Passage 1 and 2Overview of the Digestive System-translation OverviewoftheDigestiveSystem ConsiderforamomentaBigMac.ThepurposeinyoureatingaBigMac,otherthansimplehedonism,istoassimilatethenutrientsitrepresentsandmakethemavailabletobuild,repairandmaintainyourowntissues,aswellasprovideenergyforstudyingandocca-sionalotherpursuits. Youmayhaveaskedyourself-ExactlywhatnutientsarepresentinaBigMacthatIcanassimilate?MacDonaldscomesclosetofulldisclosureinthisregard,butwhattheydonttellyouisthatinordertotakeadvantageofthesenutrients,youhavetoprovidethemeanstocarefullybreakthemdownintomuchsmallermoleculesthatcanbeimportedintoblood.Luckily,yourdigestivesystemtakescareofthisverycomplexprocesssoefficientlythatmostofthetimeyoudontevenneedtothinkaboutit. Atitssimplest,thedigestivesystemitisatuberunningfrommouthtoanus.Thistubeislikeanassemblyline,ormoreproperly,adissemblyline.Itschiefgoalistobreakdownhugemacromolecules(proteins,fatsandstarch),whichcannotbeabsorbedintact,intosmallermolecules(aminoacids,fattyacidsandglucose)thatcanbeabsorbedacrossthewallofthetube,andintothecirculatorysystemfordisseminationaroundyourbody. ConsiderforamomentaBigMac.ThepurposeinyoureatingaBigMac,otherthansimplehedonism,istoassimlatethenutrientsitrepresentsandmakethemavailabletobuild,repairandmaintainyourowntissues,aswellasprovideenergyforstudyingandoccasio-nalotherpursuits. ThebreakdownoffoodstuffslikeaBigMacisaccomplishedthroughacombinationofmechanicalandenzymaticprocesses.Toaccomplishthisbreakdown,thedigestivetuberequiresconsiderableassistancefromaccessorydigestiveorganssuchasthesalivaryglands,liverandpancreas,whichdumptheirsecretionsintothetube.Thenameaccessoryshouldnotbetakentomeandispensible;indeed,withoutpancreaticenzymesyouwouldstarvetodeathinshortorder. Inmanyways,thedigestivesystemcanbethoughtofasawell-runfactoryinwhichalargenumberofcomplextasksareperformed. Thethreefundamentalprocessesthattakeplaceare: Secretion:Deliveryofenzymes,mucus,ionsandthelikeintothelumen,andhormonesintoblood Absorption:Transportofwater,ionsandnutrientsfromthelumen,acrosstheepitheliumandintoblood Motility:Contractionsofsmoothmuscleinthewallofthetubethatcrush,mixandpropelitscontents Eachpartofthedigestivetubeperformsatleastsomeofthesetasks,anddifferentregionsofthetubehaveuniqueandimportantspecializations. Likeanywell-runfactory,properfunctionofthedigestivesystemrequiresrobustcontrolsystems.Controlsystemsmustfacilitatecommunicationamongdifferentsectionsofthedigestivetract(i.e.controlonthefactoryfloor),andbetweenthedigestivetractandthebrain(i.e.betweenworkersandmanagment).Controlofdigestivefunctionisachievedthroughacombinationofelectricalandhormonalmessageswhichoriginateeitherwithinthedigestivesystemsownnervousandendocrinesystems,aswellasfromthecentralnervoussytemandfromendocrineorganssuchastheadrenalgland.Differentpartsofthesesystemsareconstantlytalkingtooneanother.ThebasicmessagesarealongthelinesofIjustreceivedanextraordinaryloadoffood,soIsuggestyougetprepared(stomachtolargeintestine)orForgoodnesssake,pleaseslowdownuntilIcancatchupwithwhatyouvealreadygivenme(smallintestinetostomach). Finally,anoteaboutdifferencesindigestiveanatomyandphysiologyamonganimals.Thedigestivesystemsofhumans,dogs,mice,horses,kangaroosandgreatwhitesharksare,toafirstapproximation,virtuallyidentical.Ifyoulookmorecarefullyhowever,itbecomesapparentthateachofthesespecieshasevolvedcertaindigestivespecializationsthathaveallowedittoadapttoaparticulardiet. Thesedifferencesbecomeparticularlyapparentwhenyoucompareacarnivorelikeacatwithaherbivorelikeagoatorahorse.Goatsandhorsesevolvedfromancestorsthatsubsistedonplantsandadaptedpartsoftheirdigestivetractsintomassivefermentationvatswhichenabledthemefficientlyutilizecellulose,themajorcarbohydrateofplants. Incontrast,catsevolvedfromanimalsthatlivedonthecarcassesofotheranimals,andhavedigestivesystemsthatreflectthishistory-extremelysmallfermentationvatsandessentiallynoabilitytoutilizecellulose.Bridgingthegapbetweencarnivoresandherbivoresareomnivoreslikehumansandpigs,whosedigestivetractsattesttoahistoricaldietthatincludedbothplantsandanimals