哈喽朋友们迟到的五一小长假快乐(*^▽^*)

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OG18的-

Interrestrialenvironments,gravityplacesspecialdemandsonthecardiovascularsystemsofanimals.Gravitationalpressurecancausebloodtopoolinthelowerregionsofthebody,makingitdifficulttocirculatebloodtocriticalorganssuchasthebrain.Terrestrialsnakes,inparticular,exhibitadaptationsthataidincirculatingbloodagainsttheforceofgravity.

Theproblemconfrontingterrestrialsnakesisbestillustratedbywhathappenstoseasnakeswhenremovedfromtheirsupportivemedium.Becausetheverticalpressuregradientswithinthebloodvesselsarecounteractedbysimilarpressuregradientsinthesurroundingwater,thedistributionofbloodthroughoutthebodyofseasnakesremainsaboutthesameregardlessoftheirorientationinspace,providedtheyremainintheocean.Whenremovedfromthewaterandtiltedatvariousangleswiththeheadup,however,bloodpressureattheirmidpointdropssignificantly,andatbrainlevelfallstozero.Thatmanyterrestrialsnakesinsimilarspatialorientationsdonotexperiencethiskindofcirculatoryfailuresuggeststhatcertainadaptationsenablethemtoregulatebloodpressuremoreeffectivelyinthoseorientations.

Onesuchadaptationisthecloserproximityoftheterrestrialsnake’shearttoitshead,whichhelpstoensurecirculationtothebrain,regardlessofthesnake’sorientationinspace.Theheartofseasnakescanbelocatednearthemiddleofthebody,apositionthatminimizestheworkentailedincirculatingbloodtobothextremities.Inarborealsnakes,however,whichdwellintreesandoftenassumeaverticalposture,theaveragedistancefromthehearttotheheadcanbeaslittleas15percentofoverallbodylength.Suchalocationrequiresthatbloodcirculatedtothetailofthesnaketravelagreaterdistancebacktotheheart,aproblemsolvedbyanotheradaptation.Whenclimbing,arborealsnakesoftenpausemomentarilytowiggletheirbodies,causingwavesofmusclecontractionthatadvancefromthelowertorsotothehead.By







































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