Tissues,Organs,andOrganSystem

Lesson

Keypoints• Humans—andothercomplexmulticellularorganisms—havesystemsoforgansthatworktogether,carryingoutprocessesthatkeepusalive.

• Thebodyhaslevelsoforganizationthatbuildoneachother.Cellsmakeuptissues,tissuesmakeuporgans,andorgansmakeuporgansystems.

• Thefunctionofanorgansystemdependsontheintegratedactivityofitsorgans.Forinstance,digestivesystemorganscooperatetoprocessfood.

• Thesurvivaloftheorganismdependsontheintegratedactivityofalltheorgansystems,oftencoordinatedbytheendocrineandnervoussystems.

IntroductionIfyouwereasingle-celledorganismandyoulivedinanutrient-richplace,stayingalivewouldbeprettystraightforward.Forinstance,ifyouwereanamoebalivinginapond,youcouldabsorbnutrientsstraightfromyourenvironment.Theoxygenyouwouldneedformetabolismcoulddiffuseinacrossyourcellmembrane,andcarbondioxideandotherwastescoulddiffuseout.Whenthetimecametoreproduce,youcouldjustdivideyourselfintwo!However,oddsareyouarenotanamoeba—giventhatyou'reusingKhanAcademyrightnow—andthingsaren’tquitesosimpleforbig,many-celledorganismslikehumanbeings.Yourcomplexbodyhasover30trillioncells,andmostofthosecellsaren’tindirectcontactwiththe

externalenvironment.^11startsuperscript,1,endsuperscriptAcelldeepinsideyourbody—inoneofyourbones,say,orinyourliver—can’tgetthenutrientsoroxygenitneedsdirectlyfromtheenvironment.How,then,doesthebodynourishitscellsandkeepitselfrunning?Let'stakeacloserlookathowtheorganizationofyouramazingbodymakesthispossible.

MulticellularorganismsneedspecializedsystemsMostcellsinlargemulticellularorganismsdon'tdirectlyexchangesubstanceslikenutrientsandwasteswiththeexternalenvironment,instead,theyaresurroundedbyaninternalenvironmentofextracellularfluid—literally,fluidoutsideofcells.Thecellsgetoxygenandnutrientsfromthisextracellularfluidandreleasewasteproductsintoit.Humansandothercomplexorganismshavespecializedsystemsthatmaintaintheinternalenvironment,keepingitsteadyandabletoprovidefortheneedsofthecells.Differentsystemsofthebodycarryoutdifferentfunctions.Forexample,yourdigestivesystemisresponsiblefortakinginandprocessingfood,whileyourrespiratorysystem—workingwithyourcirculatorysystem—isresponsiblefortakingupoxygenandgettingridofcarbondioxide.Themuscularandskeletalsystemsarecrucialformovement;thereproductivesystemhandlesreproduction;andtheexcretorysystemgetsridofmetabolicwaste.Becauseoftheirspecialization,thesedifferentsystemsaredependentoneachother.Thecellsthatmakeupthedigestive,muscular,skeletal,reproductive,andexcretorysystemsallneedoxygenfromthe

respiratorysystemtofunction,andthecellsoftherespiratorysystem—aswellasalltheothersystems—neednutrientsandmustgetridofmetabolicwastes.Allthesystemsofthebodyworktogethertokeepanorganismupandrunning.

OverviewofbodyorganizationAlllivingorganismsaremadeupofoneormorecells.Unicellularorganisms,likeamoebas,consistofonlyasinglecell.Multicellularorganisms,likepeople,aremadeupofmanycells.Cellsareconsideredthefundamentalunitsoflife.Thecellsincomplexmulticellularorganismslikepeopleareorganizedintotissues,groupsofsimilarcellsthatworktogetheronaspecifictask.Organsarestructuresmadeupoftwoormoretissuesorganizedtocarryoutaparticularfunction,andgroupsoforganswithrelatedfunctionsmakeupthedifferentorgansystems.

