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

23 The Digestive System: Part ADigestive SystemTwo groups of organsAlimentary canal (gastrointestinal or GI tract)Digests and absorbs foodMouth, pharynx, esophagus, stomach, small intestine, and large intestineDigestive SystemAccessory digestive organsTeeth, tongue, gallbladderDigestive glandsSalivary glandsLiverpancreasFigure 23.1Mouth (oral cavity)TongueEsophagusLiverGallbladderAnusDuodenumJejunumIleumSmall intestineParotid glandSublingual glandSubmandibularglandSalivaryglands PharynxStomachPancreas(Spleen)Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canalLargeintestineDigestive ProcessesSix essential activitiesIngestionPropulsionMechanical digestionChemical digestionAbsorptionDefecation Figure 23.2FoodIngestionPropulsionEsophagusStomachPharynxMechanicaldigestionChemicaldigestion• Chewing (mouth)• Churning (stomach)• Segmentation (small intestine)Smallintestine LargeintestineDefecationAnusFecesBloodvesselLymphvesselAbsorption• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)Mainly H2OFigure 23.3Frommouth(b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs.(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.GI tract regulatory mechanismsMechanoreceptors and chemoreceptors Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestionInitiate reflexes thatActivate or inhibit digestive glands Stimulate smooth muscle to mix and move lumen contents GI tract regulatory mechanismsIntrinsic and extrinsic controlsEnteric nerve plexuses (gut brain) initiate short reflexes in response to stimuli in the GI tractLong reflexes in response to stimuli inside or outside the GI tract involve CNS centers and autonomic nervesHormones from cells in the stomach and small intestine stimulate target cells in the same or different organsFigure 23.4External stimuli(sight, smell, taste,thought of food)Central nervous systemand extrinsic autonomic nervesAfferent impulsesEfferent impulsesLong reflexesInternal(GI tract)stimuliChemoreceptors,osmoreceptors, ormechanoreceptorsLocal (intrinsic)nerve plexus(“gut brain”)Effectors:Smooth muscleor glandsGastrointestinalwall (site of shortreflexes)Response:Change incontractile orsecretory activityLumen of thealimentary canalShort reflexesPeritoneum and Peritoneal CavityPeritoneum: serous membrane of the abdominal cavityVisceral peritoneum on external surface of most digestive organsParietal peritoneum lines the body wallPeritoneal cavityBetween the two peritoneumsFluid lubricates mobile organs Figure 23.5aPeritonealcavityParietalperitoneumVisceralperitoneumVentralmesenteryAbdominopelviccavityDorsalmesenteryVertebraAlimentarycanal organ(a) Schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries.LiverPeritoneum and Peritoneal CavityMesentery is a double layer of peritoneumRoutes for blood vessels, lymphatics, and nervesHolds organs in place and stores fatRetroperitoneal organs lie posterior to the peritoneumIntraperitoneal (peritoneal) organs are surrounded by the peritoneumFigure 23.5bAbdominopelviccavityMesenteryresorbedand lost(b) Some organs lose their mesentery and become retroperitoneal during development.Alimentarycanal organAlimentary canal organ ina retroperitoneal positionBlood Supply: Splanchnic CirculationArteriesHepatic, splenic, and left gastricInferior and superior mesenteric Hepatic portal circulationDrains nutrient-rich blood from digestive organsDelivers it to the liver for processing Histology of the Alimentary CanalFour basic layers (tunics)MucosaSubmucosaMuscularis externaSerosaFigure 23.6Glands in submucosaSubmucosaLumenMucosa-associatedlymphoid tissueDuct of gland outsidealimentary canal Gland in mucosaNerveArteryVeinLymphaticvessel MesenteryIntrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexusMucosa• Epithelium• Lamina propria• Muscularis mucosae Muscularis externa• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissueMucosaLines the lumen FunctionsSecretes mucus, digestive enzymes and hormonesAbsorbs end products of digestionProtects against infectious diseaseThree sublayers: