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
Chapter 25: Urinary System 2
TypesofNephrons
Figure 25.5b
CapillaryBedsoftheNephron
• Everynephronhastwocapillarybeds
• Glomerulus
• Ini9alfiltra9oninrenalcapsule
• Peritubularcapillaries
• NetworkingwithPCT/DCTforabsorp9onandsecre9on
• Eachglomerulusis:
• Fedbyanafferentarteriole
• Drainedbyanefferentarteriole
Chapter 25: Urinary System 3
CapillaryBedsoftheNephron
• Bloodpressureintheglomerulusishighbecause:
• Arteriolesarehigh‐resistancevessels
• Afferentarterioleshavelargerdiametersthanefferentarterioles
• Fluidsandsolutesareforcedoutofthebloodthroughouttheen9relengthoftheglomerulus
CapillaryBeds
• Peritubularbeds
• Low‐pressure,porouscapillariesadaptedforabsorp9onthat:
• Arisefromefferentarterioles
• Clingtoadjacentrenaltubules
• Emptyintotherenalvenoussystem
• Vasarecta
• Long,straightefferentarteriolesofjuxtamedullarynephrons
• Typicallyforreabsorp9on
Chapter 25: Urinary System 4
CapillaryBeds
Figure 25.5a
JuxtaglomerularApparatus(JGA)
• Wherethedistaltubuleliesagainsttheafferent(some9mesefferent)arteriole
• Consistsofjuxtaglomerularcellsandmaculadensa
• Regulatesrenalbloodpressure
• Arteriolewallshavejuxtaglomerular(JG)cells
• Enlarged,smoothmusclecells
• Havesecretorygranulescontainingrenin
• Actasmechanoreceptors
• Detectbloodpressure
Chapter 25: Urinary System 5
JuxtaglomerularApparatus(JGA)• Maculadensa
• Tall,closelypackeddistaltubulecells
• LieadjacenttoJGcells
• Func9onaschemoreceptorsorosmoreceptors
• Detectchangesinfiltratesoluteconcentra9ons
• Mesanglialcells:
• Havephagocy9candcontrac9leproper9es
• Influencecapillaryfiltra9on
JuxtaglomerularApparatus(JGA)
Figure 25.6
Chapter 25: Urinary System 6
MechanismsofUrineForma&on
• Thekidneysfilterthebody’sen9replasmavolume609meseachday
• Thefiltrate:
• Containsallplasmacomponentsexceptprotein
• Loseswater,nutrients,andessen9alionstobecomeurine
• Theurinecontainsmetabolicwastesandunneededsubstances
MechanismsofUrineForma&on
• 3steps
• Glomerularfiltra9on
• Tubularreabsorp9on
• Secre9on
Figure 25.8
Chapter 25: Urinary System 7
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
Chapter 25: Urinary System 8
GlomerularFiltra&onRate(GFR)• Thetotalamountoffiltrateformedperminutebythe
kidneys
• ~120‐125ml/min
• GFR~180L/day
• Factorsgoverningfiltra9onrateatthecapillarybedare:
• Totalsurfaceareaavailableforfiltra9on
• Filtra9onmembranepermeability
• Netfiltra9onpressure
• GFRisdirectlypropor9onaltotheNFP
• ChangesinGFRnormallyresultfromchangesinglomerularbloodpressure
GlomerularFiltra&onRate(GFR)
Figure 25.9
Chapter 25: Urinary System 9
Regula&onofGlomerularFiltra&on
• IftheGFRistoohigh:
• Neededsubstancescannotbereabsorbedquicklyenoughandarelostintheurine
• IftheGFRistoolow:
• Everythingisreabsorbed,includingwastesthatarenormallydisposedof
Regula&onofGlomerularFiltra&on
• ThreemechanismscontroltheGFR
• Renalautoregula9on(intrinsicsystem)
• Neuralcontrols
• Hormonalmechanism(therenin‐angiotensinsystem)
Chapter 25: Urinary System 10
IntrinsicControls• Renalautoregula9on
• Maintainsanearlyconstantglomerularfiltra9onrate
• Undernormalcondi9ons
• Autoregula9onentailstwotypesofcontrol
• Myogenic
• Respondstochangesinpressureintherenalbloodvessels
• Smoothmusclecontractsinresponsetostretching
• Tubuloglomerularfeedback(maculadensa‐lowosmolarity)
• Senseschangesinthejuxtaglomerularapparatus
• JGcellsreleaserenin
• Renin⇒angiotensin⇒aldosterone⇒⇑BP
ExtrinsicControls
• Whenthesympathe9cnervoussystemisatrest:
• Renalbloodvesselsaremaximallydilated
• Autoregula9onmechanismsprevail
Chapter 25: Urinary System 11
ExtrinsicControls• Understress:
• Norepinephrineisreleasedbythesympathe9cnervoussystem
• Epinephrineisreleasedbytheadrenalmedulla