Fromlefttoright:singlemusclecell,multiplemusclecellstogetherformingmuscletissue,organmadeupofmuscletissue(bladder),and

organsystemmadeupofkidneys,ureter,bladderandurethra.Imagecredit:modifiedfromLevelsofstructuralorganizationofthehumanbodybyOpenStaxCollege,Anatomy&Physiology,CCBY4.0

Ateachleveloforganization—cells,tissues,organs,andorgansystems—structureiscloselyrelatedtofunction.Forinstance,thecellsinthesmallintestinethatabsorbnutrientslookverydifferentfromthemusclecells

neededforbodymovement.Thestructureoftheheartreflectsitsjobofpumpingbloodthroughoutthebody,whilethestructureofthelungsmaximizestheefficiencywithwhichtheycantakeupoxygenandreleasecarbondioxide.

TypesoftissuesAswesawabove,everyorganismadeupoftwoormoretissues,groupsofsimilarcellsthatworktogethertoperformaspecifictask.Humans—andotherlargemulticellularanimals—aremadeupoffourbasictissuetypes:epithelialtissue,connectivetissue,muscletissue,andnervoustissue.

Thefourtypesoftissuesareexemplifiedinnervoustissue,stratifiedsquamousepithelialtissue,cardiacmuscletissue,andconnectivetissue

insmallintestine.Imagecredit:modifiedfromTypesoftissues:Figure1byOpenStaxCollege,Anatomy&Physiology,CCBY3.0

Epithelialtissue

Epithelialtissueconsistsoftightlypackedsheetsofcellsthatcoversurfaces—includingtheoutsideofthebody—andlinebodycavities.Forinstance,theouterlayerofyourskinisanepithelialtissue,andsoistheliningofyoursmallintestine.Epithelialcellsarepolarized,meaningthattheyhaveatopandabottomside.Theapical,top,sideofanepithelialcellfacestheinsideofacavityortheoutsideofastructureandisusuallyexposedtofluidorair.Thebasal,bottom,sidefacestheunderlyingcells.Forinstance,theapicalsidesofintestinalcellshavefinger-likestructuresthatincreasesurfaceareaforabsorbingnutrients.

Imageshowingthreecellsliningthesmallintestine.Eachcellcontainsanucleusandissurroundedbyaplasmamembrane.Thetopsofthecells

havemicrovillithatfacethecavityfromwhichsubstanceswillbeabsorbed.

Imagecredit:Eukaryoticcells:Figure3byOpenStaxCollege,Biology,CCBY3.0

Epithelialcellsaretightlypacked,andthisletsthemactasbarrierstothemovementoffluidsandpotentiallyharmfulmicrobes.Often,thecellsarejoinedbyspecializedjunctionsthatholdthemtightlytogethertoreduceleaks.

Connectivetissue

Connectivetissueconsistsofcellssuspendedinanextracellularmatrix.Inmostcases,thematrixismadeupofproteinfiberslikecollagenandfibrininasolid,liquid,orjellylikegroundsubstance.Connectivetissuesupportsand,asthenamesuggests,connectsothertissues.Looseconnectivetissue,showbelow,isthemostcommontypeofconnectivetissue.It'sfoundthroughoutyourbody,anditsupportsorgansandbloodvesselsandlinksepithelialtissuestothemusclesunderneath.Dense,orfibrous,connectivetissueisfoundintendonsandligaments,whichconnectmusclestobonesandbonestoeachother,respectively.

Looseconnectivetissueiscomposedoflooselywovencollagenandelasticfibers.Thefibersandothercomponentsoftheconnectivetissue

matrixaresecretedbyfibroblasts.Imagecredit:Animalprimarytissues:Figure6byOpenStaxCollege,Biology,CCBY4.0

Specializedformsofconnectivetissueincludeadiposetissue—bodyfat—bone,cartilage,andblood,inwhichtheextracellularmatrixisaliquidcalledplasma.

Muscletissue

Muscletissueisessentialforkeepingthebodyupright,allowingittomove,andevenpumpingbloodandpushingfoodthroughthedigestivetract.Musclecells,oftencalledmusclefibers,containtheproteinsactinandmyosin,whichallowthemtocontract.Therearethreemaintypesofmuscle:skeletalmuscle,cardiacmuscle,andsmoothmuscle.

