Mechanism of Urine formation in human beings.pdf

Useful to students

1. Mechanism of
urine formation

2. Theprocessofurineformationisknownasuropoeisis.
Urineformationisthephysiologicalprocessthrough
whichthekidneysfilterbloodtogetridofwaste
materialsandexcesssubstancesandkeepthebody’sfluid
andelectrolytebalance.
Itisanimportantprocessinthehumanbody,
isresponsibleforeliminatingwasteandmaintaininga
balanceofmanycomponentsrequiredforoverallhealth.
Urineproductionisavitalpartoftheexcretorysystem
thatinvolvesdifferentstagesthatenabletheelimination
ofmetabolicwastes,excesswaterandelectrolytesfrom
thebloodstream.

3. Urineisawateryorsemi-solidconcentrationof
wastegeneratedbybothhumansandanimalsasa
resultofphysiologicalprocesses.
Italsocontainshazardoussubstancesproducedby
excretoryorgansduringfluidcirculation.
Urineisoneofthewasteproductsthatthehuman
bodyroutinelyexcretes.
Kidneysareessentialtotheeliminationprocess.
Bothorganicandinorganicchemicalscanbefound
inurine.Urinecancontainorganiccompoundssuch
asurea,creatinineanduricacid.Chloride,sodium,
andpotassiumareexamplesofinorganicsolutes.

4. ItsacidicpHofabout6iscausedbythepresenceof
proteins.
Inhealthypeople,urobilinisprimarilyresponsible
forthecoloroftheurine.

5. Physiology / mechanism of Urine formation
•Therearethreestagesinvolvedintheprocessof
urineformation.Theyare:
1.Glomerularfiltrationorultra-filtration
2.Selectivereabsorption
3.Tubularsecretion

7. Glomerularfiltration
•Theinitialstageinthemechanismofurineformationis
glomerularfiltration
•Thistakesplacethroughthesemipermeablewallsofthe
glomerularcapillariesandBowman’scapsule.
•Theafferentarteriolessupplyingbloodtoglomerular
capsulecarriesusefulaswellasharmfulsubstances.The
usefulsubstancesareglucose,aminoacids,vitamins,
hormones,electrolytes,ionsetcandtheharmfulsubstances
aremetabolicwastessuchasurea,uricacids,creatinine,
ions,etc.
•Thediameterofefferentarteriolesisnarrowerthanafferent
arterioles.Duetothisdifferenceindiameterofarteries,
bloodleavingtheglomeruluscreatesthepressureknownas
hydrostaticpressure.

8. •Theglomerularhydrostaticpressureforcesthe
bloodtoleavestheglomerulusresultinginfiltration
ofblood.
•Acapillaryhydrostaticpressureofabout7.3kPa
(55mmHg)buildsupintheglomerulus.However
thispressureisopposedbytheosmotic
pressureoftheblood,providedmainlybyplasma
proteins,about4kPa(30mmHg),andbyfiltrate
hydrostaticpressureofabout2kPa(15mmHgin
theglomerularcapsule.
•Thenetfiltrationpressureis=
Therefore:55-(30+15)=10mmHg.

9. •Bythenetfiltrationpressureof10mmHg,bloodis
filteredintheglomerularcapsule.
•Waterandothersmallmoleculesreadilypass
throughthefiltrationslitsbutBloodcells,plasma
proteinsandotherlargemoleculesaretoolargeto
filterthroughandthereforeremaininthecapillaries.
•Thefiltratecontaininglargeamountofwater,
glucose,aminoacids,uricacid,urea,electrolytesetc
intheglomerularcapsuleisknownasnephric
filtrateofglomerularfiltrate.
•Thevolumeoffiltrateformedbybothkidneyseach
minuteiscalledtheglomerularfiltrationrate
(GFR).InahealthyadulttheGFRisabout125
mL/min,i.e.180litresoffiltrateareformedeachday
bythetwokidneys

11. Selectivereabsorption
•Reabsorptionofsomesubstanceispassive,whilesome
substancesareactivelytransported.Majorportionof
waterisreabsorbedbyOsmosis.
•Theproximalconvolutedtubule(PCT),aHenleloop,
andthedistalconvolutedtubule(DCT)arewherealmost
allofthereabsorbingprocessinthekidneyhappens.
TheProximalConvolutedTubule(PCT):
•Themajorityofreabsorptionoccursintheproximal
convolutedtubules.
•Asthefiltratepassestotherenaltubules,useful
substancesincludingsomewater,electrolytesand
organicnutrientssuchasglucose,aminoacids,vitamins
hormonesetcareselectivelyreabsorbedfromthefiltrate
backintothebloodintheproximalconvolutedtubule.

