Y khoa, y dược - The digestive system: Part B

23 The Digestive System: Part BPharynxOropharynx and laryngopharynxAllow passage of food, fluids, and airStratified squamous epithelium liningSkeletal muscle layers: inner longitudinal, outer pharyngeal constrictors EsophagusFlat muscular tube from laryngopharynx to stomachPierces diaphragm at esophageal hiatus Joins stomach at the cardiac orifice EsophagusEsophageal mucosa contains stratified squamous epitheliumChanges to simple columnar at the stomach Esophageal glands in submucosa secrete mucus to aid in bolus movementMuscularis: skeletal superiorly; smooth inferiorlyAdventitia instead of serosaFigure 23.12aMucosa(contains a stratifiedsquamous epithelium)Submucosa (areolarconnective tissue)LumenMuscularis externaAdventitia (fibrousconnective tissue)(a) • Circular layer • Longitudinal layerFigure 23.12bMucosa(contains a stratifiedsquamous epithelium)(b)Digestive Processes: Mouth IngestionMechanical digestionMastication is partly voluntary, partly reflexiveChemical digestion (salivary amylase and lingual lipase)Propulsion Deglutition (swallowing)Deglutition Involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groupsBuccal phaseVoluntary contraction of the tonguePharyngeal-esophageal phaseInvoluntaryControl center in the medulla and lower ponsFigure 23.13TongueTracheaPharynxEpiglottisGlottisBolus of foodEpiglottisEsophagusUvulaBolusBolusRelaxed musclesCircular musclescontractBolus of foodLongitudinal musclescontractStomachRelaxedmusclesGastroesophagealsphincter opensGastroesophagealsphincter closed Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins. Food is movedthrough the esophagusto the stomach byperistalsis. The gastroesophagealsphincter opens, and foodenters the stomach. The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus. The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry.12435Figure 23.13, step 1TongueTracheaPharynxEpiglottisGlottisBolus of food Upper esophageal sphincter is contracted. Duringthe buccal phase, the tongue presses against the hardpalate, forcing the food bolus into the oropharynxwhere the involuntary phase begins.1Figure 23.13, step 2EpiglottisEsophagusUvulaBolus The uvula and larynx rise to prevent food fromentering respiratory passageways. The tongue blocksoff the mouth. The upper esophageal sphincterrelaxes, allowing food to enter the esophagus.2Figure 23.13, step 3Bolus The constrictor muscles of the pharynx contract,forcing food into the esophagus inferiorly. The upperesophageal sphincter contracts (closes) after entry.3Figure 23.13, step 4Relaxed musclesBolus of foodStomachCircular musclescontractLongitudinal musclescontractGastroesophagealsphincter closed Food is moved throughthe esophagus to thestomach by peristalsis.4Figure 23.13, step 5RelaxedmusclesGastroesophagealsphincter opens The gastroesophagealsphincter opens, and foodenters the stomach.5Figure 23.13TongueTracheaPharynxEpiglottisGlottisBolus of foodEpiglottisEsophagusUvulaBolusBolusRelaxed musclesCircular musclescontractBolus of foodLongitudinal musclescontractStomachRelaxedmusclesGastroesophagealsphincter opensGastroesophagealsphincter closed Upper esophageal sphincter iscontracted. During the buccal phase, thetongue presses against the hard palate,forcing the food bolus into the oropharynxwhere the involuntary phase begins. Food is movedthrough the esophagusto the stomach byperistalsis. The gastroesophagealsphincter opens, and foodenters the stomach. The uvula and larynx rise to prevent foodfrom entering respiratory passageways. Thetongue blocks off the mouth. The upperesophageal sphincter relaxes, allowing foodto enter the esophagus. The constrictor muscles of thepharynx contract, forcing foodinto the esophagus inferiorly. Theupper esophageal sphinctercontracts (closes) after entry.12435Stomach: Gross AnatomyCardiac region (cardia) Surrounds the cardiac orificeFundusDome-shaped region beneath the diaphragmBodyMidportionStomach: Gross AnatomyPyloric region: antrum, pyloric canal, and pylorusPylorus is continuous with the