on line chapter 2 body structure.docx
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on line chapter 2 body structure.docx
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onlinechapter2bodystructure
Chapter2
BodyStructure
OrganizationoftheBody
Beforetakingalookatthewholehumanbody,weneedtoexamineitsparts.Thehumanorganismconsistsofcells,tissues,organs,andsystems.Thesecomponentsarearrangedinahierarchicalmanner.Thatis,cellscometogethertoformtissues,tissuescometogethertoformorgans,organscometogethertoformsystems,andallthesystemscometogethertoformthewholeorganism.
Cells
Anadulthumanbodyconsistsofaboutseventy-fivetrillioncells.Thesecellshavemuchincommon,yetthoseindifferenttissuesvaryinanumberofways.
Cellsvaryconsiderablyinsize.Wemeasurecellsizesinunitscalledmicrometers.Amicrometerequalsonethousandthofamillimeterandissymbolizedμm.Ahumaneggcellisabout140μmindiameterandisjustbarelyvisibletoanunaidedeye.Thisislargewhencomparedtoaredbloodcell,whichisabout7.5μmindiameter,orthemostcommonwhitebloodcells,whichvaryfrom10-12μmindiameter.Ontheotherhand,smoothmusclecellscanbebetween20and500μmlong.
Cellsalsovaryinshape,andtypicallytheirshapesarecloselyrelatedtotheirfunctions.Forinstance,nervecellsoftenhavelong,threadlikeextensionsthattransmitnerveimpulsefromonepartofthebodytoanother.Epithelialcellsthatlinetheinsideofthemoutharethin,flattened,andtightlypacked,somewhatlikefloortiles.Musclecells,whichcontracttopullstructuresclosertogether,areslenderandrod-like,withtheirendsattachedtothepartstheymove.Theyarefilledwithcontractileproteins.Anadiposecellislittlemorethanabloboffat.ABlymphocyteisanantibodyfactory.[fig.2.1]
Fig.2.1Nervecell,musclecell,redbloodcellsandspermcell
Acellconsistsoftwomajorparts—thenucleusandthecytoplasm.Thenucleusisinnermostandisenclosedbyathinmembranecalledthenuclearenvelope.Thecytoplasmisamassoffluidthatsurroundsthenucleusandisitselfencircledbyaneventhinnercellmembrane(alsocalledplasmamembrane).Withinthecytoplasmarespecializedstructurescalledcytoplasmicorganellesthatperformspecificmetabolicfunctions.Thenucleus,ontheotherhand,directstheoverallactivitiesothecell.Thenucleusisalsoconsideredtobeanorganelle.[fig.2.2]
Fig.2.2Structureofcell
Tissues
Inallcomplexorganisms,cellsareorganizedintolayersorgroupscalledtissues.Althoughthecellsofdifferenttissuesvaryinsize,shape,arrangement,andfunction,thosewithinatissuearequitesimilar.
Usuallytissuecellsareseparatedbynonliving,intercellularmaterialsthatthecellssecrete.Theseintercellularmaterialsvaryincompositionfromonetissuetoanotherandmaybesolid,semisolid,orliquid.Thetissuesofthehumanbodyincludefourmajortypes:
epithelialtissues,connectivetissues,muscletissues,andnervoustissues.Thesetissuesareorganizedintoorgansthathavespecializedfunctions.
Epithelialtissuesarewidespreadthroughoutthebody.Theycoverallbodysurfaces—insideandout—andaremajortissuesofglands.
Sinceepitheliumcoversorgans,formstheinnerliningofbodycavities,andlinesholloworgans,italwayshasafreesurface—onethatisexposedtotheoutsideortoanopenspaceinternally.Asarule,epithelialtissueslackbloodvessels.However,theyarenourishedbysubstancesthatdiffusefromunderlyingconnectivetissues,whicharewellsuppliedwithbloodvessels.[fig.2.3]
Fig.2.3Epitheliumtissue
Connectivetissuesoccurthroughoutthebodyandarethemostabundanttypeoftissuebyweight.Theybindstructurestogether,providesupportandprotection,serveasframeworks,fillspaces,storefat,producebloodcells,protectagainstinfections,andhelprepairtissuedamage.Connectivetissuecellsareusuallyabletoreproduce.Thesetissueshavevaryingdegreesofvascularity,butinmostcases,theyhavegoodbloodsuppliesandarewellnourished.Someconnectivetissues,suchasboneandcartilage,arequiterigid.Looseconnectivetissue,adiposeconnectivetissue,andfibrousconnectivetissuearemoreflexible.[fig.2.4]
Fig.2.4Connectivetissue
Muscletissuesarecontractile;thatis,theirelongatedcells,ormusclefibers,canshortenandthicken.Astheycontract,musclefiberspullattheirattachedends,whichmovebodyparts.
Skeletalmuscletissueisfoundinmusclesthatusuallyareattachedtobonesandthatwecontrolbyconsciouseffort.Forthisreason,itisoftencalledvoluntarymuscletissue.Theselongandthreadlikecellshavealternatinglightanddarkcross-markingscalledstriations.
Smoothmuscletissueiscalledsmoothbecauseitscellslackstriations.Thistissueisfoundinthewallsofhollowinternalorgans,suchasthestomach,intestines,urinarybladder,uterus,andbloodvessels.Unlikeskeletalmuscle,smoothmuscleusuallycannotbestimulatedtocontractbyconsciousefforts.Thus,itisatypeofinvoluntarymuscletissue.