Fromlefttoright.Smoothmusclecells,skeletalmusclecells,andcardiacmusclecells.Smoothmusclecellsdonothavestriations,whileskeletalmusclecellsdo.Cardiacmusclecellshavestriations,but,unlikethe

multinucleateskeletalcells,theyhaveonlyonenucleus.Cardiacmuscletissuealsohasintercalateddiscs,specializedregionsrunningalongtheplasmamembranethatjoinadjacentcardiacmusclecellsandassistin

passinganelectricalimpulsefromcelltocell.Imagecredit:Animalprimarytissues:Figure12byOpenStaxCollege,Biology,CCBY4.0

Skeletalmuscle,whichisalsocalledstriated—striped—muscle,iswhatwerefertoasmuscleineverydaylife.Skeletalmuscleisattachedtobonesbytendons,anditallowsyoutoconsciouslycontrolyourmovements.Forinstance,thequadsinyourlegsorbicepsinyourarmsareskeletalmuscle.Cardiacmuscleisfoundonlyinthewallsoftheheart.Likeskeletalmuscle,cardiacmuscleisstriated,orstriped.Butit'snotundervoluntarycontrol,so—thankfully!—youdon’tneedtothinkaboutmakingyourheartbeat.Theindividualfibersareconnectedbystructurescalledintercalateddisks,whichallowthemtocontractinsync.

Smoothmuscleisfoundinthewallsofbloodvessels,aswellasinthewallsofthedigestivetract,theuterus,theurinarybladder,andvariousotherinternalstructures.Smoothmuscleisnotstriped,striated,andit'sinvoluntary,notunderconsciouscontrol.Thatmeansyoudon'thavetothinkaboutmovingfoodthroughyourdigestivetract!

Nervoustissue

Nervoustissueisinvolvedinsensingstimuli—externalorinternalcues—andprocessingandtransmittinginformation.Itconsistsoftwomaintypesofcells:neurons,ornervecells,andglia.Theneuronsarethebasicfunctionalunitofthenervoussystem.Theygenerateelectricalsignalscalledconductednerveimpulsesoractionpotentialsthatallowtheneuronstoconveyinformationveryrapidlyacrosslongdistances.Thegliamainlyacttosupportneuronalfunction.

Pictureofneuron.Theneuronhasprojectionscalleddendritesthatreceivesignalsandprojectionscalledaxonsthatsendsignals.Alsoshownaretwotypesofglialcells:astrocytesregulatethechemicalenvironmentofthenervecell,andoligodendrocytesinsulatetheaxonsotheelectrical

nerveimpulseistransferredmoreefficiently.Imagecredit:Animalprimarytissues:Figure13byOpenStaxCollege,Biology,CCBY4.0

OrgansOrgans,suchastheheart,thelungs,thestomach,thekidneys,theskin,andtheliver,aremadeupoftwoormoretypesoftissueorganizedtoserveaparticularfunction.Forexample,theheartpumpsblood,thelungsbringinoxygenandeliminatecarbondioxide,andtheskin

providesabarriertoprotectinternalstructuresfromtheexternalenvironment.Mostorganscontainallfourtissuetypes.Thelayeredwallsofthesmallintestineprovideagoodexampleofhowtissuesformanorgan.Theinsideoftheintestineislinedbyepithelialcells,someofwhichsecretehormonesordigestiveenzymesandothersofwhichabsorbnutrients.Aroundtheepitheliallayerarelayersofconnectivetissueandsmoothmuscle,interspersedwithglands,bloodvessels,andneurons.Thesmoothmusclecontractstomovefoodthroughthegut,undercontrolofitsassociatednetworksofneurons.^22startsuperscript,2,endsuperscript

Cross-sectionoftheGItract.Fromoutsidetoinside:Bloodvessels,networksofnervesinsmoothmusclelayers,connectivetissue,more

smoothmuscle,anotherlayerofconnectivetissue,epithelialtissue,andemptyspaceinthemiddleasthepathofdigestedfood.

