease of water acids, buffers, enzymes & salts by epithelium of GI tract and glandular organsAbsorptionMovement of organic substrates, electrolytes, vitamins & water across digestive epitheliumExcretionRemoval of waste products from body fluids

  • Features and Functions of the MouthBuccal/oral cavityContains stratified squamous epitheliumVestibule: area bounded by lips and cheeks externally and teeth and gums internallyLips: posses no sweat or oil glandsPalate: forms roof of the mouth, soft and hard palate, uvula

  • Features and Functions of the TongueHelps grind food into a bolus which contains partially digested food and salivaHelps form words and is a sensory organ for tasteThree surface features:Filiform papillae (roughness and grip)Fungiform papillae (contains taste buds)Foliate papilae (contains taste buds)Circumvallate papillae (contains taste buds)

  • Papillae of the Tongue

  • Features and Functions of the Salivary GlandsMain functions:Produces and secretes salivaCleanses the mouthDissolves food chemicals so they can be tastedMoistens food, compacting it into a bolusBegins the chemical breakdown of foodSalivary amylase: starch

  • Types of Salivary GlandsSubmandibular GlandsFound underneath the mandibleSublingual GlandsFound underneath the tongueParotid GlandsFound anterior to the ear between masseter and skinSaliva travels to oral orifice via ducts from all three glands

  • Anatomy of the Salivary Glands

  • Composition of Saliva: 97-99.5% water0.5 % electrolytesSodium, potassium, chloride, phosphate, and bicarbonateMost important proteins are salivary amylase, mucus and lysozymes

  • Features and Functions of the TeethBreak food into smaller parts, increasing surface area for digestionTypes of TeethDeciduous Teeth (baby teeth)Permanent TeethIncisors- cutting and shreddingCanines- piercing and tearingMolars- grindingPremolars- grinding and crushing

  • Tooth StructureCrown: exposed portion of tooth covered by enamel which covers dentinRoot: internal portion that is beneath the gums (gingiva) and is anchored by periodontal ligaments

  • PharynxResponsible for swallowingSwallowing is divided into oropharyngeal stage and esophageal stage

  • Features and Functions of the EsophagusMuscular tube that propels food to stomach; bolus enters stomach through esophageal hiatusEsophageal glands produce mucus to lubricate bolusEsophageal sphincter prevents backflow into oral cavityCardiac sphincter- prevents backflow into esophagus

  • Features and Functions of the EsophagusA hollow muscular tubeAbout 25 cm (10 in.) long and 2 cm (0.80 in.) wideConveys solid food and liquids to the stomachBegins posterior to cricoid cartilage Esophageal glands produce mucus to lubricate bolusEsophageal sphincter prevents backflow into oral cavity

    SwallowingAlso called deglutitionCan be initiated voluntarilyProceeds automaticallyIs divided into three phasesBuccal phasePharyngeal phaseEsophageal phase*

  • Anatomy of the Esophagus

  • Digestive Processes in the Mouth, Pharynx, and EsophagusMouth processes:IngestionMechanical digestion (e.g. salivary amylase)Initiation of PropulsionMastication: chewingPharyngeal processes:Deglutition = swallowingEsophageal processes:Peristalsis (rhythmic contractions, involuntary)

  • Peristalsis

  • Features and Functions of the StomachTemporary storage area for food and allows it to mix with gastric juice to produce chymeRegions: cardiac, fundus, body, and pyloric

  • Stomach FunctionMajor Functions of the StomachStorage of ingested foodMechanical breakdown of ingested foodDisruption of chemical bonds in food material by acid and enzymesProduction of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine

  • Anatomy of the Stomach

  • Gastric Juices (hormones)Simple columnar epithelium contains gastric pits that secrete gastric juicesGoblet cells secrete mucus that coats stomach and prevents it from being digested itselfParietal cells secrete hydrochloric acid (converts pepsinogen into pepsin) and intrinsic factor (necessary for absorption of vitamin B12)Chief cells secrete pepsinogen which is converted to pepsin to aid in protein digestionEnteroendocrine cells release hormones such as:Histamine, Serotonin, Gastrin, Endorphins, and Somatostatin

