Chapter 37 - Barometric Pressure, Reduced.docxVIP专享VIP免费原创优质

Chapter37-BarometricPressure,ReducedVENTILATORYACCLIMATIZATIONTOHIGHALTITUDEJohnT.ReevesandJohnV.WeilPeopleareincreasinglyworkingathighaltitudes.Miningoperations,recreationalfacilities,modesoftransportation,agriculturalpursuitsandmilitarycampaignsareoftenathighaltitude,andalloftheserequirehumanphysicalandmentalactivity.Allsuchactivityinvolvesincreasedrequirementsforoxygen.Aproblemisthatasoneascendshigherandhigherabovesealevel,boththetotalairpressure(thebarometricpressure,PB)andtheamountofoxygenintheambientair(thatportionoftotalpressureduetooxygen,PO2)progressivelyfall.Asaresult,theamountofworkwecanaccomplishprogressivelydecreases.Theseprinciplesaffecttheworkplace.Forexample,atunnelinColoradowasfoundtorequire25%moretimetocompleteatanaltitudeof11,000ftthancomparableworkatsealevel,andaltitudeeffectswereimplicatedinthedelay.Notonlyisthereincreasedmuscularfatigue,butalso,deteriorationofmentalfunction.Memory,computation,decisionmakingandjudgementallbecomeimpaired.ScientistsdoingcalculationsattheMonaLoaObservatoryatanaltitudeabove4,000montheislandofHawaiihavefoundtheyrequiremoretimetoperformtheircalculationsandtheymakemoremistakesthanatsealevel.Becauseoftheincreasingscope,magnitude,varietyanddistributionofhumanactivitiesonthisplanet,morepeopleareworkingathighaltitude,andeffectsofaltitudebecomeanoccupationalissue.Fundamentallyimportanttooccupationalperformanceataltitudeismaintainingtheoxygensupplytothetissues.We(andotheranimals)havedefencesagainstlowoxygenstates(hypoxia).Chiefamongtheseisanincreaseinbreathing(ventilation),whichbeginswhentheoxygenpressureinthearterialblood(PaO2)decreases(hypoxemia),ispresentforallaltitudesabovesealevel,isprogressivewithaltitudeandisourmosteffectivedefenceagainstlowoxygenintheenvironment.Theprocesswherebybreathingincreasesathighaltitudeiscalledventilatoryacclimatization.Theimportanceoftheprocesscanbeseeninfigure37.1,whichshowsthattheoxygenpressureinthearterialbloodishigherinacclimatizedsubjectsthaninunacclimatizedsubjects.Further,theimportanceofacclimatizationinmaintainingthearterialoxygenpressureincreasesprogressivelywithincreasingaltitude.Indeed,theunacclimatizedpersonisunlikelytosurviveaboveanaltitudeof20,000ft,whereasacclimatizedpersonshavebeenabletoclimbtothesummitofMountEverest(29,029ft,8,848m)withoutartificialsourcesofoxygen.Figure37.1VentilatoryacclimatizationMechanismThestimulusfortheincreaseinventilationathighaltitudelargelyandalmostexclusivelyarisesinatissuewhichmonitorstheoxygenpressureinthearterialbloodandiscontainedwithinanorgancalledthecarotidbody,aboutthesizeofapinhead,locatedatabranchpointineachofthetwocarotidarteries,attheleveloftheangleofthejaw.Whenthearterialoxygenpressurefalls,nerve-likecells(chemoreceptorcells)inthecarotidbodysensethisdecreaseandincreasetheirfiringratealongthe9thcranialnerve,whichcarriestheimpulsesdirectlytotherespiratorycontrolcentreinthebrainstem.Whentherespiratorycentrereceivesincreasednumbersofimpulses,itstimulatesanincreaseinthefrequencyanddepthofbreathingviacomplexnervepathways,whichactivatethediaphragmandthemusclesofthechestwall.Theresultisanincreasedamountofairventilatedbythelungs,figure37.2whichinturnactstorestorethearterialoxygenpressure.Ifasubjectbreathesoxygenorairenrichedwithoxygen,thereversehappens.Figure37.2SequenceofeventsinacclimatizationThatis,thechemoreceptorcellsdecreasetheirfiringrate,wh...