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1、CirculatorySystem循环系统CirculatorySystem,orcardiovascularsystem,inhumans,thecombinedfunctionoftheheart,blood,andbloodvesselstotransportoxygenandnutrientstoorgansandtissuesthroughoutthebodyandcarryawaywasteproducts.Amongitsvitalfunctions,thecirculatorysysteminc
2、reasestheflowofbloodtomeetincreasedenergydemandsduringexerciseandregulatesbodytemperature.Inaddition,whenforeignsubstancesororganismsinvadethebody,thecirculatorysystemswiftlyconveysdisease-fightingelementsoftheimmunesystem,suchaswhitebloodcellsandantibodies,
3、toregionsunderattack.Also,inthecaseofinjuryorbleeding,thecirculatorysystemsendsclottingcellsandproteinstotheaffectedsite,whichquicklystopbleedingandpromotehealing.Thehumancirculatorysystemiscomposedofthemuscularheartandanintricatenetworkofelasticbloodvessels
4、knownasarteries,veins,andcapillaries.Thesestructuresworktogethertocirculatebloodthroughoutthebody,intheprocessdeliveringlife-preservingoxygenandnutrientstotissuecellswhilealsoremovingwasteproducts.Theheart,blood,andbloodvesselsarethethreestructuralelementsth
5、atmakeupthecirculatorysystem.Theheartistheengineofthecirculatorysystem.Itisdividedintofourchambers:therightatrium,therightventricle,theleftatrium,andtheleftventricle.Thewallsofthesechambersaremadeofaspecialmusclecalledmyocardium,whichcontractscontinuouslyand
6、rhythmicallytopumpblood.Thepumpingactionoftheheartoccursintwostagesforeachheartbeat:diastole,whentheheartisatrest;andsystole,whentheheartcontractstopumpdeoxygenatedbloodtowardthelungsandoxygenatedbloodtothebody.Duringeachheartbeat,typicallyabout60to90ml(abou
7、t2to3oz)ofbloodarepumpedoutoftheheart.Iftheheartstopspumping,deathusuallyoccurswithinfourtofiveminutes.Bloodconsistsofthreetypesofcells:oxygen-bearingredbloodcells,disease-fightingwhitebloodcells,andblood-clottingplatelets,allofwhicharecarriedthroughbloodves
8、selsinaliquidcalledplasma.Plasmaisyellowishandconsistsofwater,salts,proteins,vitamins,minerals,hormones,dissolvedgases,andfats.Threetypesofbloodvesselsformacomplexnetworkoftubesthroughoutthebody