duodenum through the pyloric valve (sphincter)Greater curvatureConvex lateral surfaceLesser curvatureConcave medial surfaceFigure 23.14aCardiaEsophagusPyloric sphincter(valve) at pylorusPyloriccanalPyloricantrumRugae ofmucosaBody LumenSerosaFundusLessercurvatureGreatercurvatureMuscularisexterna • Longitudinal layer • Circular layer • Oblique layer(a)DuodenumStomach: Gross AnatomyLesser omentum From the liver to the lesser curvatureGreater omentum Drapes from greater curvatureAnterior to the small intestineFigure 23.30aFalciform ligamentLiverGallbladderSpleenStomachLigamentum teresGreater omentumSmall intestineCecum(a)Figure 23.30bLiverLesser omentumGallbladderStomachDuodenumTransverse colonSmall intestineCecumUrinary bladder(b)Stomach: Gross AnatomyANS nerve supply Sympathetic via splanchnic nerves and celiac plexusParasympathetic via vagus nerveBlood supply Celiac trunkVeins of the hepatic portal systemStomach: Microscopic Anatomy Four tunicsMuscularis and mucosa are modifiedMuscularis externaThree layers of smooth muscleInner oblique layer allows stomach to churn, mix, move, and physically break down food Figure 23.15aMucosaSurfaceepitheliumLamina propriaMuscularismucosaeOblique layerCircular layerLongitudinallayerSerosa(a) Layers of the stomach wall (l.s.)Stomach wallMuscularis externa(contains myentericplexus)Submucosa(contains submucosalplexus)Stomach: Microscopic Anatomy MucosaSimple columnar epithelium composed of mucous cellsLayer of mucus traps bicarbonate-rich fluid beneath itGastric pits lead into gastric glandsFigure 23.15b(b) Enlarged view of gastric pits and gastric glandsMucous neck cellsParietal cellSurface epithelium(mucous cells) Gastric pitsChief cellEnteroendocrine cellGastric pitGastric glandGastric GlandsCell typesMucous neck cells (secrete thin, acidic mucus)Parietal cells Chief cells Enteroendocrine cells Figure 23.15c(c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric glandPepsinogenMitochondriaPepsinHClChief cellEnteroendocrinecell Parietal cellGastric Gland SecretionsGlands in the fundus and body produce most of the gastric juiceParietal cell secretionsHCl  pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many bacteriaIntrinsic factorGlycoprotein required for absorption of vitamin B12 in small intestineGastric Gland SecretionsChief cell secretionsInactive enzyme pepsinogen Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism)Gastric Gland SecretionsEnteroendocrine cellsSecrete chemical messengers into the lamina propriaParacrinesSerotonin and histamineHormonesSomatostatin and gastrinMucosal BarrierLayer of bicarbonate-rich mucus Tight junctions between epithelial cells Damaged epithelial cells are quickly replaced by division of stem cellsHomeostatic ImbalanceGastritis: inflammation caused by anything that breaches the mucosal barrierPeptic or gastric ulcers: erosion of the stomach wallMost are caused by Helicobacter pylori bacteria Figure 23.16BacteriaMucosalayer ofstomach(a) A gastric ulcer lesion(b) H. pylori bacteriaDigestive Processes in the StomachPhysical digestionDenaturation of proteinsEnzymatic digestion of proteins by pepsin (and rennin in infants)Secretes intrinsic factor required for absorption of vitamin B12 Lack of intrinsic factor  pernicious anemiaDelivers chyme to the small intestineRegulation of Gastric SecretionNeural and hormonal mechanisms Stimulatory and inhibitory events occur in three phases:Cephalic (reflex) phase: few minutes prior to food entryGastric phase: 3–4 hours after food enters the stomachRegulation of Gastric SecretionIntestinal phase: brief stimulatory effect as partially digested food enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones) Figure 23.17 Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty Distension;presence offatty, acidic,partiallydigested foodin theduodenumBriefeffectIntestinal(enteric)gastrinreleaseto bloodEntero-gastricreflexRelease of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)LocalreflexesVagalnucleiin medullaPyloricsphincterStimulateInhibit112Stomachsecretoryactivity