PHYS20008–Lecturenotes–Part3Lecture26–Tubularfunction
• Dependingonwhatourbodyneedsatthetimesolutescanbefiltrated,reabsorbed,secretedorexcreted• Creatinineisatruewasteproduct
ChangesinFiltratealongnephron
• Onlyconsiderthe20%ofplasma
thatisfilteredàassumetheother80%doesn’texistso100%=alltheplasmathatgetsfiltered
• 300mOsm=isosmotic• Proximaltubule–bulkabsorption
andisosmoticabsorption(reabsorb70%ofourfluidbutitstaysthesameosmolarity)àmadepossiblebythemicrovilliintheproximaltubule
• OncepasttheloopofHenlewefurtherreabsorb20%ofthetotalfiltratebutitisonly100mOsm(hyposmoticàwehavereabsorbedalotmoresolutethanwehavefluid)–Controlledbycountercurrentmultiplier(discussedlater)
• Attheendofthecollectingductwereabsorbanother9%(approx.)–hormonecontrolled• Widerangeofosmolarityforthefluidcomingoutofthecollectingductandessentiallyendsupbeingurine–
osmolaritycanbefrom50-1200mOsmàcangreatlydiluteorconcentrateoururineo Thisisimportantdependingonourdiet(i.e.eatenalotofsaltetcandgetridofwithoutafcetingthe
3
Silverthorn: Fig 19.1i
Glomerulus &
4
Filtration Reabsorption Secretion Excretion
Water + + 1%
Na+ + + 0.5%
K+ + + + 10%
Ca2+ + + 2%
Phosphate + + 20%
Glucose + + 0%
Creatinine + 100%
Urea + + 50%
PRS:TheplasmaisMOSTsimilarinchemicalcompositiontothefluidinthe:A. ProximaltubuleB. CollectingductC. DistaltubuleD. Bowman’scapsuleE. AscendinglimboftheloopofHenle
8
Changes in Filtrate along nephron
Silverthorn : Adapted from Fig 19.2
restofthebodymuch)PathwaysforTubularReabsorption
• Transepithelialtransporto Substancescrossbothapicalandbasolateral
membraneo Hastoacrossacello Usuallyanactiveprocessbecausewehaveto
createconcentrationgradients• Paracellularpathway
o Substancespassthroughthejunctionbetweentwoadjacentcells
o CreatetightjunctionsStepsofTransepithelialTransport
• Firstneedtogothroughluminalmembrane• Either4or5
o 4–stayininterstitialfluido 5–butiffullreabsorptionneedstohappencrosstheinterstitialfluidintotheplasma
• Positionofbloodvesselsaroundthenephron• Efferentarterioleformsalargepartofourperitubularcapillary–itsitsintertwinedveryclosetothetubule
whichdecreasesdiffusiondistanceandhelpstodrivereabsorption
10
Steps of Transepithelial Transport
Sherwood: Fig 14.14
11
Silverthorn: Fig 19.1g
9Adapted from Silverthorn: Fig 19.8a
Pathways for Tubular Reabsorption
Na+
Tubularepithelium Extracellular fluidTubule lumen
Filtrate is similar to interstitial fluid.
1
! Transepithelial transport
" Substances cross both apical and basolateralmembrane
! Paracellular pathway
" Substances pass through the junction between two adjacent cells
Paracellular Pathway
Transepithelial Transport
Apical / Luminal Membrane
Basolateral Membrane
ReabsorptionintoPeritubularCapillaries
• Hydrostaticpressureismuchlowerwhenwegodowntheefferenttubulethaninourbowman’scapsule
PrinciplesgoverningTubularReabsorption
• Naisthefirstthingthatmovesinandithelpstomovealltheotherstuff• Nareabsorption=ACTIVEPROCESS• WhenNaisreabsorbed,thereisanetpositivechargeintheECFsoanionsnaturallywanttofollowà
electrochemicalgradient• NowwehaveanosmolaritygradientastheosmolarityintheECFhasincreasedduetothereabsorptionof
thesoluteso For3–i.e.inproximaltubule(howevernotinallpartsofnephronài.e.incollectingductwhichare
usuallyimpermeabletowatercanbecomepermeabletowaterdependingonwhathormonesarecirculatingandcontactingthem)
12
Reabsorption into Peritubular Capillaries
Silverthorn: Fig 19.11
16Silverthorn: Fig 19.8a
Principles governing Tubular Reabsorption
PRS:Whichone/softhesesolutesis/aredependentonactivetransporttobereabsorbed?(activereabsorption)
A. AminoacidsB. GlucoseC. SodiumD. Anions
Explanation:ActivereabsorptionThereare2typesofactivetransport:primaryandsecondary
• NaundergoesprimaryactivetransportasitisthemoleculethatistransportedbytheATPasebutitistheenergycreatedbytheNa(i.e.electrochemicalandosmoticgradients)thatforcesthemovementoftheothersolutes