生理學(xué)雙語課件-呼吸-2

第二節(jié)呼吸氣體的交換一、氣體交換原理（一）氣體的擴散機體內(nèi)的氣體交換就是以擴散方式進行的。氣體擴散速率受下列因素的影響：11．氣體的分壓差

在混合氣體中，每種氣體分子運動所產(chǎn)生的壓力為各該氣體的分壓，它不受其它氣體存在的影響，只決定于它自身的濃度。兩個區(qū)域之間的分壓差（△P）是氣體擴散的動力，分壓差大，擴散快。22.氣體的分子量和溶解度擴散速率和該氣體分子量（MW）的平方根成反比，與溶解度成正比。33．?dāng)U散面積和距離氣體擴散速率與擴散面積（A）成正比。與擴散距離(d)成反比。4．溫度

擴散速率與溫度（T）成正比。4inwhichDisthediffusionrate,△Pisthepressuredifferencebetweenthetwoendsofthediffusionpathway,Aisthecross-sectionalareaofthepathway,Sisthesolubilityofthegas,disthedistanceofdiffusion,andMWisthemolecularweightofthegas.5Inadditiontothepressuredifference,severalotherfactorsaffecttherateofgasdiffusioninafluid.Theyare(1)thesolubilityofthegasinthefluid,(2)thecross-sectionalareaofthefluid,(3)thedistancethroughwhichthegasmustdiffuse,6(4)themolecularweightofthegas,and(5)thetemperatureofthefluid.Inthebody,thelastofthesefactors,thetemperature,remainsreasonablyconstantandusuallyneednotbeconsidered.7A.IntroductionGasescanmovefromonepointtoanotherbydiffusionandthatthecauseofthismovementisalwaysapres-suredifferencefromthefirstpointtothenext.Thus,oxygendiffusesfromthealveoliintothepulmonarycapillarybloodbecausetheoxygenpressure(PO2)inthealveoliisgreaterthanthePO2inthepulmonaryblood.8Then,inthetissues,ahigherPO2inthecapillarybloodthaninthetissuescausesoxygentodiffuseintothesurroundingcells.Conversely,whenoxygenismetabolizedinthecellstoformcarbondioxide,theintracellularcarbondioxidepressure(PCO2)risestoahighvalue,whichcausescarbondioxidetodiffuseintothetissuecapillaries.9Similarly,itdiffusesoutofthebloodintothealveolibecausethePCO2inthepulmonarycapillarybloodisgreaterthanthatinthealveoli.Basically,then,thetransportofoxygenandcarbondioxidebytheblooddependsonbothdiffusionandthemovementofblood.10呼吸膜{含表面活性物質(zhì)的液體層肺泡上皮細胞層肺泡上皮細胞基膜層間隙毛細血管基膜層毛細血管內(nèi)皮細胞層三、肺換氣1112（二）影響肺換氣的因素1、呼吸膜（1）厚度（2）面積2、通氣/血流比值每分肺泡通氣量和每分肺血流量之間的比值。為0.84。（V/Q)13

