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NROSCI/BIOSC1070(2017)

RenalPhysiology1

Itisanessentialneedofthebodytomaintainaconstancyofthefluidenvironmentinthebody.Youhavealreadyspentawhilediscussingtheimportanceofthecirculatorysysteminthisprocess.[ClaudeBernard,a19thcenturyFrenchphysiologist,wrote"Itisthefixityoftheinternalenvironmentthatistheconditionoffreeandindependentlife....Allvitalmechanisms,howevervariedtheymaybe,haveonlyoneobject,thatofpreservingconstantconditionsoflifeintheinternalenvironment".]Clearly,inorderforinterstitialfluidlevelsofsubstancestobekeptatan"appropriate"level,bloodlevelsmustalsobekeptatanappropriatelevel.Youdiscussedhowtherespiratorysystemaccomplishesthisforgasses-bothgassesneededformetabolicsubstrates(i.e.,02)andwastegasses(i.e.,C02).Thekidneys,alongwiththedigestivesystem,serveasimilarfunctionfornon-gaseoussubstances.Forexample,duringmetabolismweproduceorganicwasteproducts.Howdoesthebodyeliminatethese?Asweshalldiscussingreatdetailoverthenextfewclasses,theyareremovedfromthebloodbythekidneys.

Let’sconsiderforamoment,twocriticalcomponentsoftheextracellularfluid,waterandNa+.ItisimportantthatwemaintainconstantamountsofwaterandNa+inourbody,butourdailyintakesandlossesofthesesubstancesvarygreatly.Somedayswemighteatalotofsalt,whereasonotherdaysweeatless.Somedayswelosemoresaltandwaterinsweat,otherdayswelooseless.Butthebodycannottoleratelargechangesineitherofthesesubstances.Thebodyadjuststomaintainconstantlevelsofthese-BALANCE.(slides3and4)

FunctionsoftheKidneys:(slide5)•excretionofmetabolicwastes•excretionofforeignsubstances(e.g.,drugs,toxins)•regulationofbodyfluidosmolarityandelectrolyteconcentrations(e.g.,Na+,K+,Ca++)•maintenanceofwaterbalanceandelectrolytebalance•contributestomaintenanceofbodypH•regulationofredbloodcellnumber(erythropoietin)•regulationofbloodpressure•glucoseproduction

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Inordertodiscusshowthekidneysperformthesefunctions,weneedtounderstandtheanatomyofthekidney.(slides6and7)

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Sothefunctionalunitofthekidneyisanephron(Slides7,8,9,10).Thereareroughly1millionoftheseperkidney,andwewillspendthenextfewclassesdiscussinghownephronswork.We'llstartwithabriefoverviewofwhatnephronsdo,andthengobackandlookateachstepinmoredetail.Whatgetsexcretedbythekidneysresultsfromfiltrationofbloodintheglomeruliandthensubsequentreabsorptionorsecretionofsubstancesinthedifferenttubularsegments.Soforanysubstance,wecanexpressitsexcretionrateintermsofhowmuchisfiltered,howmuchisreabsorbed,andhowmuchissecreted.(Don'tconfusesecretionandexcretion.)(slides11,12)urinaryexcretionrate=filtrationrate-reabsorptionrate+secretionrate

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Let'sconsider5hypotheticalsubstances(slide13):1.Filtered,butnotreabsorbedorsecreted.Filtration=excretion.Ifwehadasubstancelikethis,thenwecouldeasilydeterminefiltrationratebysimplymeasuringexcretionrate.Inulinisasubstancenotnormallyfoundinthebodythatisneitherreabsorbedorsecretedbythekidney,andsoitisoftenusedtoassessrenalfiltration.2.filteredandpartiallyreabsorbed,soexcretion<filtration.Manysubstancesarehandledinthiswaybythekidney.3.filteredandtotallyreabsorbed;therefore,noneisusuallylostintotheurine.Atnormalbloodlevels,glucoseishandledlikethis.4.filteredandsecreted;excretion>filtration.5.notfiltered;notallsubstancesarefilteredfrombloodintheglomeruli.

soforeachsubstanceweconsider,wewillneedtoask4questions(slide14):1.towhatextentisitfilteredintheglomeruli?2.isitreabsorbed?(ifso,where?how?)3.isitsecreted?(ifso,where,how?)4.whatfactorshomeostaticallyregulatethefiltration,reabsorption,andsecretionofthesubstance?