Imagecredit:modifiedfromLayersoftheGItractbyGorantek-en,[CCBY-SA3.0](https://creativecommons.org/licenses/by-sa/3.0/deed.en;themodifiedimageislicensedunderaCCBY-SA3.0license

OrgansystemsOrgansaregroupedintoorgansystems,inwhichtheyworktogethertocarryoutaparticularfunctionfortheorganism.Forexample,theheartandthebloodvesselsmakeupthecardiovascularsystem.Theyworktogethertocirculatetheblood,bringingoxygenandnutrientstocellsthroughoutthebodyandcarryingawaycarbondioxideandmetabolicwastes.Anotherexampleistherespiratorysystem,whichbringsoxygenintothebodyandgetsridofcarbondioxide.Itincludesthenose,mouth,pharynx,larynx,trachea,andlungs.

Twodiagrams.Ontheleft,adiagramoftherespiratorysystemshowingnasalpassages,trachea,andlungs.Ontheright,adiagramofthe

circulatorysystemshowingheartandbloodvessels.

Imagecredit:Structuralorganizationofthehumanbody:Figures2and3byOpenStaxCollege,Anatomy&Physiology,CCBY4.0

Majororgansystemsofthehumanbody

Organsystem FunctionOrgans,tissues,andstructuresinvolved

Cardiovascular

Transportsoxygen,nutrients,andothersubstancestothecellsandtransportswastes,carbondioxide,andothersubstancesawayfromthecells;itcanalsohelpstabilizebodytemperatureandpH

Heart,blood,andbloodvessels

Lymphatic

Defendsagainstinfectionanddiseaseandtransferslymphbetweentissuesandthebloodstream

Lymph,lymphnodes,andlymphvessels

Digestive

Processesfoodsandabsorbsnutrients,minerals,vitamins,andwater

Mouth,salivaryglands,esophagus,stomach,liver,gallbladder,exocrinepancreas,smallintestine,andlargeintestine

Organsystem FunctionOrgans,tissues,andstructuresinvolved

Endocrine

Providescommunicationwithinthebodyviahormonesanddirectslong-termchangeinotherorgansystemstomaintainhomeostasis

Pituitary,pineal,thyroid,parathyroids,endocrinepancreas,adrenals,testes,andovaries.

Integumentary

Providesprotectionfrominjuryandfluidlossandprovidesphysicaldefenseagainstinfectionbymicroorganisms;involvedintemperaturecontrol Skin,hair,andnails

MuscularProvidesmovement,support,andheatproduction

Skeletal,cardiac,andsmoothmuscles

Nervous

Collects,transfers,andprocessesinformationanddirectsshort-termchangeinotherorgansystems

Brain,spinalcord,nerves,andsensoryorgans—eyes,ears,tongue,skin,andnose

Reproductive

Producesgametes—sexcells—andsexhormones;ultimatelyproducesoffspring

Fallopiantubes,uterus,vagina,ovaries,mammaryglands(female),testes,vasdeferens,

Organsystem FunctionOrgans,tissues,andstructuresinvolved

seminalvesicles,prostate,andpenis(male)

RespiratoryDeliversairtositeswheregasexchangecanoccur

Mouth,nose,pharynx,larynx,trachea,bronchi,lungs,anddiaphragm

Skeletal

Supportsandprotectssofttissuesofthebody;providesmovementatjoints;producesbloodcells;andstoresminerals

Bones,cartilage,joints,tendons,andligaments

Urinary

Removesexcesswater,salts,andwasteproductsfromthebloodandbodyandcontrolspH

Kidneys,ureters,urinarybladder,andurethra

Immune

Defendsagainstmicrobialpathogens—disease-causingagents—andotherdiseases

Leukocytes,tonsils,adenoids,thymus,andspleen

ThistableismodifiedfromMajororgansystemsofthehumanbodybyCK-12Foundation,CCBY-NC3.0.Althoughweoftentalkaboutthedifferentorgansystemsasthoughtheyweredistinct,partsofonesystemmayplayaroleinanothersystem.The

mouth,forinstance,belongstoboththerespiratorysystemandthedigestivesystem.There'salsoalotoffunctionaloverlapamongthedifferentsystems.Forinstance,whilewetendtothinkofthecardiovascularsystemasdeliveringoxygenandnutrientstocells,italsoplaysaroleinmaintainingtemperature.Thebloodalsotransportshormonesproducedbytheglandsoftheendocrinesystem,andwhitebloodcellsareakeycomponentoftheimmunesystem.