V/Qratioistheratioofalveolarventilation(V)topulmonarybloodflow(Q).MatchingventilationandperfusionisimportanttoachievetheidealexchangeofO2andCO2.Ifthefrequency,tidalvolume,andcardiacoutputarenormal,theV/Qratioisapproximately0.8.ThissituationresultsinanarterialPO2of100mmHgandanarterialPCO2of40mmHg.14如果VA/Q比值增大，這就意味著通氣過剩，血流不足，部分肺泡氣未能與血液氣充分交換，致使肺泡無效腔增大。15反之，VA/Q下降，則意味著通氣不足，血流過剩，部分血液流經(jīng)通氣不良的肺泡，未能得到充分更新,血就流回了心臟。猶如發(fā)生了功能性的動-靜脈短路。16肺泡無效腔增大動靜脈短路171.V/QratioinairwayobstructionIftheairwaysarecompletelyblocked(e.g.,byapieceofsteakcaughtinthetrachea),thenventilationiszero.Ifbloodflowisnormal,thenV/Qiszero.18Thereisnogasexchangeinalungthatisperfusedbutnotventilated.ThePO2andPCO2ofpulmonarycapillaryblood(and,therefore,ofsystemicarterialblood)willapproachtheirvaluesinmixedvenousblood.192.V/QratioinbloodflowobstructionIfbloodflowtoalungiscompletelyblocked(e.g.,byanembolismoccludingapulmonaryartery),thenbloodflowtothatlungiszero.Ifventilationisnormal,thenV/Qisinfinite.Thereisnogasexchangeinalungthatisventilatedbutnotperfused.20動脈靜脈O2(30mmHg)CO2(50mmHg)O2(100mmHg)CO2(40mmHg)O2(40mmHg)CO2(46mmHg)組織細胞三、組織換氣21DiffusionofOxygenfromthePeripheralCapillariesintotheTissueFluidWhenthearterialbloodreachestheperipheraltissues,itsPO2inthecapillariesisstill95mmHg.YetthePO2intheinterstitialfluidthatsurroundsthetissuecellsaveragesonly40mmHg.22Thus,thereisatremendousinitialpressuredifferencethatcausesoxygentodiffuserapidlyfromthebloodintothetissues,sorapidlythatthecapillaryPO2fallsalmosttoequalthe40mmHgpressureintheinterstitial.Therefore,thePO2ofthebloodleavingthetissuecapillariesandenteringtheveinsisalsoabout40mmHg.23Whenoxygenisusedbythecells,mostofitbecomescarbondioxide,andthisincreasestheintracellularPCO2BecauseofthehightissuecellPCO2,carbondioxidediffusesfromthecellsintothetissuecapillariesandisthencarriedbythebloodtothelungs.Inthelungs,itdiffusesfromthepulmonarycapillariesintothealveoli.Thus,ateachpointinthegastransportchain,carbondioxidediffusesinadirectionexactlyoppositethatofthediffusionofoxygen.24Yetthereisonemajordifferencebetweenthediffusionofcarbondioxideandthatofoxygen:carbondioxidecandiffuseabout20timesasrapidlyasoxygen.Therefore,thepressuredifferencesrequiredtocausecarbondioxidediffusionare,ineachinstance,farlessthanthepressuredifferencesrequired.25第三節(jié)氣體在血液中的運輸26一、氧和二氧化碳在血液中運輸?shù)幕拘问剑ㄒ唬┪锢砣芙獗壤。ǘ┗瘜W(xué)結(jié)合主要O2溶解的O2化學(xué)結(jié)合的O2溶解的O2O2CO2溶解的CO2化學(xué)結(jié)合的CO2溶解的CO2

CO227二、氧的運輸（一）物理溶解占血液總氧含量的約1.5%（二）化學(xué)結(jié)合占血液總氧含量的約98.5%與血紅蛋白（Hb）進行化學(xué)結(jié)合28TransportOfOxygenInTheBloodNormally,about97percentoftheoxygentransportedfromthelungstothetissuesiscarriedinchemicalcombinationwithhemoglobinintheredbloodcells.Theremaining3percentistransportedinthedissolvedstateinthewateroftheplasmaandcells.Thus,undernormalconditions,oxygeniscarriedtothetissuesalmostentirelybyhemoglobin.291.

Hb的分子結(jié)構(gòu){1個珠蛋白4個血紅素四條多肽鏈22每個血紅素有4個吡咯環(huán)，其中心有一個Fe2+一條多肽鏈和一個血紅素相連形成單體（亞單位）。30312.Hb與O2的結(jié)合特點（1）反應(yīng)快，可逆，不需要酶的催化，受O2分壓的影響。32（2）Fe2+與O2的結(jié)合是氧合，不是氧化。（3）1分子的Hb可以結(jié)合4分子的O2。33ReversibleCombinationofOxygenwithHemoglobinTheoxygenmoleculecombineslooselyandreversiblywiththehemeportionofthehemoglobin.WhenPO2ishigh,asinthepulmonarycapillaries,oxygenbindswiththehemoglobin,butwhenPO2islow,asinthetissuecapillaries,oxygenisreleasedfromthehemoglobin.Thisisthebasisforalmostalloxygentransportfromthelungstothetissues.34

A.HemoglobinCharacteristicsglobularproteinoffoursubunitsEachsubunitcontainsaheme,whichisiron-containingporphyrin.卟啉Theironisintheferrousstate(Fe2+),whichbindsO2,[Ifironisintheferricstate(Fe3+),itismethemoglobin高鐵血紅蛋白,whichdoesnotbindO2]35Eachsubunithasapolypeptidechain.Twoofthesubunitshaveαchainsandtwoofthesubunitshaveβchains;thus,normaladulthemoglobiniscalledα2β2.36

