动物生理学6.消化与吸收(二)

6.3.1.2Compositionandfunctionofgastricjuice•纯净的胃液是一种无色而呈酸性反应的液体。pH0.9-1.5•正常人每日分泌的胃液量约为1.5-2.5L。•胃液的成分无机物，如盐酸、钠和钾的氯化物等，有机物，如粘蛋白、胃蛋白酶原等。Hydrochloricacid•Hydrochloricacidproducedinstomachisalsocalledgastricacid.•有两种形式：游离酸和结合酸（与蛋白质结合）•胃酸排出量：基础酸排出量0~5mmol/h最大排出量20~25mmol/h胃酸的生理作用•激活胃蛋白酶原，为胃蛋白酶提供酸性环境•杀灭细菌•促进Ca2+和Fe2+的吸收•进入十二指肠后，可促进secretin和CCK的释放MechanismofhydrochloricacidsecretionparietalcellProtonpumpMechanismofhydrochloricacidsecretionPepsinogen[pɛp,sɪnədʒən]•Pepsin的最适pH为2.0~3.5，当pH5.0时便失活•主要作用于蛋白质及多肽分子中含苯丙氨酸或酪氨酸的肽键上，主要分解产物是眎和胨以及少量的多肽和氨基酸。PepsinogenpepsinHClMucusandbicarbonateMucus由粘膜表面的上皮细胞、粘颈细胞、贲门、幽门的腺细胞产生。作用：①润滑②保护胃粘膜免受机械损伤③降低胃酸的酸度、降低胃蛋白酶活性防止酸和胃蛋白酶对胃粘膜的损害④减慢H+向胃壁的扩散MucusbicarbonatebarrierGastricmucosalbarrierGastricMucus-BicarbonateBarrierViewofableedingulcerbyusinganendoscope(atubularinstrumentbearingatinylensandalightsource)thatcanbeinsertedintotheabdominalcavity.Intrinsic[in’trinsik]factorIntrinsicfactorisaglycoproteinproducedbytheparietalcellsofthestomach.ItisnecessaryfortheabsorptionofvitaminB12intheterminalileum.6.3.1.3Regulationofgastricsecretion消化间期胃液分泌消化期胃液分泌Endogenoussubstancespromotinggastricsecretion•Acetylcholine(vagusreflexes)–Stimulatessmoothmusclecontraction–AlsostimulatesChief,Parietal,ELCandGcellsAch→→Mreceptor→ECLcells→→→Histamine→→H2receptorGastrin(endocrine)•Effects①Stimulateschief,parietalcells②StimulatesECLcells③StimulatesthegrowthofGImucosa,andthesynthesisofprotein④StimulatesthemotilityofGItract,contractionofgallbladder,andthesecretionofbileandpancreas.•BiggastrinG34•SmallgastrinG17•ArtificialG5Histamine(paracrine)•ProducedbyECLcells•Stronglystimulatesgastricsecretion•BindtoH2receptor•BeControlledbyAchandgastrin协同作用Endogenoussubstancesinhibitinggastricsecretion•SomatostatinSS(过去也称GHIH)生长抑素•Secretin促胰液素•prostaglandin前列腺素•Cephalicphase头期Neuralregulation•Gastricphase胃期Bothneuralandhumoral•Intestinalphase肠期mainlyhumoralGastricsecretionindigestingphaseCephalicphaseConditionedreflexUnconditionedreflexAccountfor10%-15%totalvolumeofsecretionLargeamountofHLandpepsinogen,highdigestiveabilityGastricPhaseDistensionofstomach(arrivaloffood)Vagal/EntericreflexesParietalcellsAChPeptidesinlumenGcellsGastrinGastrin/AChECLcellsHistamine•Startswhenfoodreachesthestomach•Provides2/3ofthejuicereleased•Therearetwoparts(neuralandchemical)tothisphaseNeuralpart•Activatedbystretchreceptors•Initiatesbothlocalneuralreflexesaswellasthelongervago-vagalreflex•BothreflexesresultinreleaseofAChatstomachsynapseswhichstimulatessecretorycells•ThisbranchisinhibitedbySympatheticaction(emotionalupset)GastricPhaseChemicalpart•AnincreaseinpH(thus,‘less’acidity),presenceofpeptides,andcaffeineactivatetheG-cells•ThisresultsinGastrinbeingreleased•GastrinactsonParietalcellsthatstartsecretingHCl•GastrinalsostimulatesHistaminerelease,whichinturnstimulatesParietalcells•TheincreaseinHClpromotespepsinproductionandproteindegradation•ThereleaseofGastrinispartlyregulatedbyacidity,IncreasedacidityinhibitstheG-cells.Increasedpresenceofproteinsinamealtendstobufferproton.ThisinturnkeepsthepHfrombecomingtooacidicandallowsmoregastrintobereleasedIntestinalPhaseAccountforabout5%ofsecretionPrimarilyhormonal–denervatedstomachwillbestimulatedtosecreteacidbyproteininduodenumHormonestillunknownVerysmalllnumberofG-cellsinduodenumalsoreleasegastrininresponsetoaminoacidsRegulationofGastricSecretionsoccursvia3phasesFactorsinhibitinggastricsecretionduringdigesdtion•AcidinhibitreleaseofgastrinstimulatesreleaseofSSinanrumstimulatesreleaseofsecretinandbulbogastrone球抑胃素indoudenum•Fatstimulatesreleaseofenterogastrone(notisolated)•Hypertonicsolution高张溶液（高渗溶液）(duodenal)osmoticreceptor→→→enterogastricreflex