Organsinasystemworktogether.Justlikeworkersonanassemblyline,theorgansofanorgansystemmustworktogetherforthesystemtofunctionasawhole.Forinstance,thefunctionofthedigestivesystem—takinginfood,breakingitdownintomoleculessmallenoughtobeabsorbed,absorbingit,andeliminatingundigestedwasteproducts—dependsoneachsuccessiveorgandoingitsindividualjob.^{3,4}3,4startsuperscript,3,comma,4,endsuperscriptDigestionisthebreakdownoffoodsothatitsnutrientscanbeabsorbed.Itincludesbothmechanicaldigestionandchemicaldigestion.Inmechanicaldigestion,chunksoffoodarebrokenintosmallerpieces.Inchemicaldigestion,largemoleculeslikeproteinsandstarchesarebrokenintosimplerunitsthatcanbereadilyabsorbed.Mechanicaldigestion,alongwithsomeinitialchemicaldigestion,takesplaceinthemouthandstomach.Chewingbreaksfoodintosmallerpieces,andthestomachchurnsthefoodupintoafluidmixture.Thestomachalsoactsasastoragetank,releasingpartiallydigestedfoodinto

thesmallintestineataratethesmallintestinecanhandle.^44startsuperscript,4,endsuperscript

Digestivesystem.Startsatmouth,whichconnectstostomach.Theliverandpancreasareadjacenttothestomach,whichleadstothesmall

intestineandthenthelargeintestine.Imagecredit:modifiedfromDigestivesystems:Figure5byOpenStaxCollege,Biology,CCBY4.0

Thesmallintestineisthemajorsiteofchemicaldigestion,whichiscarriedoutbyenzymesreleasedfromthepancreasandliver.Thesmallintestineisalsothemainsiteofnutrientabsorption;moleculeslike

sugarsandaminoacidsaretakenupbycellsandtransportedintothebloodstreamforuse.Themouth,stomach,smallintestine,andotherdigestivesystemorgansworktogethertomakedigestingfoodandabsorbingnutrientsefficient.Digestionwouldn’tsoworkwellifyourstomachstoppedchurningorifoneofyourenzyme-producingglands—likethepancreas—decidedtotakethedayoff!

Organsystemsworktogether,too.Justastheorgansinanorgansystemworktogethertoaccomplishtheirtask,sothedifferentorgansystemsalsocooperatetokeepthebodyrunning.Forexample,therespiratorysystemandthecirculatorysystemworkcloselytogethertodeliveroxygentocellsandtogetridofthecarbondioxidethecellsproduce.Thecirculatorysystempicksupoxygeninthelungsanddropsitoffinthetissues,thenperformsthereverseserviceforcarbondioxide.Thelungsexpelthecarbondioxideandbringinnewoxygen-containingair.Onlywhenbothsystemsareworkingtogethercanoxygenandcarbondioxidebesuccessfullyexchangedbetweencellsandenvironment.Therearemanyotherexamplesofthiscooperationinyourbody.Forinstance,thebloodinyourcirculatorysystemhastoreceivenutrientsfromyourdigestivesystemandundergofiltrationinyourkidneys,oritwouldn'tbeabletosustainthecellsofyourbodyandremovethewastestheyproduce.

Controlandcoordination

Manybodyfunctionsarecontrolledbythenervoussystemandtheendocrinesystem.Thesetworegulatorysystemsusechemicalmessengerstoaffectthefunctionoftheotherorgansystemsandtocoordinateactivityatdifferentlocationsinthebody.Howdotheendocrineandnervoussystemsdiffer?

• Intheendocrinesystem,thechemicalmessengersarehormonesreleasedintotheblood.

• Inthenervoussystem,thechemicalmessengersareneurotransmitterssentstraightfromonecelltoanotheracrossatinygap.Sincehormoneshavetotravelthroughthebloodstreamtotheirtargets,theendocrinesystemusuallycoordinatesprocessesonaslowertimescalethanthenervoussysteminwhichmessagesaredelivereddirectlytothetargetcell.Insomecases,suchasthefight-or-flightresponsetoanacutethreat,thenervousandendocrinesystemsworktogethertoproducearesponse.