O2capacityisthemaximumamountofO2thatcanbeboundtohemoglobin.O2capacityisdeterminedbythehemoglobinconcentrationinblood.37O2capacitylimitstheamountofO2thatcanbecarriedinblood.O2capacityismeasuredat100%saturation.38O2contentO2contentisthetotalamountofO2carriedinblood,includingboundanddissolvedO2,dependsonthehemoglobinconcentrationandthePo2.39Saturationofthehemoglobin.thepercentageofhemoglobinboundwithoxygeniscalledthepercentsaturationofthehemoglobin.40100ml血液中，Hb所能結(jié)合的最大O2量稱為氧容量。而實際結(jié)合的O2量稱為氧含量。氧含量和氧容量的百分比為氧飽和度。41?HbO2呈鮮紅色，去氧Hb呈紫藍色。當(dāng)表淺毛細血管血液中去氧Hb含量達5g/100ml血液以上時，皮膚、粘膜呈淺藍色，稱紫紺。423、氧離曲線（oxygendissociationcurve）是表示PO2與氧飽和度關(guān)系的曲線。呈S形。43上段：PO2100~60mmHg中段：PO260~40mmHg下段：PO240~15mmHg44上段PO2100~60mmHg：曲線平坦，受O2分壓影響小。是Hb與O2的結(jié)合階段。意義：當(dāng)外環(huán)境或吸入氣中的O2分壓下降，造成血PO2降低時，仍能為機體攝取和攜帶足夠的O2。45中段：PO260~40mmHg曲線較陡，是HbO2釋放O2的部分。意義：當(dāng)動脈血流經(jīng)組織時，可釋放出適量的O2，滿足機體安靜狀態(tài)下對O2

的需求。46下段：PO240~15mmHg曲線很陡，表明當(dāng)O2分壓稍有下降時，血氧飽和度將明顯降低。也是Hb與O2的解離階段。意義：當(dāng)動脈血流經(jīng)活動增強的組織時，可釋放足夠的O2，滿足活動增強組織對O2需求。471.HemoglobincombinesrapidlyandreversiblywithO2toformoxyhemoglobin. 2.Thehemoglobin-O2dissociationcurveisaplotofsaturationofhemoglobinasafunction函數(shù)ofPO2. a.AtaPO2of100mmHg(e.g.,arterialblood)hemoglobinisalmost100%saturated;O2isboundtoallfourhemegroupsonallhemoglobinmolecules.48ThePO2at50%saturationistheP50.Fiftypercentsaturationmeansthat,onaverage,twoofthefourhemegroupsofeachhemoglobinmoleculehaveO2bound.49ThesigmoidshapeofthecurveistheresultofachangeintheaffinityofhemoglobinaseachsuccessiveO2moleculebindstoahemesite(calledpositivecooperativity).BindingofthefirstO2moleculeincreasestheaffinityforthesecondO2molecule,andsoforth.50TheaffinityforthefourthO2moleculeisthehighest.ThischangeinaffinityfacilitatestheloadingofO2inthelungs(flatportionofthecurve)andtheunloadingofO2atthetissues(steepportionofthecurve).51

InthelungsAlveolargashasaPO2of100mmHg.Pulmonarycapillarybloodis"arterialized"bythediffusionofO2fromalveolargasintoblood,sothatthePO2ofpulmonarycapillarybloodalsobecomes100mmHg.52ThecurveisalmostflatwhenthePO2isbetween60mmHgand100mmHg.Thus,humanscantoleratechangesinatmosphericpressure(andPO2)withoutcompromiseoftheO2-carryjngcapacityofhemoglobin.53IntheperipheraltissuesO2diffusesfromarterialbloodtothecells.ThegradientforO2diffusionismaintainedbecausethecellsconsumeO2foraerobicmetabolism,keepingthetissuePO2low.Theloweraffinity-ofhemoglobinforO2inthissteepportionofthecurvefacilitatestheunloadingofO2tothetissues.544、影響氧離曲線的因素通常用P50表示Hb對O2的親和力。P50是使Hb氧飽和度達50%時的PO2。P50增大，表明Hb對O2的親和力降低，曲線右移；P50降低，說明Hb對O2的親和力增加，曲線左移。5556（1）PCO2和pH的影響pH降低或PCO2升高，P50增大，曲線右移，促使O2的釋放；反之相反。影響因素：5758波爾效應(yīng)（Bohreffect)：當(dāng)酸度增大時，Hb對氧的親和力下降。當(dāng)酸度減小時相反。59生理意義可促進肺毛細血管血液的氧合（因為CO2從血進入肺泡，酸度下降）。又有利于組織毛細血管血液氧氣的釋放（因為CO2從組織入血，酸度上升）。60（2）溫度的影響