epithelium, lamina propria, and muscularis mucosaeMucosaEpitheliumSimple columnar epithelium and mucus-secreting cellsMucusProtects digestive organs from enzymesEases food passageMay secrete enzymes and hormones (e.g., in stomach and small intestine)MucosaLamina propriaLoose areolar connective tissueCapillaries for nourishment and absorption Lymphoid follicles (part of MALT) Muscularis mucosae: smooth muscle that produces local movements of mucosaSubmucosa and Muscularis ExternaSubmucosaDense connective tissueBlood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexusSubmucosa and Muscularis ExternaMuscularis externaResponsible for segmentation and peristalsis Inner circular and outer longitudinal layersMyenteric nerve plexusSphincters in some regionsSerosaVisceral peritoneumReplaced by the fibrous adventitia in the esophagus Retroperitoneal organs have both an adventitia and serosaFigure 23.6Glands in submucosaSubmucosaLumenMucosa-associatedlymphoid tissueDuct of gland outsidealimentary canal Gland in mucosaNerveArteryVeinLymphaticvessel MesenteryIntrinsic nerve plexuses• Myenteric nerve plexus• Submucosal nerve plexusMucosa• Epithelium• Lamina propria• Muscularis mucosae Muscularis externa• Longitudinal muscle • Circular muscleSerosa• Epithelium• Connective tissueEnteric Nervous SystemIntrinsic nerve supply of the alimentary canalSubmucosal nerve plexusRegulates glands and smooth muscle in the mucosaMyenteric nerve plexusControls GI tract motilityEnteric Nervous SystemLinked to the CNS via afferent visceral fibers Long ANS fibers synapse with enteric plexusesSympathetic impulses inhibit secretion and motilityParasympathetic impulses stimulateMouthOral (buccal) cavityBounded by lips, cheeks, palate, and tongue Oral orifice is the anterior openingLined with stratified squamous epitheliumFigure 23.7aUvulaSoft palate Palatoglossal archPalatine tonsilHard palateOral cavityTongueLingual tonsilOropharynxEpiglottisHyoid boneLaryngopharynxEsophagusTrachea(a) Sagittal section of the oral cavity and pharynxLips and CheeksContain orbicularis oris and buccinator musclesVestibule: recess internal to lips and cheeks, external to teeth and gums Oral cavity proper lies within the teeth and gumsLabial frenulum: median attachment of each lip to the gumFigure 23.7bUvulaPalatine tonsilSublingual foldwith openings ofsublingual ductsTongueUpper lipLower lipVestibuleGingivae (gums)Gingivae (gums)Hard palateSoft palateLingual frenulumOpening ofsubmandibular ductPalatine rapheInferior labialfrenulumPosterior wallof oropharynxPalatopharyngealarchSuperior labialfrenulumPalatoglossal arch(b) Anterior viewPalateHard palate: palatine bones and palatine processes of the maxillaeSlightly corrugated to help create friction against the tongueSoft palate: fold formed mostly of skeletal muscleCloses off the nasopharynx during swallowingUvula projects downward from its free edgeTongueFunctions includeRepositioning and mixing food during chewing Formation of the bolusInitiation of swallowing, speech, and tasteIntrinsic muscles change the shape of the tongueExtrinsic muscles alter the tongue’s positionLingual frenulum: attachment to the floor of the mouthTongueSurface bears papillaeFiliform—whitish, give the tongue roughness and provide friction Fungiform—reddish, scattered over the tongue Circumvallate (vallate)—V-shaped row in back of tongueThese three house taste budsFoliate—on the lateral aspects of the posterior tongueTongueTerminal sulcus marks the division betweenBody: anterior 2/3 residing in the oral cavityRoot: posterior third residing in the oropharynxFigure 23.8EpiglottisPalatopharyngealarchPalatine tonsilLingual tonsilPalatoglossalarch Foliate papillaeCircumvallatepapilla Terminal sulcus Dorsum of tongueMidline grooveof tongue Filiform papillaFungiform papillaSalivary GlandsExtrinsic salivary glands (parotid, submandibular, and sublingual)Salivary GlandsIntrinsic (buccal) salivary glands are scattered in the oral mucosaSecretion (saliva) Cleanses the mouthMoistens and dissolves food chemicals Aids in bolus formationContains