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1、CirculatorySystem循环系统CirculatorySystem,orcardiovascularsystem,inhumans,thecombinedfunctionoftheheart,blood,andbloodvesselstotransportoxygenandnutrientstoorgansandtissuesthroughoutthebodyandcarryawaywasteproducts.Amongitsvitalfunctions,thecirculatorysyste
2、mincreasestheflowofbloodtomeetincreasedenergydemandsduringexerciseandregulatesbodytemperature.Inaddition,whenforeignsubstancesororganismsinvadethebody,thecirculatorysystemswiftlyconveysdisease-fightingelementsoftheimmunesystem,suchaswhitebloodcellsandant
3、ibodies,toregionsunderattack.Also,inthecaseofinjuryorbleeding,thecirculatorysystemsendsclottingcellsandproteinstotheaffectedsite,whichquicklystopbleedingandpromotehealing.Thehumancirculatorysystemiscomposedofthemuscularheartandanintricatenetworkofelastic
4、bloodvesselsknownasarteries,veins,andcapillaries.Thesestructuresworktogethertocirculatebloodthroughoutthebody,intheprocessdeliveringlife-preservingoxygenandnutrientstotissuecellswhilealsoremovingwasteproducts.Theheart,blood,andbloodvesselsarethethreestru
5、cturalelementsthatmakeupthecirculatorysystem.Theheartistheengineofthecirculatorysystem.Itisdividedintofourchambers:therightatrium,therightventricle,theleftatrium,andtheleftventricle.Thewallsofthesechambersaremadeofaspecialmusclecalledmyocardium,whichcont
6、ractscontinuouslyandrhythmicallytopumpblood.Thepumpingactionoftheheartoccursintwostagesforeachheartbeat:diastole,whentheheartisatrest;andsystole,whentheheartcontractstopumpdeoxygenatedbloodtowardthelungsandoxygenatedbloodtothebody.Duringeachheartbeat,typ
7、icallyabout60to90ml(about2to3oz)ofbloodarepumpedoutoftheheart.Iftheheartstopspumping,deathusuallyoccurswithinfourtofiveminutes.Bloodconsistsofthreetypesofcells:oxygen-bearingredbloodcells,disease-fightingwhitebloodcells,andblood-clottingplatelets,allofwh
8、icharecarriedthroughbloodvesselsinaliquidcalledplasma.Plasmaisyellowishandconsistsofwater,salts,proteins,vitamins,minerals,hormones,dissolvedgases,andfats.Threetypesofbloodvesselsformacomplexnetworkoftubesthroughoutthebody