• Afferentarteriolesconstrict
• reducesNFP
• Sympathe9cnervoussystemalsos9mulatestherenin‐angiotensinmechanism
• IncreasesBP
Renin‐AngiotensinMechanism
• TriggeredwhentheJGcellsreleaserenin
• Inresponsetolowosmolarity
• ReninactsonangiotensinogentoreleaseangiotensinI
• AngiotensinIisconvertedtoangiotensinII
• AngiotensinII:
• Causesmeanarterialpressuretorise
• S9mulatestheadrenalcortextoreleasealdosterone
• Asaresult,bothsystemicandglomerularhydrosta9cpressurerise
Chapter 25: Urinary System 12
ReninRelease
• Reninreleaseistriggeredby:
• ReducedstretchofthegranularJGcells
• S9mula9onoftheJGcellsbyac9vatedmaculadensacells
• Directs9mula9onoftheJGcellsviaβ1‐adrenergicreceptorsbyrenalnerves
• AngiotensinII
TubularReabsorp&on• Transepithelialprocess
• Tubulecontentsarereturnedtotheblood
• Transportedsubstancesmovethroughthreemembranes
• Luminalandbasolateralmembranesoftubulecells
• Endotheliumofperitubularcapillaries
Chapter 25: Urinary System 13
TubularReabsorp&on
• Reducesfiltratetourine
• Urea,uricacid,andmetabolicwastes
• 1‐2L/day
• From180L/dayfiltrate!
• Allorganicnutrientsarereabsorbed
• Waterandionreabsorp9onishormonallycontrolled
• Inresponsetobodilyneeds
• Reabsorp9onmaybeanac9ve(requiringATP)orpassiveprocess
• Roughly99%offiltrate
RoutesofWaterandSoluteReabsorp&on
Figure 25.11
Chapter 25: Urinary System 14
Reabsorp&onbyPCTCells• Ac9vepumpingofNa+drivesreabsorp9onof:
• Waterbyosmosis,aidedbywater‐filledporescalledaquaporins
• Ca9onsandfat‐solublesubstancesbydiffusion
• Organicnutrientsandselectedca9onsbysecondaryac9vetransport
Reabsorp&onbyPCTCells
Figure 25.12
Chapter 25: Urinary System 15
NonreabsorbedSubstances
• Atransportmaximum(Tm):
• Reflectsthenumberofcarriersintherenaltubulesavailable
• Existsfornearlyeverysubstancethatisac9velyreabsorbed
• Whenthecarriersaresaturated,excessofthatsubstanceisexcreted
NonreabsorbedSubstances
• Substancesarenotreabsorbedifthey:
• Lackcarriers
• Arenotlipidsoluble
• Aretoolargetopassthroughmembranepores
• Urea,crea9nine,anduricacidarethemostimportantnonreabsorbedsubstances
Chapter 25: Urinary System 16
• 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)
Chapter 25: Urinary System 17
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
Chapter 25: Urinary System 18
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
Chapter 25: Urinary System 19
CountercurrentMechanism
• Interac9onbetweentheflowof…
• FiltratethroughloopofHenle
• countercurrentmul9plier
• Bloodthroughthevasarecta
• countercurrentexchanger
• Soluteconcentra9onintheloopofHenle
• Rangesfrom300mOsmto1200mOsm
• Dissipa9onofthemedullaryosmo9cgradientispreventedbecausethebloodinthevasarectaequilibrateswiththeinters99alfluid
Osmo&cGradientintheRenalMedulla
Figure 25.13
Chapter 25: Urinary System 20
LoopofHenle:CountercurrentMul&plier
• ThedescendingloopofHenle:
• Isrela9velyimpermeabletosolutes
• Ispermeabletowater
• TheascendingloopofHenle:
• Ispermeabletosolutes
• Isimpermeabletowater
• Collec9ngductsinthedeepmedullaryregionsarepermeabletourea
LoopofHenle:CountercurrentExchanger
• Thevasarectaisacountercurrentexchangerthat:
• Maintainstheosmo9cgradient
• Deliversbloodtothecellsinthearea
Chapter 25: Urinary System 21
LoopofHenle:CountercurrentMechanism
Figure 25.14
Forma&onofDiluteUrine
• FiltrateisdilutedintheascendingloopofHenle
• Diluteurineiscreatedbyallowingthisfiltratetocon9nueintotherenalpelvis
• Thiswillhappenaslongasan9diure9chormone(ADH)oraldosteroneisnotbeingsecreted
Chapter 25: Urinary System 22
Forma&onofDiluteUrine
• Collec9ngductsremainimpermeabletowater
• Nofurtherwaterreabsorp9onoccurs
• Sodiumandselectedionscanberemovedbyac9veandpassivemechanisms
• Urineosmolalitycanbeaslowas50mOsm(one‐sixththatofplasma)