溫度升高，氧離曲線右移，促使O2的釋放；溫度降低，曲線左移，不利于O2釋放。61626364（3）2、3-二磷酸甘油酸（2、3-DPG)2,3-DPG濃度升高，氧離曲線右移：2,3-DPG濃度升降低，曲線左移。65AnumberoffactorscandisplacethedissociationcurveinonedirectionortheotherinthemannershowninFigurebelow.Thisfigureshowsthatwhenthebloodbecomesslightlyacidic,withthepHdecreasingfromthenormalvalueof7.4to7.2,theoxygen-hemoglobindissociationcurveshifts,onaverage,about15percenttotheright.66Conversely,anincreaseinthepHfromthenormal7.4to7.6shiftsthecurveasimilaramounttotheleft.67allofwhichshiftthecurvetotheright,are(1)increasedcarbondioxideconcentration,(2)increasedbloodtemperature,and(3)increased2,3-diphosphoglycerate(DPG),ametabolicallyimportantphosphatecompoundpresentinthebloodbutindifferentconcentrationsunderdifferentmetabolicconditions.68TheBohrEffect:IncreasesinPCO2orhydrogenions(decreasesinpH) shiftthecurvetotheright,decreasingtheaffinityofhemoglobinforO2andIncreasingDeliveryofOxygentotheTissuesO2).69Forexample,duringexercise,thetissuesproducemoreCO2,whichdecreasestissuepHand,throughtheBohreffect,stimulatesO2deliverytotheexercisingmuscle.70三、二氧化碳的運輸（一）物理溶解每100ml靜脈血，物理溶解的CO2約占總量的的5%。71（二）化學(xué)結(jié)合{碳酸氫鹽氨基甲酸血紅蛋白88%7%72Transportofcarbondioxidebythebloodisnotnearlysogreataproblemastransportofoxygen,becauseeveninthemostabnormalconditions,carbondioxidecanusuallybetransportedinfargreaterquantitiesthancanoxygen.73However,theamountofcarbondioxideintheblooddoeshavemuchtodowithacid-basebalanceofthebodyfluids.741.TransportofCarbonDioxideintheDissolvedStateTobegintheprocessofcarbondioxidetransport,carbondioxidediffusesoutofthetissuecellsinthedissolvedmolecularcarbondioxideform.Asmallportionofthecarbondioxideistransportedinthedissolvedstatetothelungs,onlyabout0.3milliliterofcarbondioxideistransportedintheformofdissolvedcarbondioxidebyeach100millilitersofblood.Thisisonlyabout7percentofallthecarbondioxidenormallytransported.75

碳酸氫鹽紅細胞內(nèi)碳酸氫鉀血漿內(nèi)碳酸氫鈉76TransportofCarbonDioxideintheFormofBicarbonateIonThedissolvedcarbondioxideinthebloodreactswithwatertoformcarbonicacid.Insidetheredbloodcarbonicanhydrase碳酸酐酶catalyzesthereactionbetweencarbondioxideandwater,thenthecarbonicacidformedintheredcells(H2CO3)dissociatesintohydrogenandbicarbonateions(H+andHCO3-).77Mostofthehydrogenionsthencombinewiththehemoglobinintheredbloodcells.Inturn,manyofthebicarbonateionsdiffusefromtheredcellsintotheplasmawhilechlorideionsdiffuseintotheredcellstotaketheirplace(thechlorideshift).78Thereversiblecombinationofcarbondioxidewithwaterintheredbloodcellsundertheinfluenceofcarbonicanhydraseaccountsforabout70percentofthecarbondioxidetransportedfromthetissuestothelungs.Thus,thismeansoftransportingcarbondioxideisbyfarthemostimportantofallthemethodsfortransport.792、HbNHCOOHHbNH2O2+H++CO2HbNHCOOH+O2肺組織80TransportofCarbonDioxideinCombinationwithHemoglobinandPlasmaProteins-Carbaminohemoglobin.Inadditiontoreactingwithwater,carbondioxidereactsdirectlywithamineradicalsofthehemoglobinmoleculetoformthecompoundcarbaminohemoglobin氨基甲酰血紅蛋白