  • Histamine

    - activates parietal cells to release HCl Serotonin

    - contraction of stomach muscle Gastrin

    - gastric glands to increase secretionEndorphins

    - natural opiates Somatostatin

    - inhibits gastric secretion- inhibits gastric emptying

  • Digestive Processes in the StomachThree PhasesCephalic PhaseStimulated by the thought, sight, taste, or aroma or foodInputs from olfactory receptors and taste buds travel to parasympathetic enteric ganglia which then stimulate stomach glandsGastric PhaseStomach distension activates stretch receptorsFood chemicals (e.g. peptides, rising pH) activate chemoreceptors which activate G cells which secrete gastrinIntestinal PhasePresence of low pH and partially digested foods in duodenum stimulates intestinal gastrin secretion

  • Digestive Processes in the Stomach

  • Gastric Motility and EmptyingPeristaltic waves approach stomach and become stronger near pyloric regionPyloric sphincter allows ~ 3 mL of chyme to pass to duodenum and the rest to return to stomach for further mixing

  • Small Intestine90% of absorption occurs in the small intestine

  • Small IntestineThe Duodenum The segment of small intestine closest to stomach25 cm (10 in.) longMixing bowl that receives chyme from stomach and digestive secretions from pancreas and liverFunctions of the duodenum To receive chyme from stomachTo neutralize acids before they can damage the absorptive surfaces of the small intestine

  • Small IntestineThe Jejunum Is the middle segment of small intestine2.5 meters (8.2 ft) longIs the location of mostChemical digestionNutrient absorptionSmall villi

  • Small IntestineThe IleumThe final segment of small intestine3.5 meters (11.48 ft) long Ends at the ileocecal valve, a sphincter that controls flow of material from the ileum into the large intestine

  • Small IntestineIntestinal SecretionsWatery intestinal juice1.8 liters per day enter intestinal lumenMoisten chymeAssist in buffering acidsKeep digestive enzymes and products of digestion in solution

    Intestinal MovementsChyme arrives in duodenumWeak peristaltic contractions move it slowly toward jejunum

  • Microscopic Anatomy of Small IntestineVilli: fingerlike projections that increase the surface area of the SIMicrovilli: tiny projections on the plasma membranes of columnar cells that appear fuzzy (i.e. brush border cells)Crypts of Lieberkuhn: secrete intestinal juice and special lysozymes that protect against bacteriaPeyers Patches: aggregated lymphoid tissues containing lymphocytes

  • Microscopic Anatomy of the Small Intestine

  • Secretions of the Small IntestineSecretin: released by enteroendocrine cells when acidic chyme enters SI; causes release of bicarbonate-rich pancreatic juicesSomatostatin: slows gastric motility and emptying and inhibits production of gastric secretionsCholecystokinin (CCK): released when fatty, protein-rich chyme enters SI; causes release of enzyme-rich pancreatic juices and bileBrush border enzymes: process long peptides, nucleic acids, and sugars into smaller ones

  • After Digestion: AbsorptionProtein Fats

  • Functions of the LiverLargest internal organFunctions: Filters and processes nutrient-rich blood of carbohydrates, proteins, and lipids from intestineProduction and regulation of cholesterolProduction of bile which emulsifies fatsRemoves drugs and hormones from circulationStorage of vitamins and minerals

  • Anatomy of the LiverRight and Left Lobes: separated by falciform ligamentCaudate and Quadrate Lobes: found on posterior sideBlood vessels:Hepatic artery/vein and hepatic portal veinGallbladder: found underneath left lobe, stores bile

