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

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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

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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)

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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