Week6 FluidandElectrolyteImbalanceFluidmovementthroughthecapillarymembraneisduetopressureplacedonthemembranebytheblood(hydrostaticpressure)
- Thehydrostaticpressurepushesthefluidoutthecapillary,whereasosmoticpressurepullsfluidintothecapillary
Thereareseveralmechanismsthatcreatethebalancebetweenfluidintakeandoutput:ð Thirstð Antidiuretichormone(ADH)ð RAASð Arterialnatureticpeptide(ANP)
HypovolemiaThethirstmechanism:
- Thirstcentreisactivatedwhenthebloodvolumedropsduetowaterlossorwhenserumosmolalityincreases.Thismechanismishighlyeffectiveinregulatingextracellularsodiumlevels
- Increasedsodiumlevelsintheextracellularfluid,increaseserumosmolalityandthereforestimulatesthethirstcentre
- Anincreaseof2-3%inplasmaosmolalitytriggersthethirstcentreofthehypothalamus
- Secondarily,a10-15%dropinbloodvolumealsotriggersthirst- Thisisasignificantlyweakerstimulus
ADH- Regulatorofthewaterexcretionbythekidneys- Osmoreceptorslocatedinthehypothalamusrespondtoanincreaseinserum
osmolalityandtheassociateddecreaseinthebloodvolume;thisactivatesADHproductionandrelease
- WithincreasedwaterabsorptionthankstotheADH,theurineoutputfalls,bloodvolumeisrestored,andserumosmolalitydropsasthewaterdilutesbodilyfluid
RAAS
- Workstomaintainthebalanceofintravascularfluidandbloodpressure- Whenthereisadecreasetothebloodflowandpressuretothekidneys,special
receptorsarestimulatedinthenephronstoproducethereninenzyme- ReninconvertsangiotensintoangiotensinI- ANGIthentravelsthroughthebloodstreamintothelungs,whereitsconvertedinto
ANGII
- ANGIIhelpstoincreasetheBP,asit’sapowerfulvasoconstrictor
- Theadrenalcortexisactivatedandaldosteroneisreleasebecauseofthis- Aldosteronehelpswithsodiumandwaterretentioninthedistalnephrons,which
helpstorestorebloodvolumeinthebodyANP
- ANPisahormonereleasedbyatrialmusclecellsafterthedistentionassociatedwith
fluidoverload- Ithasaneffectonmanybodysystems,butprimarilytheRAAS
- ANPopposesthiscompensatorysystembyinhibitingreninsecretionandblockingcapabilitiesofaldosterone
- Becauseofthisprocess,thesodiumwastinganddiuresispromotionresultsinvasodilationinthebody
- Hypovolaemiaorfluidvolumedeficitisadecreaseinintravascular,interstitialand/orintracellularfluidinthebody