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Undercertaincircumstances,thehumanbodymustcopewithgasesatgreater-than-normalatmosphericpressure.Forexample,gaspressuresincreaserapidlyduringadivemadewithscubagearbecausethebreathingequipmentallowsdiverstostayunderwaterlongeranddivedeeper.
Thepressureexertedonthehumanbodyincreasesby1atmosphereforevery10metersofdepthinseawater,sothatat30metersinseawateradiverisexposedtoapressureofabout4atmospheres.Thepressureofthegasesbeingbreathedmustequaltheexternalpressureappliedtothebody,otherwisebreathingisverydifficult.Thereforeallofthegasesintheairbreathedbyascubadiverat40metersarepresentatfivetimestheirusualpressure.Nitr ogen,whichcomposes80percentoftheairwebreathe,usuallycausesabalmyfeelingofwell-beingatthispressure.Atadepthof5atmospheres,nitr ogencausessymptomsresemblingalcoholintoxication,knownasnitr ogennarcosis.Nitr ogennarcosisapparentlyresultsfromadirecteffectonthebrainofthelargeamountsofnitr ogencauseunderthesepressurizedheliumdoesnotexertasimilarnarcoticeffect.
Asascubadiverdescends,thepressureofnitr ogeninthelungsincreases.Nitr ogenthendiffusesfromthelungstotheblood,andfromthebloodtobodytissues.Thereverseoccurswhenthediversurfaces;thenitr ogenpressureinthelungsfallsandthenitr ogendiffusesfromthetissuesintotheblood,andfromthebloodintothelungs.Ifthereturntothesurfaceistoorapid,nitr ogeninthetissuesandbloodcannotd iffuseoutr apidlyenoughandnitr ogenbubblesareformed.Theycancauseseverepains,particularlyaroundthejoints.
Anothercomplicationmayresultifthebreathisheldduringascent.Duringascentfromadepthof10meters,thevolumeofairinthelungswilldoublebecausetheairpressureatthesurfaceisonlyhalfofwhatitwasat10meters.Thischangeinvolumemaycausethelungstodistendandevenrupture.Thisconditioniscalledairembolism.Toavoidthisevent,adivermustascendslowly,neveratarateexceedingtheriseoftheexhaledairbubbles,andmustexhaleduringas-cent.
31.Whatd oesthepassagemainlydiscuss?
A)Theequipmentd iversuse.
B)Theeffectsofpressureongasesinthehumanbody.
C)Howtoprepareforadeepdive.
D)Thesymptomsofnitr ogenbubblesinthebloodstr eam.
正确答案：B答案：B
[试题分析]文章主旨题。
[详细解答]文章第一段提出话题，接下来通过原理结合实际来讲述压力的改变对身体的影响。还详细介绍了潜水过程中，下潜和上升过程中会出现的情况，因此，答案为B较为合理。