Cardiacmuscletissueoccursonlyintheheart.Itscells,whicharestriated,arejoinedendtoend.Theresultingmusclefibersarebranchedandinterconnectedincomplexnetworks.Eachcellwithinacardiacmusclefiberhasasinglenucleus.Atitsend,whereittouchesanothercell,isaspecializedintercellularjunctioncalledanintercalateddisk,whichisseenonlyincardiactissue.Cardiacmuscle,likesmoothmuscle,iscontrolledinvoluntarilyand,infact,cancontinuetofunctionwithoutbeingstimulatedbynerveimpulses.Thistissuemakesupthebulkoftheheartandpumpsbloodthroughtheheartchambersandintobloodvessels.[fig.2.5]
Fig.2.5Skeletalmuscle,cardiacmuscleandsmoothmuscle
NervousTissues
Nervoustissuesarefoundinthebrain,spinalcord,andperipheralnerves.Thebasiccellsarecallednervecells,orneurons,andtheyareamongthemorehighlyspecializedbodycells.Neuronsaresensitivetocertaintypesofchangesintheirsurroundings.Theyrespondbytransmittingnerveimpulsesalongcytoplasmicextensions(nervefibers)tootherneuronsortomusclesorglands.Asaresultoftheextremelycomplexpatternsbywhichneuronsareconnectedwitheachotherandwithmuscleandglandcells,theyareabletocoordinate,regulate,andintegratemanybodyfunctions.[fig.2.6]
Fig.2.6Nervoustissue
OrganSystems
Thehumanorganismconsistsofseveralorgansystems.Eachsystemincludesasetofinterrelatedorgansthatworktogethertoprovidespecializedfunctions.Themaintenanceofhomeostasisdependsontheseorgansystemsworkingtogetherinacoordinatedmanner.Majororgansineachsystemandtheirfunctionsareasfollows:
(Table2.1)
Table2.1OrganSystems
System
MajorOrgans
Majorfunctions
integumentary
skin,hair,nails,sweatglands,sebaceousglands
protecttissues,regulatebodytemperature,supportsensoryreceptors
musculoskeletal
bones,ligaments,cartilages
provideframework,protectsofttissues,provideattachmentsformuscles,producebloodcells,storeinorganicsalts;causemovements,maintainposture,producebodyheat
cardiovascular
heart,arteries,veins,capillaries
movebloodthroughbloodvesselsandtransportsubstancesthroughoutbody
respiratory
nasalcavity,pharynx,larynx,trachea,bronchi,lungs
intakeandoutputofair,exchangeofgasesbetweenairandblood
digestive
mouth,tongue,teeth,salivaryglands,pharynx,esophagus,stomach,liver,gallbladder,pancreas,smallandlargeintestines
receive,breakdown,andabsorbfood;eliminateunabsorbedmaterial
urinary
kidneys,ureters,urinarybladder,urethra
removewastesfromblood,maintainwaterandelectrolytebalance,storeandtransporturine
reproductive
male:
scrotum,testes,epididymides,vasadeferntia,seminalvesicles,prostategland,bulbourethralglands,urethra,penis
female:
ovaries,uterinetubes,uterus,vagina,clitoris,vulva
produceandmaintainspermcells,transferspermcellsintofemalereproductivetract
produceandmaintaineggcell;receiveandtransportspermcells;supportdevelopmentofanembryoandfunctioninbirthprocess
endocrine
glandsthatsecretehormones
controlmetabolicactivitiesofbodystructures
nervous
brain,spinalcord,nerves,senseorgans
detectchanges,receiveandinterpretsensoryinformation,stimulatemusclesandglands
lymphatic
lymphaticvessels,lymphnodes,thymus,spleen
returntissuefluidtotheblood,carrycertainabsorbedfoodmolecules,defendthebodyagainstinfection
AnatomicalTerminology
AnatomicalPosition
Theanatomicalpositionisusedwhendescribingthepositionsandrelationshipsofastructureinthehumanbody.Abodyintheanatomicalpositionisstandingerectwiththearmsatthesideofthebody,thepalmsofthehandsfacingforwardandtheeyeslookingstraightahead.Inaddition,thelegsareparallelwiththefeetandthetoespointingforward.Fordescriptivepurposestheassumptionisalwaysthatthepersonisintheanatomicalpositionevenifthebodyorpartsofthebodyareinanyotherposition.[fig.2.7]
Fig.2.7Anatomicalpositionandrelativeposition
RelativePosition
Termsofrelativepositionareusedtodescribethelocationofonebodypartwithrespecttoanother.Theyincludethefollowing:
1.Superior(orcephalic)meansapartisaboveanotherpart,orclosertothehead.(Thethoraciccavityissuperiortotheabdominopelviccavity.)
2.inferior(orcaudal)meanssituatedbelowanotherpartortowardthefeet.(Theneckisinferiortothehead.)
3.Anterior(orventral)meanstowardthefront.(Theeyesareanteriortothebrain.)
4.Posterior(ordorsal)istheoppositeofanterior,itmeanstowardtheback.(Thepharynxisposteriortotheoralcavity.)
5.Medialrelatestoan
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