Sight and thoughtof food Stomachdistensionactivatesstretchreceptors Stimulation oftaste and smellreceptors Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors Loss ofappetite,depression EmotionalupsetLack ofstimulatoryimpulses toparasym-patheticcenterCerebralcortexCerebral cortexConditioned reflexVagovagalreflexesLocalreflexesMedullaG cellsHypothalamusand medullaoblongataVagusnerveVagusnerveGastrinreleaseto bloodGastrinsecretiondeclinesG cellsOverridesparasym-patheticcontrolsSympatheticnervoussystemactivation1111222Stimulatory eventsInhibitory eventsCephalicphaseGastricphaseIntestinalphase Excessiveacidity (pH <2) in stomach Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenumRegulation and Mechanism of HCl SecretionThree chemicals (ACh, histamine, and gastrin) stimulate parietal cells through second-messenger systemsAll three are necessary for maximum HCl secretionAntihistamines block H2 receptors and decrease HCl releaseFigure 23.18Stomach lumenChief cellParietal cellInter-stitialfluidCarbonicanhydraseAlkalinetideHCO3–BloodcapillaryCO2Cl–CO2 + H2OH2CO3HCO3–- Cl–antiporterHCO3–H+Cl–Cl–lK+K+H+H+-K+ATPaseHCIResponse of the Stomach to FillingStretches to accommodate incoming foodReflex-mediated receptive relaxationCoordinated by the swallowing center of the brain stemGastric accommodationPlasticity (stress-relaxation response) of smooth muscleGastric Contractile ActivityPeristaltic waves move toward the pylorus at the rate of 3 per minuteBasic electrical rhythm (BER) initiated by pacemaker cells (cells of Cajal)Distension and gastrin increase force of contractionGastric Contractile ActivityMost vigorous near the pylorusChyme is eitherDelivered in ~ 3 ml spurts to the duodenum, orForced backward into the stomach Figure 23.191 Propulsion: Peristaltic waves move from the fundus toward the pylorus.23 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. Retropulsion: The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach.PyloricvalveclosedPyloricvalveclosedPyloricvalveslightlyopenedRegulation of Gastric EmptyingAs chyme enters the duodenumReceptors respond to stretch and chemical signalsEnterogastric reflex and enterogastrones inhibit gastric secretion and duodenal fillingCarbohydrate-rich chyme moves quickly through the duodenum Fatty chyme remains in the duodenum 6 hours or moreFigure 23.20Presence of fatty, hypertonic,acidic chyme in duodenumDuodenal entero-endocrine cellsChemoreceptors andstretch receptorsEnterogastrones(secretin, cholecystokinin,vasoactive intestinalpeptide)DuodenalstimulideclineVia shortreflexesVia longreflexesEntericneuronsInitial stimulusPhysiological responseResultContractile force andrate of stomachemptying declineCNS centers sympathetic activity; parasympathetic activityStimulateInhibitSecreteTargetSmall Intestine: Gross AnatomyMajor organ of digestion and absorption2–4 m long; from pyloric sphincter to ileocecal valve Subdivisions Duodenum (retroperitoneal)Jejunum (attached posteriorly by mesentery)Ileum (attached posteriorly by mesentery)Figure 23.1Mouth (oral cavity)TongueEsophagusLiverGallbladderAnusDuodenumJejunumIleumSmall intestineParotid glandSublingual glandSubmandibularglandSalivaryglands PharynxStomachPancreas(Spleen)Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canalLargeintestineDuodenumThe bile duct and main pancreatic ductJoin at the hepatopancreatic ampullaEnter the duodenum at the major duodenal papilla Are controlled by the hepatopancreatic sphincterFigure 23.21JejunumMucosawith foldsCystic ductDuodenumHepatopancreaticampulla and sphincterGallbladderRight and lefthepatic ducts of liverBile duct and sphincterMain pancreatic ductand sphincter PancreasTail of pancreasHead of pancreasCommon hepatic ductMajor duodenalpapillaAccessory pancreatic ductStructural ModificationsIncrease surface area of proximal part for nutrient absorptionCircular folds (plicae circulares)VilliMicrovilliStructural ModificationsCircular folds Permanent (~1 cm deep) Force chyme to slowly spiral through lumenFigure 23.22aVein carrying blood tohepatic