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GMAT–RC结论说明类2AmyHou
OG上结论说明文章范例文章精讲结论说明文章特征
LineInterrestrialenvironments,gravityplaces
specialdemandsonthecardiovascularsystemsof
animals.Gravitationalpressurecancausebloodto
poolinthelowerregionsofthebody,makingit(5)difficulttocirculatebloodtocriticalorganssuchas
thebrain.Terrestrialsnakes,inparticular,exhibit
adaptationsthataidincirculatingbloodagainstthe
forceofgravity.
Theproblemconfrontingterrestrialsnakesisbest(10)illustratedbywhathappenstoseasnakeswhenremovedfromtheirsupportivemedium.Becausethe
verticalpressuregradientswithinthebloodvessels
arecounteractedbysimilarpressuregradientsinthe
surroundingwater,thedistributionofblood(15)throughoutthebodyofseasnakesremainsabout
thesameregardlessoftheirorientationinspace,
providedtheyremainintheocean.Whenremoved
fromthewaterandtiltedatvariousangleswiththeOG
headup,however,bloodpressureattheirmidpoint(20)dropssignificantly,andatbrainlevelfallstozero.
Thatmanyterrestrialsnakesinsimilarspatial
orientationsdonotexperiencethiskindofcirculatory
failuresuggeststhatcertainadaptationsenablethem
toregulatebloodpressuremoreeffectivelyinthose(25)orientations.Onesuchadaptationisthecloserproximityofthe
terrestrialsnakeshearttoitshead,whichhelpsto
ensurecirculationtothebrain,regardlessofthe
snakesorientationinspace.Theheartofseasnakes(30)canbelocatednearthemiddleofthebody,a
positionthatminimizestheworkentailedin
circulatingbloodtobothextremities.Inarboreal
snakes,however,whichdwellintreesandoften
assumeaverticalposture,theaveragedistance(35)fromthehearttotheheadcanbeaslittleas15OG
percentofoverallbodylength.Suchalocation
requiresthatbloodcirculatedtothetailofthe
snaketravelagreaterdistancebacktotheheart,
aproblemsolvedbyanotheradaptation.When(40
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