13. LoopofHenle:
•Only60–70%offiltratereachestheHenleloop.
Muchofthis,especiallywater,sodiumandchloride,
isreabsorbedintheloop,
•Thedescendinglimbreabsorbstheremaining
water.Chlorideandsodiumionsarereabsorbed
fromtheascendinglimb.
TheDistalConvolutedTubule(DCT):
•Thisstructureiscapableofreabsorbingparticular
substancesthathaveremainedinthefiltrate.
•Sothatonly15–20%oftheoriginalfiltratereaches
thedistalconvolutedtubule,Moreelectrolytesare
reabsorbedhere,especiallysodium,sothefiltrate
enteringthecollectingductsisactuallyquitedilute.

14. •Aldosteroneisoneofthehormonesthatcontrolit.
Aldosteronealsocontrolsthereabsorptionof
sodiumionsinDCT.
Collectingducts
•Themainfunctionofthecollectingductsisto
reabsorbasmuchwaterasthebodyneeds.
•Nutrientssuchasglucose,aminoacids,and
vitaminsarereabsorbedbyactivetransport.
Positivechargedionsionsarealsoreabsorbedby
activetransportwhilenegativechargedionsare
reabsorbedmostoftenbypassivetransport.Water
isreabsorbedbyosmosis,andsmallproteinsare
reabsorbedbypinocytosis.

15. Tubularsecretion
•Tubularsecretiontakesplacefromthebloodinthe
peritubularcapillariestothefiltrateintherenal
tubulesandcanensurethatwastessuchas
creatinineorexcessH+orexcessK+ionsare
activelysecretedintothefiltratetobeexcreted.
•ExcessK+ionissecretedinthetubulesandin
exchangeNa+ionisreabsorbedotherwiseit
causesaclinicalconditioncalledHyperkalemia.
•Tubularsecretionofhydrogenions(H+)isvery
importantinmaintainingnormalbloodpH.

16. •Substancessuchas,e.g.drugsincludingpenicillin
andaspirin,maynotbeentirelyfilteredoutofthe
bloodbecauseoftheshorttimeitremainsinthe
glomerulus.Suchsubstancesareclearedby
secretionfromtheperitubularcapillariesintothe
filtratewithintheconvolutedtubules.
•Thetubularfiltrateisfinallyknownasurine.Human
urineisusuallyhypertonic.

18. Composition of human urine
–Water –96%
–Urea –2%
–Uric acids, creatinine, pigments-0.3%
–Inorganic salts –2%
BadsmellisduetoUrinoid
Paleyellowcolorduetourochromeorurobillin
(whichisabreakdownproductofhaemoglobin)
Micturation:
•Theprocessoftimetotimecollectionandremoval
ofurinefromurinarybladderisknownas
micturition.Collectionofmorethan300mlofurinein
urinarybladdercreatespressureonthewall.The
pressurestimulatesthedesireforurination.

19. SignificanceofUrineFormation
•Thefollowingaresomeimplicationsofurine
formation:
Theproductionofurineaidsineliminatingwastes
likeureaandcreatinine.
Ithelpscontrolextracellularfluidvolumeby
removingextrabodilyfluid.
pHisregulatedbyurine,whichaidsinremoving
excessacidicbloodplasmacomponents.
Thekidneysmaintainosmolarityortheelectrolyte-
waterbalancebyconcentratingordilutingtheurine.

20. UrineFormationPerDay
•Thevolumeoffiltrateproducedbybothkidneys
eachminuteisknownastheglomerularfiltration
rate(GFR).
•Mengenerallycreate125mL/minoffiltrate,
whereaswomenmake105mL/min.
•Reabsorptioncauses99%ofthegeneratedfiltrate
toreturntocirculation.Therefore,ahealthyhuman
bodyonlyhas1-2litresofurineeveryday.

21. Conclusion
Inconclusion,theformationofurineistheresultofan
organizedsetofphysiologicalprocessesthattakeplace
insidethekidneys.Itactsasacrucialmechanismfor
themanagement offluidbalance,electrolyte
equilibrium,andwastedisposal,allofwhichcontribute
totheoverallhealthandequilibriumofthebody’s
internalenvironment.
Severalstepsareinvolvedinurineformation,suchas
glomerularfiltration,selectivereabsorption,andtubular
secretion.Thenephronisthebasicstructuralunitofthe
kidneyandeverynephronpassesthroughallthree
phasesofurinecreation.First,itproducesconcentrated
urinecarriedtotheurinarybladderviatheuretersand
subsequentlyexpelledfromthebodybythekidneys
andotherorgans.