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However,beforewedealwiththemovementofmoleculesintoandoutoftherenaltubulesystem(i.e.,reabsorbtionandsecretioninthenephron)let'sspendalittletimetalkingingeneraltermsabouthowmoleculesgetacrossmembranes.TransportofmoleculesacrossmembranesThereareessentially4waysthatmoleculescangetacrosscellmembranes(slide15):•Simplediffusionthroughthemembrane-limitedtolipidsolublesubstances•Simplediffusionthroughholes(i.e.,pores)inthemembrane-thisislimitedtoselectedsmallions(e.g.,K+passingthroughK+channels;orNa+orCl-)•Carriermediatedtransport(ormediatedtransportforshort)-asdescribedinmoredetailbelow,thereare3generalclassesofmediatedtransport•Endocytosis/exocytosis=vesiclemediatedtransportMuchofwhatgoesoninthekidneysisviamediatedtransport,andsoweneedtospendalittletimediscussingsomeofthemechanismsofmediatedtransportandthefactorsthatinfluencethem.Withmediatedtransport,moleculesaretransportedacrossthemembranebyaspecifictransportproteinembeddedinthemembrane.Theseproteinsarereferredtoastransporters,andtheybindtotheirligandononesideofthemembraneandreleasethemontheother.Threefactorsinfluencetherateofmediatedtransport(slide16):•relativeaffinityoftheligandforthetransporter(howmuchversushowhighanaffinity)•howmanytransporters•howfastthetransporterworksNotethatunlikediffusion,withmediatedtransportthefluxofamoleculeacrossamembranecanbecomesaturatedandreachamaximum(transportmaximum).Also,therecanbecompetitionfortransportbetweendifferentmoleculesthatshareacommontransporter.Typesofmediatedtransport(slide17)Mediatedtransportcanbedividedintotwocategoriesbasedonwhetherornotenergyisrequired.Facilitateddiffusionismediatedtransportthatissolelydrivenbythediffusiongradient;butmakenomistake-itisnotaprocessofdiffusion.GlucoseentersmostcellsbyfacilitateddiffusionActivetransportutilizesenergyderivedfromcellularmetabolismtodrivethetransportprocess.Thisuseofenergyactuallyallowsmoleculestobemovedfrom[low]to[high];topumpmoleculesagainstaconcentrationgradient.Thus,thetransportproteininthiscaseisfrequentlyreferredtoasa"pump".Basedonhowthepumpiscoupledtoenergyderivedfrom

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cellmetabolism(i.e.,ATP),activetransportcanbedividedintotwocategories:primary,inwhichthehydrolysisofATPisinvolveddirectly,andsecondary,inwhichthetransportisdrivenbyanionconcentrationgradientthatwassetupbyaprimaryactivetransportprocess.PrimaryActiveTransport-thetransporterisitselfanATP-ase-4havebeenidentified:Na,K-ATPase;Ca-ATPase;H-ATPase;H,K-ATPase.-theNa,K-ATPaseispresentinallcellsandleadstothecharacteristichighintracellularK(150mMvs4or5mMout)andlowintracellularNa(15vs145).ForeachATPused,3Naout,2Kin.SecondaryActivetransportTheenergyfortransportisderivedfromaconcentrationgradientestablishedbyactivetransport.-manyaredrivenbytheNagradientestablishedbytheNa,K-ATPase.-asmuchas50%ofATPusagebyacellmaybeusedtodrivetheNaKATP-ase-co-transport(samedirectionasNa,orwhateveristheiondrivingthetransport)vscountertransport(oppositedirection);someauthorsuseco-transportindependentofdirection,andthensymportandantiporttoindicaterelativedirectionofthetransportedmolecules.(Slide18)

directionofnetfluxCovsClatsteadystate?useofintegralmembraneprotein?saturation?chemicalspecificity?energy;?sourceexamplesofmolecules(slide19)

