week 6 fluid and electrolyte imbalance there are several … · - hypovolaemia or fluid volume...

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