(CO2Hgb).81Thiscombinationofcarbondioxidewiththehemoglobinisareversiblereactionthatoccurswithaloosebond,sothatthecarbondioxideiseasilyreleasedintothealveoli,wherethePCO2islowerthaninthetissuecapillaries.82Thiscarbaminohemoglobinmechanismprovidestransportofabout20percentofthetotalquantityofcarbondioxide.83（三）CO2的解離曲線（carbondioxidedissociationcurve）表示血液中CO2含量與PCO2關(guān)系的曲線，幾乎呈線形關(guān)系。8485ThecurveBELOWdepictsthisdependenceoftotalbloodcarbondioxideinallitsformsonPCO2;thiscurveiscalledthecarbondioxidedissociationcurve.86（四）Hb的結(jié)合對CO2運輸?shù)挠绊慜2與Hb結(jié)合將促使CO2釋放，這一效應(yīng)稱為

何爾登效應(yīng)（Haldaneeffect)87TheHaldaneEffect:WhenOxygenBindswithHemoglobin,CarbonDioxideisReleasedtoIncreaseCO2Transport88Earlierinthechapter,itwaspointedoutthatanincreaseincarbondioxideinthebloodcausesoxygentobedisplacedfromthehemoglobin(theBohreffect)andthatthisisanimportantfactorinincreasingoxygentransport.89Thereverseisalsotrue:bindingofoxygenwithhemoglobintendstodisplacecarbondioxidefromtheblood.Indeed,thiseffect,calledtheHaldaneeffect,isquantitativelyfarmoreimportantinpromotingcarbondioxidetransportthanistheBohreffectinpromotingoxygentransport.90第四節(jié)呼吸運動的調(diào)節(jié)91一.呼吸中樞與呼吸節(jié)律的形成（一）呼吸中樞921、脊髓脊髓中支配呼吸肌的運動神經(jīng)元位于第3-5頸段（支配膈?。┖托囟危ㄖ淅唛g肌和腹肌等）前角。932、下位腦干

延髓和腦橋呼吸節(jié)律產(chǎn)生于低(下)位腦干。9495腦橋上部有呼吸調(diào)整中樞：相對集中于臂旁內(nèi)側(cè)核（NPBM)和KF核（合稱PBKF核群）。其主要的作用是限制吸氣，促使吸氣轉(zhuǎn)為呼氣。96關(guān)于長吸中樞（腦橋中下部）目前趨于否定。973、高位腦腦橋以上部位特別是大腦皮層大腦皮層對呼吸的調(diào)節(jié)系統(tǒng)是通過軀體運動神經(jīng)。。下位腦干的呼吸調(diào)節(jié)系統(tǒng)是自主節(jié)律呼吸調(diào)節(jié)系統(tǒng).它們的下行通路是分開的。98Itisdividedintothreemajorcollectionsofneurons:1．Adorsalrespiratorygroup,locatedinthedorsalportionofthemedulla,whichmainlycausesinspiration;992.Aventralrespiratorygroup,locatedintheventrolateralpartofthemedulla,whichcancauseeitherexpirationorinspiration,dependingonwhichneuronsinthegrouparestimulated;1003Thepneumotaxiccenter呼吸調(diào)整中樞,locateddorsallyinthesuperiorportionofthepons,whichhelpscontroltherateandpatternofbreathing.Thedorsalrespiratorygroupofneuronsplaysthemostfundamentalroleinthecontrolofrespiration.101

NeuralControlofBreathingThenervoussystemnormallyadjuststherateofalveolarventilationalmostexactlytothedemandsofthebodysothattheoxygenpressure(PO2)andcarbondioxidepressure(PCO2)inthearterialbloodarehardlyalteredevenduringexerciseandmostothertypesofrespiratorystress.102CentralcontrolofbreathingRespiratoryCenterTherespiratorycenteriscomposedofseveralgroupsofneuronslocatedbilaterallyinthemedullaoblongataandpons,asshownforonesideinFigurebelow.103Cerebralcortex Breathingcanbeundervoluntarycontrol;therefore,apersoncanvoluntarilyhyperventilateorhypoventilate.104二、呼吸的反射性調(diào)節(jié)肺牽張反射（黑-伯反射）(Hering-Breuerreflex)1051、肺擴張反射是肺充氣或肺擴張引起的吸氣抑制。感受器位于氣管到支氣管的平滑肌中。