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MovementofwaterRemember,waterisamoleculetoo,anditisveryabundantinthebody.Approximately60%ofbodyweightiswater(fora70kgman,that's42L;womenhaveasomewhathigherfatcontent,soasomewhatlowerwatercontent).Ofthis,approx.2/3ofthewaterisinsidecells(intracellularfluid).Ofthe1/3thatisoutsidecells(extracellularfluid),approx80%isinterstitialfluid,withtherestbeingthefluidphaseofblood(bloodplasma).Despiteitspolarnature,waterreadilydiffusesthroughbiologicalmembranesthroughchannelsformedbyafamilyofproteinscalledaquaporins.Theprocessofwatermovingdownsitsconcentrationgradientisosmosis.Wetypicallytalkaboutsolutesnotwater,butasthesoluteconcentrationgoesup,thewaterconcentration(bydefinition)goesdown.Thus,inthinkingabouttheconcentrationofwater,itisusefultohaveawayofreferringtothetotalsoluteconcentrationitin(notjustofaspecificmolecule)andthisiscalledosmolarity;1osmoleisequalto1moleofsolute(e.g.,1moleofglucoseor1moleofNa;1moleofNaCl-becauseitfullyionizesinwater-istwoosmoles).Osmolarityisthenumberofmolesofsoluteperliter.(Arelatedterm,osmolality,isthenumberofmolesofsoluteperkilogramofwater.Withinthecontextofphysiologicalconcentrationsofthings,thetwovaluesareessentiallythesame.However,duetothewayitistypicallymeasured,theappropriatetermisosmolality.)Thehighertheosmolarity,thelowerthewaterconcentration.Iftheosmolarityofonesolutionisequaltothatofanother,thentheyaresaidtobeisosmotic.Ifthesecondsolutionhasahigherosmolarity,itissaidtobehyperosmotic.Conversely,ifitisLower,itishyposmotic.Anormalphysiologicalsolutionisapproximately290milliosmolar.(Forthepurposeofthiscourse,wewillcallis300mOsm.)Thus,asolutionof0.9%NaCl,whichis150mMNaCl,isisosmotictonormalsolutions.5%Dextroseinwater(asolutioncommonlyreferredtoasD5W)isslightlyhyposmotic(approx275mM!).Nowconsidertwosolutionsseparatedbyamembranethatispermeabletowaterbutnotthesolute.Whathappens?Howisthisdifferentfromwhathappensifthemembraneispermeanttothesolute?Thusweneedtodistinguishbetweenpermeantandnon-permeantsolutes.Tonicityandosmoticpressure(slide20)Osmoticpressureisthepressure(inmmHg)thatmustbeappliedtocounteracttheosmoticflowofwater.Tonicityisanexpressionofrelativeosmoticpressure.ThewordtonicityderivesfromtheGreektonikos,meaningpertainingtostretch.Inphysiologicalterms,asolutionof<300mOsmishypotonic,sincewaterwilldiffusefromthatsolutionintoaphysiologicallynormalsolution.,andthereforedecreasethestretchofthatcompartmentfromwhichitisdiffusing.Withasolutionof300mOsmorgreaterwecan'tknowthetonicityuntilweknowwhetherthesolutemoleculeswillpermeatethemembraneseparatingthetwosolutions.Inanisotonicsolution,thetotalconcentrationofnon-permeantsoluteswouldbe300mM.(slide21)

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dependinguponwhatothersolutesmaybepresentinthatsolution,itcouldbeeitherisomotic(i.e.,noadditionalnon-permeantsolutes),orhyperosmotic.Puttingalivingcellinahypertonicsolution,willcauseittoshrink(i.e.,thefluidinsidethecellishypotonicrelativetotheoutside).EpithelialtransportNotethataswehavebeendiscussingthemovementofmolecules,wehavepicturedlinearpiecesofmembrane.Incontrast,let'sconsideracell,inwhichthelocalizationoftransportersandporesarenotrandomlylocalized,butratherpresentinspecificregionsofthemembrane.Forexample,let'sconsideracellwithasodiumchannelononesideandasodiumpump(pumpingoutward)ontheother.Inthismanner,cellscanmovesubstancesacrossacellularliningofacompartment!Forthenextseveralclasseswewilltalkaboutthetransportofmoleculesacrosstheepithelialcellliningofthetubulesystemofthenephron.

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GLOMERULARFlLTRATION(slides22and23transitioningbacktothistopic)FiltrationofsubstancesfromglomerularcapillariesintoBowman'scapsule(thefirstpartoftherenaltubulesystem)isbasedontheprinciplesthatwehavealreadycoveredonthepassageofsubstancesfromcapillariesintointerstitialspace:hydrostaticpressuresandosmoticpressures.Inaddition,theamountofasubstancethatcanreadilypassthroughthecapillarieswilldependonthesurfaceareaavailableforexchangeandhow"leaky"thecapillarybarrieris,whichcanbedescribedinatermcalledthefiltrationconstant(Kf).filtration=KfXnetfiltrationpressure(slide24,25)

netfiltrationpressure=thesumofthehydrostaticandosmoticpressures.

