chapter 26 fluid, electrolyte, and acid base balance · chapter 26: fluid, electrolyte, and...
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Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 1
Chapter26
Bi233
Fluid,Electrolyte,andAcidBaseBalance
BodyWaterContent
• Largestcomponentofthebody
• Infantshavelowbodyfat,lowbonemass,andare73%ormorewater
• Healthymalesareabout60%water;healthyfemalesarearound50%
• Thisdifferencereflectsfemales’:
• Higherbodyfat
• Smalleramountofskeletalmuscle
• Totalwatercontentdeclinesthroughoutlife
• Inoldage,onlyabout45%ofbodyweightiswater
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 2
FluidCompartments• Wateroccupiestwo(orthree)mainfluidcompartments
• Intracellularfluid(ICF)–abouttwothirdsbyvolume,containedincells
• ~25L
• Extracellularfluid(ECF)–consistsoftwomajorsubdivisions
• ~15L
• IntersXXalfluid(IF)–fluidinspacesbetweencells
• 80%ofECF~10‐12L
• Plasma–thefluidporXonoftheblood
• 20%ofECF~3‐5L
FluidCompartments
Figure 26.1
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 3
Composi4onofBodyFluids• Wateristheuniversalsolvent
• Solutesarebroadlyclassifiedinto:
• Electrolytes–ionic(caXonsoranions)
• Inorganicsalts,allacidsandbases,andsomeproteins
• YieldtwoormoresoluteparXcleswhendissolvedinsoluXon
• NaCl⇒Na++Cl‐
• CaCl2⇒Ca+++2Cl‐
• Nonelectrolytes–covalent
• Glucose,lipids,creaXnine,andurea
• ElectrolyteshavegreaterosmoXcpowerthannonelectrolytes
• WatermovesaccordingtoosmoXcgradients
ExtracellularandIntracellularFluids
• EachfluidcompartmentofthebodyhasadisXncXvepaaernofelectrolytes
• Extracellularfluidsaresimilar(exceptforthehighproteincontentofplasma)
• Na+primarycaXon
• AlsoCa2+
• Cl‐primaryanion
• AlsoHCO3‐
• PlasmahasmanymoreproteinandsodiumanionsbutfewerchlorideionsthanintersXXalfluid
• Moreproteinsinintracellularthanextracellularfluid
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 4
ExtracellularandIntracellularFluids
• Intracellularfluidshavelowsodiumandchloride
• PotassiumprimarycaXon
• AlsomagnesiumMg2+
• Phosphateprimaryanion
• Alsohydrogenphosphate
• HPO4‐
• SodiumandpotassiumconcentraXonsinextra‐andintracellularfluidsarenearlyopposites
• DuetocellularATP‐dependentsodium‐potassiumpumps
ExtracellularandIntracellularFluids
• Proteins,phospholipids,cholesterol,andneutralfatsaccountfor:
• 90%ofthemassofsolutesinplasma
• 60%ofthemassofsolutesinintersXXalfluid
• 97%ofthemassofsolutesintheintracellularcompartment
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 5
ElectrolyteComposi4onofBodyFluids
Figure 26.2
FluidMovementAmongCompartments• FluidexchangebetweencompartmentsisconXnuous
• RegulatedbyosmoXcandhydrostaXcpressures
• AnychangeinsoluteconcentraXonleadstonetwatermovement
• ECFgreaterimpactthanICF
• MovementbetweenplasmaandintersXXalfluid(withinECF)solutesmoveinbothdirecXons
• Dependentuponfourpressures
• CapillaryhydrostaXcpressure
• CapillaryosmoXcpressure
• IntersXXalfluidhydrostaXcpressure
• IntersXXalfluidosmoXcpressure
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 6
ExtracellularandIntracellularFluids• IonfluxesarerestrictedandmoveselecXvelybyacXve
transport
• Nutrients,respiratorygases,andwastesmoveunidirecXonally
• Two‐wayosmoXcwatermovement
• DependentonNa+movementoutofcellandK+intothecell
• AlsoaldosteroneandADHsecreXon
• LowNa+concentraXoninintersXXalfluid(lowosmoXcpressure),watermayleavethecellbutthenreturnresulXnginwaterintoxicaXon
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 7
Con4nuousMixingofBodyFluids
Figure 26.3
WaterBalanceandECFOsmolality
• Toremainproperlyhydrated,waterintakemustequalwateroutput
• Waterintakesources
• Ingestedfluid(60%)andsolidfood(30%)
• MetabolicwaterorwaterofoxidaXon(10%)
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 8
