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