portal vesselMusclelayersCircularfoldsVilli(a)LumenStructural ModificationsVilli Motile fingerlike extensions (~1 mm high) of the mucosaVillus epitheliumSimple columnar absorptive cells (enterocytes)Goblet cells Structural ModificationsMicrovilli Projections (brush border) of absorptive cellsBear brush border enzymesIntestinal CryptsIntestinal crypt epitheliumSecretory cells that produce intestinal juiceEnteroendocrine cells Intraepithelial lymphocytes (IELs)Release cytokines that kill infected cellsPaneth cellsSecrete antimicrobial agents (defensins and lysozyme)Stem cellsFigure 23.22b(b)Absorptive cellsLactealIntestinal cryptMucosaassociatedlymphoid tissueMuscularismucosaeDuodenal glandSubmucosaEnteroendocrinecellsVenuleLymphatic vesselGoblet cellBloodcapillariesVilusMicrovilli(brush border)SubmucosaPeyer’s patches protect distal part against bacteria Duodenal (Brunner’s) glands of the duodenum secrete alkaline mucus Intestinal JuiceSecreted in response to distension or irritation of the mucosaSlightly alkaline and isotonic with blood plasmaLargely water, enzyme-poor, but contains mucusFacilitates transport and absorption of nutrientsLiverLargest gland in the bodyFour lobes—right, left, caudate, and quadrateLiverFalciform ligamentSeparates the (larger) right and (smaller) left lobes Suspends liver from the diaphragm and anterior abdominal wallRound ligament (ligamentum teres)Remnant of fetal umbilical vein along free edge of falciform ligamentFigure 23.24aSternumNippleLiverRight lobeof liver Gallbladder(a)Bare areaFalciformligament Left lobe of liverRound ligament(ligamentum teres)Figure 23.24bLesser omentum(in fissure)Left lobe of liver(b)Porta hepatiscontaining hepaticartery (left) andhepatic portal vein(right)Quadrate lobeof liverLigamentum teresGallbladderHepatic vein (cut)Sulcus forinferiorvena cavaCaudate lobeof liverBare areaBile duct (cut)Right lobe ofliver SternumNippleLiverLiver: Associated StructuresLesser omentum anchors liver to stomachHepatic artery and vein at the porta hepatisBile ductsCommon hepatic duct leaves the liverCystic duct connects to gallbladderBile duct formed by the union of the above two ductsFigure 23.21JejunumMucosawith foldsCystic ductDuodenumHepatopancreaticampulla and sphincterGallbladderRight and lefthepatic ducts of liverBile duct and sphincterMain pancreatic ductand sphincter PancreasTail of pancreasHead of pancreasCommon hepatic ductMajor duodenalpapillaAccessory pancreatic ductLiver: Microscopic AnatomyLiver lobulesHexagonal structural and functional unitsFilter and process nutrient-rich blood Composed of plates of hepatocytes (liver cells) Longitudinal central vein Figure 23.25a, b(a)(b)LobuleCentral veinConnectivetissue septumLiver: Microscopic AnatomyPortal triad at each corner of lobuleBile duct receives bile from bile canaliculi Portal arteriole is a branch of the hepatic artery Hepatic venule is a branch of the hepatic portal vein Liver sinusoids are leaky capillaries between hepatic platesKupffer cells (hepatic macrophages) in liver sinusoidsFigure 23.25c(c)Interlobular veins(to hepatic vein)Central veinSinusoidsPortal triadPlates ofhepatocytesPortal veinFenestratedlining (endothelial cells) of sinusoidsBile duct (receivesbile from bile canaliculi)Bile ductPortal arteriolePortal venuleHepaticmacrophagesin sinusoid wallsBile canaliculiLiver: Microscopic AnatomyHepatocyte functionsProcess bloodborne nutrientsStore fat-soluble vitaminsPerform detoxification Produce ~900 ml bile per dayBileYellow-green, alkaline solution containing Bile salts: cholesterol derivatives that function in fat emulsification and absorptionBilirubin: pigment formed from heme Cholesterol, neutral fats, phospholipids, and electrolytesBileEnterohepatic circulationRecycles bile saltsBile salts  duodenum  reabsorbed from ileum  hepatic portal blood  liver  secreted into bileThe GallbladderThin-walled muscular sac on the ventral surface of the liverStores and concentrates bile by absorbing its water and ionsReleases bile via the cystic duct, which flows into the bile duct