WaterBalanceandECFOsmolality
• Wateroutput
• Urine(60%)andfeces(4%)
• Insensiblelosses(28%),sweat(8%)
• IncreasesinplasmaosmolalitytriggerthirstandreleaseofanXdiureXchormone(ADH)
WaterIntakeandOutput
Figure 26.4
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 9
Regula4onofWaterIntake
• ThehypothalamicthirstcenterissXmulated:
• Byadeclineinplasmavolumeof10%–15%
• Byincreasesinplasmaosmolalityof1–2%
• ⇓fluidinintersXXumandplasmacells‐⇓saliva‐drymouthandthroat/sXmulaXonofosmoreceptorsandcrenaXonofsupraopXcnuclei‐thirst
Regula4onofWaterIntake
• Thirstisquenchedassoonaswebegintodrinkwater
• Feedbacksignalsthatinhibitthethirstcentersinclude:
• Moisteningofthemucosaofthemouthandthroat
• AcXvaXonofstomachandintesXnalstretchreceptors
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 10
Regula4onofWaterIntake:ThirstMechanism
Figure 26.5
Regula4onofWaterOutput• Obligatorywaterlossesinclude:
• Insensiblewaterlossesfromlungsandskin(28%)
• Waterthataccompaniesundigestedfoodresiduesinfeces(4%),urine(60%),sweaXng(8%)
• Obligatorywaterlossreflectsthefactthat:
• Kidneysexcrete900‐1200mOsmofsolutestomaintainbloodhomeostasis
• Urinesolutesmustbeflushedoutofthebodyinwater
• ~1500ml
• FluidsmayalsobelostduetovomiXng,diarrhea,extensiveskinburns,increasedbloodpressure,ordiet
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 11
DisordersofWaterBalance:Dehydra4on• Waterlossexceedswaterintake
• BodyisinnegaXvefluidbalance
• Causesinclude:
• hemorrhage,severeburns,prolongedvomiXngordiarrhea,profusesweaXng,waterdeprivaXon,anddiureXcabuse
• Signsandsymptoms:
• Coaonmouth,thirst,dryflushedskin,andoliguria
• Treatwithsaltpills
• ProlongeddehydraXonmayleadtoweightloss,fever,andmentalconfusion
• Otherconsequencesincludehypovolemicshockandlossofelectrolytes
Figure 26.7a
DisordersofWaterBalance:Dehydra4on
Excessive loss of H2O fromECF
1 2 3ECF osmoticpressure rises
Cells lose H2Oto ECF byosmosis; cellsshrink
(a) Mechanism of dehydration
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 12
• HypotonichydraXon
• CanleadtodiluXonalhyponatremia,orwaterintoxicaXon
• ECFisdiluted–sodiumcontentisnormalbutexcesswaterispresent
• Orsodiumislowandwaterisotherwisenormal
• Causes:
• Renalinsufficiencyoranextraordinaryamountofwateringestedquickly
• Symptoms:
• Muscleweakness,headaches,hypotension,tachycardia,circulatoryshock
• Severe:mentalconfusion,stupor,coma
Disorders:HypotonicHydra4on
Figure 26.7b
Disorders:HypotonicHydra4on
Excessive H2O entersthe ECF
1 2 ECF osmoticpressure falls
3 H2O moves intocells by osmosis;cells swell
(b) Mechanism of hypotonic hydration
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 13
DisordersofWaterBalance:Edema
• AtypicalaccumulaXonoffluidintheintersXXalspace
• LeadingtoXssueswelling
• Requires30%increasetobedetectable
• Symptoms:
• IncreasedfluidinintersXXalspace
• ImpairedXssuefuncXonduetoincreaseddistancebetweencapillariesandcells
• Diminishesperfusion
DisordersofWaterBalance:Edema• Causes:
• Anythingthatincreasesflowoffluidsoutofthebloodstreamorhinderstheirreturn
• Factorsthatacceleratefluidlossinclude:
• Increasedcapillarypermeabilityasinallergic/inflammatoryresponses
• Increasedintra‐capillarypressure(hydrostaXc)
• DuetoincreasedarterialdilaXon,blockageofveins,increasedvenouspressure
• IncreasedintersXXalfluidcolloidosmoXcpressureduetoblockedlymphaXcs
• ExtraproteinsinintersXXalspaceandreducXoninplasma
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 14
Edema
• Hinderedfluidreturn
• UsuallyreflectsanimbalanceincolloidosmoXcpressures
• Mayresultinlowbloodvolume/pressure
• Hypoproteinemia–lowlevelsofplasmaproteins
• Forcesfluidsoutofcapillarybedsatthearterialends
• Fluidsfailtoreturnatthevenousends
• ResultsfromproteinmalnutriXon,liverdisease,orglomerulonephriXs