肺擴張肺牽張感受器興奮————呼吸中樞吸氣停止，引起呼氣。迷走神經(jīng)106意義：防止吸氣過深，促進吸氣向呼氣轉(zhuǎn)化，調(diào)節(jié)呼吸頻率和深度。1072、肺縮小反射肺縮小或肺萎陷引起的吸氣反射。肺縮小肺泡及細支氣管牽張感受器迷走神經(jīng)呼吸中樞呼氣停止，轉(zhuǎn)為吸氣108TheHering-BreuerInflationReflexStretchreceptorsarelocatedinthemuscularportionsofthewallsofthebronchiandbronchiolesthroughoutthelungs.theytransmitsensorynervesignalsthroughthevagiintothedorsalrespiratorygroupofneuronswhenthelungsbecomeoverstretched.109Thesesignalsaf-fectinspirationinmuchthesamewayassignalsfromthepneumotaxiccenter;thatis,whenthelungsbecomeoverlyinflated,thestretchreceptorsactivateanappropriatefeedbackresponsethat"switchesoff"theinspiratoryprocessandthusstopsfurtherinspiration.ThisiscalledtheHering-Breuerinflationreflex.Thisreflexalsoincreasestherateofrespiration,thesameasistrueforsignalsfromthepneumotaxiccenter.110三、呼吸的化學(xué)反射性調(diào)節(jié)（一）化學(xué)感受器1、外周化學(xué)感受器

111頸動脈體和主動脈體：對PO2降低，PCO2升高及pH下降敏感,受到刺激，反射性地引起呼吸加深加快。112

2、中樞化學(xué)感受器位于延髓腹外側(cè)，其有效刺激是腦脊液和局部細胞外液的H+，不感受局部CO2血氫離子和缺O(jiān)2的刺激。113（二）CO2對呼吸的影響

一定水平的PCO2對維持呼吸和呼吸中樞的興奮性是必要的，CO2是調(diào)節(jié)呼吸的最重要的生理性體液因子。114吸入氣CO24%肺通氣量增加1倍吸入氣CO27%CO2

麻醉現(xiàn)象115CO2刺激呼吸是通過兩條途徑實現(xiàn)的，一是通過刺激中樞化學(xué)感受器,二是刺激外周化學(xué)感受器反射性地使呼吸加深、加快。但兩條途徑中前者是主要的。116血液中的CO2能迅速通過血腦屏障，使化學(xué)感受器周圍液體中的[H+]升高，從而刺激中樞化學(xué)感受器，再引起呼吸中樞的興奮。117PCO2{外周化學(xué)感受器（20%）中樞化學(xué)感受器（80%）呼吸中樞呼吸加深，加快118119H+對呼吸的調(diào)節(jié)也是通過外周化學(xué)感受器和中樞化學(xué)感受器實現(xiàn)的,以前者為主。H+通過血液屏障的速度慢，限制了它對中樞化學(xué)感受器的作用。（三）H+對呼吸的影響120pH外周化學(xué)感受器呼吸中樞呼吸加深，加快121低O2對呼吸的刺激作用完全是通過外周化學(xué)感受器實現(xiàn)的。122當(dāng)缺氧不甚嚴重時，低O2對中樞的直接作用是壓抑。低O2可以通過對外周化學(xué)感受器的刺激而興奮呼吸中樞以對抗低O2的直接壓抑作用。123不過在嚴重低O2時，外周化學(xué)感受性反射已不足以克服低O2對中樞的壓抑作用，終將導(dǎo)致呼吸障礙。124（四）低O2對呼吸的影響PO2外周化學(xué)感受器呼吸中樞+-當(dāng)40mmHg≤PO2≤80mmHg呼吸加深加快

當(dāng)PO2≤40mmHg呼吸抑制125CHEMICALCONTROLOFRESPIRATIONTheultimategoalofrespirationistomaintainproperconcentrationsofoxygen,carbondioxide,andhydrogenionsinthetissues.Itisfortunate,therefore,thatrespiratoryactivityishighlyresponsivetochangesineachofthese.126Excesscarbondioxideorexcesshydrogenionsinthebloodmainlyactdirectlyontherespiratorycenteritself,causinggreatlyincreasedstrengthofboththeinspiratoryandtheexpiratorymotorsignalstotherespiratorymuscles.127Oxygen,incontrast,doesnothaveasignificantdirecteffectontherespiratorycenterofthebrainincontrollingrespiration.Instead,itactsalmostentirelyonperipheralchemoreceptorslocatedinthecarotidandaorticbodies,andtheseinturntransmitappropriatenervoussignalstotherespiratorycenterforcontrolofrespiration.128

ChemoreceptorsforCO2,H+,andO21.Centralchemoreceptorsinthemedullaasensitivech