Bi 233 Ch25 Urinary System part 2 -...

Chapter 25: Urinary System 1

Collec&ngDucts

• Receivefiltratefromdistalconvolutedtubule

• Manynephronsdraintoasinglecollec9ngduct

• Papillaryducts(DuctsofBellini)

• Convergenceofmanycollec9ngducts

• Drainthroughrenalpapillatominorcalyx

TypesofNephrons

• Cor9calnephrons

• 85%ofnephrons

• Locatedinthecortex

• Mayjustbarelyentermedulla

• Juxtamedullarynephrons:

• Arelocatedatthecortex‐medullajunc9on

• HaveloopsofHenlethatdeeplyinvadethemedulla

• Haveextensivethinsegments

• Areinvolvedintheproduc9onofconcentratedurine


TypesofNephrons

Figure 25.5b

CapillaryBedsoftheNephron

• Everynephronhastwocapillarybeds

• Glomerulus

• Ini9alfiltra9oninrenalcapsule

• Peritubularcapillaries

• NetworkingwithPCT/DCTforabsorp9onandsecre9on

• Eachglomerulusis:

• Fedbyanafferentarteriole

• Drainedbyanefferentarteriole


CapillaryBedsoftheNephron

• Bloodpressureintheglomerulusishighbecause:

• Arteriolesarehigh‐resistancevessels

• Afferentarterioleshavelargerdiametersthanefferentarterioles

• Fluidsandsolutesareforcedoutofthebloodthroughouttheen9relengthoftheglomerulus

CapillaryBeds

• Peritubularbeds

• Low‐pressure,porouscapillariesadaptedforabsorp9onthat:

• Arisefromefferentarterioles

• Clingtoadjacentrenaltubules

• Emptyintotherenalvenoussystem

• Vasarecta

• Long,straightefferentarteriolesofjuxtamedullarynephrons

• Typicallyforreabsorp9on


CapillaryBeds

Figure 25.5a

JuxtaglomerularApparatus(JGA)

• Wherethedistaltubuleliesagainsttheafferent(some9mesefferent)arteriole

• Consistsofjuxtaglomerularcellsandmaculadensa

• Regulatesrenalbloodpressure

• Arteriolewallshavejuxtaglomerular(JG)cells

• Enlarged,smoothmusclecells

• Havesecretorygranulescontainingrenin

• Actasmechanoreceptors

• Detectbloodpressure


JuxtaglomerularApparatus(JGA)• Maculadensa

• Tall,closelypackeddistaltubulecells

• LieadjacenttoJGcells

• Func9onaschemoreceptorsorosmoreceptors

• Detectchangesinfiltratesoluteconcentra9ons

• Mesanglialcells:

• Havephagocy9candcontrac9leproper9es

• Influencecapillaryfiltra9on

JuxtaglomerularApparatus(JGA)

Figure 25.6


MechanismsofUrineForma&on

• Thekidneysfilterthebody’sen9replasmavolume609meseachday

• Thefiltrate:

• Containsallplasmacomponentsexceptprotein

• Loseswater,nutrients,andessen9alionstobecomeurine

• Theurinecontainsmetabolicwastesandunneededsubstances

MechanismsofUrineForma&on

• 3steps

• Glomerularfiltra9on

• Tubularreabsorp9on

• Secre9on

Figure 25.8


GlomerularFiltra&on

• Func9onssimilartotypicalcapillarybeds

• Theglomerulusismoreefficientthanothercapillarybedsbecause:

• Itsfiltra9onmembraneissignificantlymorepermeable

• Glomerularbloodpressureishigher

• Ithasahighernetfiltra9onpressure

• Plasmaproteinsarenotfiltered

• Usedtomaintainonco9cpressureoftheblood

• Notcompletelynonselec9ve

• NoRBC’s,WBC’s,ormolecules>7‐9microns

NetFiltra&onPressure(NFP)• Thepressureresponsibleforfiltrateforma9on

• 3processes

• Glomerularhydrosta9cpressure(HPg)‐(onco9cpressureofglomerularblood(OPg)+capsularhydrosta9cpressure(HPc))

NFP = HPg – (OPg + HPc)

GHP ~ 15 mm HgGOP ~ 30 mm HgCHP ~ 15 mm Hg

NFP = 10 mm Hg


GlomerularFiltra&onRate(GFR)• Thetotalamountoffiltrateformedperminutebythe

kidneys

• ~120‐125ml/min

• GFR~180L/day

• Factorsgoverningfiltra9onrateatthecapillarybedare:

• Totalsurfaceareaavailableforfiltra9on

• Filtra9onmembranepermeability

• Netfiltra9onpressure

• GFRisdirectlypropor9onaltotheNFP

• ChangesinGFRnormallyresultfromchangesinglomerularbloodpressure

GlomerularFiltra&onRate(GFR)

Figure 25.9


Regula&onofGlomerularFiltra&on

• IftheGFRistoohigh:

• Neededsubstancescannotbereabsorbedquicklyenoughandarelostintheurine

• IftheGFRistoolow:

• Everythingisreabsorbed,includingwastesthatarenormallydisposedof

Regula&onofGlomerularFiltra&on

• ThreemechanismscontroltheGFR

• Renalautoregula9on(intrinsicsystem)

• Neuralcontrols

• Hormonalmechanism(therenin‐angiotensinsystem)


IntrinsicControls• Renalautoregula9on

• Maintainsanearlyconstantglomerularfiltra9onrate

• Undernormalcondi9ons

• Autoregula9onentailstwotypesofcontrol

• Myogenic

• Respondstochangesinpressureintherenalbloodvessels

• Smoothmusclecontractsinresponsetostretching

• Tubuloglomerularfeedback(maculadensa‐lowosmolarity)

• Senseschangesinthejuxtaglomerularapparatus

• JGcellsreleaserenin

• Renin⇒angiotensin⇒aldosterone⇒⇑BP

ExtrinsicControls

• Whenthesympathe9cnervoussystemisatrest:

• Renalbloodvesselsaremaximallydilated

• Autoregula9onmechanismsprevail


ExtrinsicControls• Understress:

• Norepinephrineisreleasedbythesympathe9cnervoussystem

• Epinephrineisreleasedbytheadrenalmedulla

• Afferentarteriolesconstrict

• reducesNFP

• Sympathe9cnervoussystemalsos9mulatestherenin‐angiotensinmechanism

• IncreasesBP

Renin‐AngiotensinMechanism

• TriggeredwhentheJGcellsreleaserenin

• Inresponsetolowosmolarity

• ReninactsonangiotensinogentoreleaseangiotensinI

• AngiotensinIisconvertedtoangiotensinII

• AngiotensinII:

• Causesmeanarterialpressuretorise

• S9mulatestheadrenalcortextoreleasealdosterone

• Asaresult,bothsystemicandglomerularhydrosta9cpressurerise


ReninRelease

• Reninreleaseistriggeredby:

• ReducedstretchofthegranularJGcells

• S9mula9onoftheJGcellsbyac9vatedmaculadensacells

• Directs9mula9onoftheJGcellsviaβ1‐adrenergicreceptorsbyrenalnerves

• AngiotensinII

TubularReabsorp&on• Transepithelialprocess

• Tubulecontentsarereturnedtotheblood

• Transportedsubstancesmovethroughthreemembranes

• Luminalandbasolateralmembranesoftubulecells

• Endotheliumofperitubularcapillaries


TubularReabsorp&on

• Reducesfiltratetourine

• Urea,uricacid,andmetabolicwastes

• 1‐2L/day

• From180L/dayfiltrate!

• Allorganicnutrientsarereabsorbed

• Waterandionreabsorp9onishormonallycontrolled

• Inresponsetobodilyneeds

• Reabsorp9onmaybeanac9ve(requiringATP)orpassiveprocess

• Roughly99%offiltrate

RoutesofWaterandSoluteReabsorp&on

Figure 25.11


Reabsorp&onbyPCTCells• Ac9vepumpingofNa+drivesreabsorp9onof:

• Waterbyosmosis,aidedbywater‐filledporescalledaquaporins

• Ca9onsandfat‐solublesubstancesbydiffusion

• Organicnutrientsandselectedca9onsbysecondaryac9vetransport

Reabsorp&onbyPCTCells

Figure 25.12


NonreabsorbedSubstances

• Atransportmaximum(Tm):

• Reflectsthenumberofcarriersintherenaltubulesavailable

• Existsfornearlyeverysubstancethatisac9velyreabsorbed

• Whenthecarriersaresaturated,excessofthatsubstanceisexcreted

NonreabsorbedSubstances

• Substancesarenotreabsorbedifthey:

• Lackcarriers

• Arenotlipidsoluble

• Aretoolargetopassthroughmembranepores

• Urea,crea9nine,anduricacidarethemostimportantnonreabsorbedsubstances


• SubstancesreabsorbedinPCTinclude:

• Sodium,allnutrients,ca9ons,anions,andwater

• Ureaandlipid‐solublesolutes

• Smallproteins

• LoopofHenlereabsorbs:

• H2O,Na+,Cl−,K+inthedescendinglimb

• Ca2+,Mg2+,andNa+intheascendinglimb

Absorp&veCapabili&esofRenalTubulesandCollec&ngDucts(notinnotes)

• DCTabsorbs:

• Ca2+,Na+,H+,K+,andwater

• HCO3−andCl−

• Collec9ngductabsorbs:

• Waterandurea

Absorp&veCapabili&esofRenalTubulesandCollec&ngDucts(notinnotes)


MovementintoTubuleCells• Ac9veentry

• Na+‐K+ATPasepump

• Movessodiumandfiltrateintotubulecells

• Thentointers99alfluidanddiffusesintoperitubularcapillaries

• PCT

• Facilitateddiffusionusingsymportandan9portcarriersorproteinchannels

• Sodiumandchlorideorglucose

• Obligatory

• Ac9vesodiumreabsorp9onallowswatertopassivelyfollow

• PCTcannotcontrolwatermovement

MovementintoTubuleCells

• Facilitateddiffusion

• AscendingloopofHenle

• FacilitateddiffusionviaNa+‐K+‐2Cl−symportsystem

• Impermeabletowater

• Descendinglimbpermeabletowater

• Faculta9ve

• ADHincreasesmembranepermeabilityincreasingwaterreabsorp9on

• Diffusionthroughmembranepores

• DCTandCollec9ngDucts

• BloodpHwilldictatetypeandquan9tyofanionsreabsorbed


TubularSecre&on

• Essen9allyreabsorp9oninreverse

• Substancesmovefromperitubularcapillariesortubulecellsintofiltrate

• Tubularsecre9onisimportantfor:

• Disposingofsubstancesnotalreadyinthefiltrate

• Elimina9ngundesirablesubstances

• Urea,uricacid,hydrogenions,ammonium,crea9ne

• Riddingthebodyofexcesspotassiumions

• ControllingbloodpH

Regula&onofUrineConcentra&onandVolume

• Osmolality

• Thenumberofsolutepar9clesdissolvedin1Lofwater

• Reflectsthesolu9on’sabilitytocauseosmosis

• Bodyfluidsaremeasuredinmilliosmols(mOsm)

• Kidneyskeepthesoluteloadofbodyfluidsconstantatabout300mOsm

• Accomplishedbythecountercurrentmechanism


CountercurrentMechanism

• Interac9onbetweentheflowof…

• FiltratethroughloopofHenle

• countercurrentmul9plier

• Bloodthroughthevasarecta

• countercurrentexchanger

• Soluteconcentra9onintheloopofHenle

• Rangesfrom300mOsmto1200mOsm

• Dissipa9onofthemedullaryosmo9cgradientispreventedbecausethebloodinthevasarectaequilibrateswiththeinters99alfluid

Osmo&cGradientintheRenalMedulla

Figure 25.13


LoopofHenle:CountercurrentMul&plier

• ThedescendingloopofHenle:

• Isrela9velyimpermeabletosolutes

• Ispermeabletowater

• TheascendingloopofHenle:

• Ispermeabletosolutes

• Isimpermeabletowater

• Collec9ngductsinthedeepmedullaryregionsarepermeabletourea

LoopofHenle:CountercurrentExchanger

• Thevasarectaisacountercurrentexchangerthat:

• Maintainstheosmo9cgradient

• Deliversbloodtothecellsinthearea


LoopofHenle:CountercurrentMechanism

Figure 25.14

Forma&onofDiluteUrine

• FiltrateisdilutedintheascendingloopofHenle

• Diluteurineiscreatedbyallowingthisfiltratetocon9nueintotherenalpelvis

• Thiswillhappenaslongasan9diure9chormone(ADH)oraldosteroneisnotbeingsecreted


Forma&onofDiluteUrine

• Collec9ngductsremainimpermeabletowater

• Nofurtherwaterreabsorp9onoccurs

• Sodiumandselectedionscanberemovedbyac9veandpassivemechanisms

• Urineosmolalitycanbeaslowas50mOsm(one‐sixththatofplasma)

Bi 233 Ch25 Urinary System part 2 -...

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