ElectrolyteBalance
• Electrolytesaresalts,acids,andbases
• Butelectrolytebalanceusuallyrefersonlytosaltbalance
• Saltsareimportantfor:
• ProvisionofessenXalminerals
• ControlofosmoXcmovementbetweencompartments
• Maintainacid‐basebalance
• Controllingfluidmovements
• SaltsenterthebodybyingesXonandarelostviaperspiraXon,feces,andurine
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 15
SodiuminFluidandElectrolyteBalance• SodiumholdsacentralposiXoninfluidandelectrolyte
balance
• Sodiumsalts:
• Accountfor90‐95%ofallsolutesintheECF
• Contribute280mOsmofthetotal300mOsmECFsoluteconcentraXon
• SinglemostabundantcaXonintheECF
• Alongwithpotassium,criXcalfornerveimpulsetransmission
• PrimarycaXonexerXngsignificantosmoXcpressure
• Effectsallthreecompartments
• Assodiumbicarbonate,centraltopHbuffering
Regula4onofSodiumBalance:Aldosterone
• SodiumreabsorpXon
• 65%ofsodiuminfiltrateisreabsorbedintheproximaltubules
• 25%isreclaimedintheloopsofHenle
• Whenaldosteronelevelsarehigh
• AllremainingNa+isacXvelyreabsorbed
• Waterfollowssodium
• IftubulepermeabilityhasbeenincreasedwithADH
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 16
Regula4onofSodium:AtrialNatriure4cPep4de
• Atrialcellsdetectincreasedbloodpressure
• Detectedasstretching
• ReleaseANP
• ANPtravelstothekidneytotriggersodiumexcreXon
• InhibitsreleaseofADH,aldosterone,angiotensin,andrenin
Figure 26.10
MechanismsandConsequencesofANPRelease
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 17
Regula4onofSodium:ADH
• IfsodiumconcentraXonislow
• HypotonicsoluXon
• ADHreleaseisinhibited
• DiluteurineproducXonincreases
• ADHdictatesattheDCTandcollecXngductswhetherurinebecomesdiluteorconcentrated
CardiovascularSystemBaroreceptors
• Baroreceptors
• Alertthebrainofincreasesinbloodvolume(henceincreasedbloodpressure)
• Inheart,aorta,caroXdarteries
• Sendmessagetohypothalamus
• SympatheXcnervoussystemimpulsestothekidneysdecline
• Afferentarteriolesdilate
• GlomerularfiltraXonraterises
• Sodiumandwateroutputincrease
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 18
TypicalSodiumLevels• Normal
• 135‐145mEq/L
• Abnormal
• Hyponatremia<135mEq/L
• Duetoincreased
• waterintake
• ADHsecreXon
• Hypernatremia>145mEq/L
• Diabetesinsipidus
• Increasedwaterlossinurine
Figure 26.9
MaintenanceofBloodPressureHomeostasis
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 19
Regula4onofPotassiumBalance
• PrimaryintercellularcaXon
• HelpsmaintainfluidvolumeincellsandcontrolpH
• ThroughexchangewithNa+orH+
• HighextracellularK+leadstoacidosis
• ResultsinhighH+concentraXoninsidecells
• LowextracellularK+leadstoalkalosis
• K+hasmovedintocells
• ThereforeH+hasbeenpumpedout
Regula4onofPotassiumBalance
• RelaXveICF‐ECFpotassiumionconcentraXonaffectsacell’sresXngmembranepotenXal
• ExcessiveECFpotassiumdecreasesmembranepotenXal
• ToolialeK+causeshyperpolarizaXonandnonresponsiveness
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 20
Regula4onofPotassiumBalance
• Hyperkalemiaandhypokalemiacan:
• DisruptelectricalconducXonintheheart
• Leadtosuddendeath
• Hydrogenionsshioinandoutofcells
• LeadstocorrespondingshiosinpotassiumintheoppositedirecXon
• InterfereswithacXvityofexcitablecells
InfluenceofAldosterone
• AldosteronesXmulatespotassiumionsecreXonbyprincipalcells
• IncorXcalcollecXngducts
• ForeachNa+reabsorbed,aK+issecreted
• IncreasedK+intheECFaroundtheadrenalcortexcauses:
• Releaseofaldosterone
• PotassiumsecreXon
• PotassiumcontrolsitsownECFconcentraXon
• ViafeedbackregulaXonofaldosteronerelease
Chapter 26: Fluid, Electrolyte, and Acid-Base Balance 21
TypicalPotassiumLevels
• Normal
• 3‐5mEq/L
• Abnormal
• Hypokalemia<3mEq/L
• Duetodiarrhea
• Hyperkalemia>5mEq/L
• Fromsevereburnsorrenalfailure