enzymes that begin the breakdown of starchSalivary GlandsParotid glandAnterior to the ear external to the masseter muscle Parotid duct opens into the vestibule next to second upper molarSubmandibular gland Medial to the body of the mandibleDuct opens at the base of the lingual frenulumSalivary GlandsSublingual glandAnterior to the submandibular gland under the tongueOpens via 10–12 ducts into the floor of the mouthFigure 23.9TeethDucts ofsublingualglandSublingualglandSubmandibularductPosterior bellyof digastric muscleParotid ductMasseter muscleBody ofmandible (cut) Parotidgland TongueSubmandibulargland(a)Frenulumof tongueMylohyoidmuscle (cut)Anterior belly ofdigastric muscleMucouscells (b)Serous cellsforming demilunesComposition of SalivaSecreted by serous and mucous cells 97–99.5% water, slightly acidic solution containingElectrolytes—Na+, K+, Cl–, PO4 2–, HCO3–Salivary amylase and lingual lipaseMucinMetabolic wastes—urea and uric acidLysozyme, IgA, defensins, and a cyanide compound protect against microorganismsPLAYAnimation: Rotatable headControl of SalivationIntrinsic glands continuously keep the mouth moistExtrinsic salivary glands produce secretions when Ingested food stimulates chemoreceptors and mechanoreceptors in the mouthSalivatory nuclei in the brain stem send impulses along parasympathetic fibers in cranial nerves VII and IXStrong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia)TeethPrimary and permanent dentitions are formed by age 21 20 deciduous teeth erupt (6–24 months of age)Roots are resorbed, teeth fall out (6–12 years of age) as permanent teeth develop32 permanent teethAll except third molars erupt by the end of adolescenceFigure 23.10bDeciduous teethPermanent teeth(b) Classes of TeethIncisorsChisel shaped for cutting Canines Fanglike teeth that tear or piercePremolars (bicuspids) and molars Have broad crowns with rounded cusps for grinding or crushingFigure 23.10aIncisorsCentral (6–8 mo)IncisorsCentral (7 yr)Canine (eyetooth)(16–20 mo)Canine (eyetooth)(11 yr)Premolars(bicuspids)First premolar(11 yr) MolarsFirst molar(10–15 mo)MolarsFirst molar (6–7 yr)Lateral (8–10 mo)Lateral (8 yr)Second molar(about 2 yr)Second molar(12–13 yr)Third molar(wisdom tooth)(17–25 yr)(a)PermanentteethDeciduous(milk) teethSecond premolar(12–13 yr)Dental FormulasA shorthand way of indicating the number and relative position of teethRatio of upper to lower teeth for one-half of the mouthPrimary: 2I,1C, 2MPermanent: 2I,1C, 2PM, 3M Tooth StructureCrown: the exposed part above the gingiva (gum)Covered by enamel—the hardest substance in the body (calcium salts and hydroxyapatite crystals)Root: portion embedded in the jawboneConnected to crown by neckTooth StructureCementum: calcified connective tissue Covers root and attaches it to the periodontal ligamentPeriodontal ligamentForms fibrous joint called a gomphosisGingival sulcus: groove where gingiva borders the toothTooth StructureDentin: bonelike material under enamelMaintained by odontoblasts of pulp cavityPulp cavity: cavity surrounded by dentin Pulp: connective tissue, blood vessels, and nervesRoot canal: extends from pulp cavity to the apical foramen of the rootFigure 23.11CrownNeckRootEnamelDentinDentinal tubulesPulp cavity (containsblood vessels and nerves)Gingiva (gum)CementumRoot canalPeriodontalligamentApical foramenBoneTooth and Gum DiseaseDental caries (cavities): gradual demineralization of enamel and dentin Dental plaque (sugar, bacteria, and debris) adheres to teethAcid from bacteria dissolves calcium saltsProteolytic enzymes digest organic matter Prevention: daily flossing and brushing Tooth and Gum DiseaseGingivitisPlaque calcifies to form calculus (tartar)Calculus disrupts the seal between the gingivae and the teeth Anaerobic bacteria infect gumsInfection reversible if calculus removed Tooth and Gum DiseasePeriodontitisImmune cells attack intruders and body tissuesDestroy periodontal ligamentActivate osteoclastsConsequencesPossible tooth loss, promotion of atherosclerosis and clot formation in coronary and cerebral arteries