  • Gross Anatomy of the Liver

  • Microscopic Anatomy of the LiverLiver Lobules: structural unit of liverHepatocytes: liver cells contained within the lobulesHepatic portal vein & Hepatic Artery: the circulation of the liver, they bring blood into the liver where it is filtered through the liver sinusoidal capillariesKupffer cells: remove debrisFiltered blood drains into the central vein, then to the hepatic vein, and eventually to the inferior vena cavaBile (produced by hepatocytes) drains into the bile duct after passing through portal triadBile then shipped to gallbladder for storage

  • Microscopic Anatomy of the Liver

  • Function and Regulation of BileBile ducts are present at every portal triadBile flows down bile canaliculi (tiny canals) between adjacent hepatocytes towards bile duct branches at every portal triadBile enters the bile ducts which drain into the common hepatic ductBile emulsifies fats, separating them into smaller partsBilirubin: the chief bile pigment, a waste product of the heme of hemoglobin formed during the breakdown of worn-out erythrocytes

  • Regulation of Bile ProductionBile exits cystic duct upon stimulationCCK released when acidic, fatty chyme enters intestinesCauses:Gallbladder ContractionPancreatic Juice SecretionRelaxation of hepatopancreatic sphincter

  • Features and Functions of the PancreasPancreatic Juice secreted by acinar cellsIslets of Langerhans release insulin and glucagon (important in glucose metabolism)Pancreatic Juice contains:Sodium Bicarbonate (buffers HCl in stomach)Proteases (break down polypeptides)Pancreatic amylase (digests oligosaccarides and disaccharides into monosaccharides)Pancreatic lipases (break down lipids into fatty acids and glycerol)Pancreatic nucleases (break down nucleic acids)

  • Anatomy of the Pancreas

  • Features and Functions of the Large IntestineFunctions:Reabsorption of remaining water and electrolytesProduction and absorption of Vitamins B and KElimination of fecesDiameter is only 7 cm but is larger than that of the small intestine

  • Gross Anatomy of the Large IntestineTeniae Coli: bands of smooth muscle that create pocket-like sacs (haustra)Cecum: sac-like connection between the small and large intestinesAppendix: small structure containing lymphoid tissue; small immune functionAscending, Descending, Transverse, and Sigmoid ColonSplenic and hepatic flexureRectum: storage areaAnus: regulates defecation with two sphincter muscles; internal and external

  • Anatomy of the Colon

  • Microscopic Anatomy of the Large IntestineSimple columnar epithelium for absorption except in the anal canal where there is stratified squamousNo villi, no digestive-secreting cellsGoblet cells produce mucus for lubrication of fecesBacterial flora synthesize vitamin B and most of the vitamin K needed for blood clotting

  • The Process of Absorption

  • Clinical CornerGastritis- Inflammation of gastric mucosaCaused by chronic bacterial infectionIngestion of irritating substances e.g. alcohol/asprine

    Achlorhydria :absence of HCL secretion Hypochlorhydria: diminished acid secretionsPernicious Anemia: occurs when intrinsic factor is not secreted. Absorption of Vit B12 does not occur leading to Pernicious anemia (large immature red blood ells)

  • Clinical CornerPeptic Ulcer: The damaged are of stomach or upper part of small intestine mucosa caused by the digestive action of gastric or small intestinal secretions. Causes: i) imbalance between gastric and intestinal secretion and the protective coating of mucosaii)Helicobacter bacterial infectioniii) smoking, alcohol and aspirine

  • Clinical CornerSprue: Decreased or malabsorption of nutrientsCirrohsis - scarred liver due to chronic inflammation Hepatitis - A,B,C,D, and E Biliary calculi - gall stones - crystals of cholesterol in bile Borborygmus - rumbling noise caused by gas through intestines

  • Cholecystitis - inflammation of gall bladder Colitis - inflammation of colon Dysphagia - difficulty in swallowing Enteritis - inflammation of the intestines ConstipationDiarrhea

    Clinical corner

    SwallowingAlso called deglutitionCan be initiated voluntarilyProceeds automaticallyIs divided into three phasesBuccal phasePharyngeal phaseEsophageal phase*