Kfreflectsthesurfaceareaandthehydraulicconductivityofglomerularcapillaries.Thehydraulicconductivityofglomerularcapillariesisseveralhundredtimesgreaterthaninothercapillaries:glomerularcapillariesareveryleaky.

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InpassingfromtheinsideofglomerularcapillariesintoBowman'scapsule,substancesneedtocross3layers(slide26-29):1.capillaryendothelialcells;thesearemore"fenestrated"(i.e.,haveholes)thancapillariesinotherareas.2.basallamina-anon-cellularproteinmatrix3.podocytes-specializedepithelialcellsthatformliningaroundglomerularcapillaries.Slitsbetweenpodocytefootprocessesserveasathirdlevelof"filtration"ofsubstancesastheypassfromcapillariestoBowman'sspace.(slide27-29)AlthoughwedonottypicallythinkofKfasbeingaregulatedvariable,thereissomesuggestionsthatmesangialcellsneighboringthepodocytescanregulatethesizeoftheslitsbetweenthepodocytefootprocessesandthereforealterKf.(slide30)

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Let'sconsiderwhatcontributestonetfiltrationpressure(slide31):Thekeyfactoristhedifferenceinhydrostaticpressures:about50mmHgpushingfluidoutofcapillariesversus15pushingfluidbackin.Wheredothesenumberscomefrom?Andwhatcaninfluencethesenumbers?PressureinsideBowman'scapsuledoesn'tchangemuchunlessthereisanobstructionsomewhereinthetubulesystem.Capillarybloodpressurecanchange.Let'sconsiderwhathappensaseithertheafferentorefferentarteriolesconstrictordilate(slide32).

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Normalglomerularfiltrationrate(GFR)isapprox.125ml/minor180Lperday.(rememberthesenumbers)(slide33).Consideringthatplasmavolumeisapprox3L,wearefilteringtheentireplasmavolume60timesadayoronceevery25minutes!Obviously,mostofthatfiltrateisreabsorbed;urinevolumeisonlyabout1.5liter/day.IntermsofNa+,normalplasmaNa+is3.5g/L,sothefilteredloadofNa+is630g/day.(We'lltalkabouthowNa+andwaterarereabsorbedduringthenextclass.)Butgiventhefiltrationpressureswediscussedabove,GFRshouldbeverysensitivetochangeinperfusionpressure,i.e.,bloodpressure?Itisn't.(slide34)Thereisverypowerfulautoregulationoftherenalcirculation,suchthatGFRdoesnotchangeoverawiderangeofmeanarterialpressures.Therearetwomajorcontributorstorenalautoregulation(slide35):myogenicmechanismsandtubuloglomerularfeedback.Therelativeimportanceofeachisdebated,andthemechanismofeachisnotfullyunderstood.Myogeniccontrol:asdiscussedpreviouslyforothervascularbeds,increasedpressure(i.e.stretch)causesarteriolestocontract,therebymaintainingaconstantflow.Thisprocessisoperationalatafferentarterioles(andpreglomerulararterioles).Tubuloglomerularfeedback(slide36):Na+deliverytothedistaltubuleis"sensed"inthemaculadensa,whichinfluencestheafferentandefferentarteriolesviabothparacrineandendocrine(angiotensin)signaling.

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Considerasubstanceinbloodthatisfilteredbutneitherreabsorbedorsecreted;itissubsequentlyexcreted.Therefore,GFRXPs=UsXV(wherePsistheplasmaconcentrationofthesubstance,Usistheurinaryconcentrationofthesubstance,andVistheurinevolume)sowecoulddetermineGFRby:GFR=VXUs/Ps(slide37)inulin,afructose-basedpolysaccharide,issuchanidealcompound,anditcanthereforebeusedtomeasureGFR.Itisinfusedtoaconstantplasmalevel,andthenplasmaandurinelevelaremeasuredtocalculateGFR.(slide38)creatinine,anormalproductofmusclemetabolismisclose;itissecretedalittlebit,butitsexcretionraterelativetoplasmaconcentrationisagoodestimateofGFR(itslightlyoverestimatesGFR,duetothelittlebitofsecretion).