fisiología en ambientes calurosos

7/30/2019 fisiologa en ambientes calurosos

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HumanAdaptationto Hot Environments

C.BruceWenger,M.D.,Ph.D.,Research Pharmacologist

USArmyResearchInstituteofEnvironmental MedicineNatick,Massachusetts01760-5007

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HumanAdaptationtoHotEnvironments(TMM)/20/97I .MPORTANCEOF TISSUETEMPERATURE II.ODYTEMPERATURESANDHEAT TRANSFERINTHE BODY

A.oretemperatureB.kin temperature

I I I .ALANCE BETWEEN HEAT PRODUCTION AND HEAT LOSSA.eatproduction

1 .etabolicrateandsitesofheatproductionatrest2.easurementofmetabolicrate3 .keletalmusclemetabolismandmuscularworkB.eatexchangewiththeenvironment1 .lothing

C.eatstorageIV .EAT DISSIPATION

A.vaporationB.kincirculationanddry (convectiveandradiative)heatexchange1 .ole ofskinbloodflowinheattransfer2.ympatheticcontrolofskincirculation V .HERMOREGULATORYCONTROL

A.ehavioralthermoregulationB.hysiologicalthermoregulationC.ntegrationofthermalinformation1 .elationofeffectorsignalstothermoregulatory setpoint

2.on-thermalinfluencesonthermoregulatoryresponses


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HumanAdaptationtoHotEnvironments(TMM)/20/97D.hysiologicalandpathologicalchangestothethermoregulatorysetpointE.eripheralmodificationofskinvascularandsweatglandresponses

VI.HERMOREGULATORYRESPONSESDURING EXERCISEA.hallenge ofexerciseintheheattocardiovascularhomeostasis1 .mpairmentofcardiacfilling2.ompensatorycardiovascularresponses

V I I .ACTORS THATALTERHEATTOLERANCEA.eatAcclimatization

1 .cquisitionandloss2.hangesinthermoregulatory responses3 .on-thermoregulatorychanges4.ffectsonheatdisorders

B.hysicalfitness,age,drugs,anddiseaseVIII.EFERENCESIV .UGGESTEDREADING


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HumanAdaptationtoHotEnvironments(TMM)/20/97IMPORTANCEOFTISSUETEMPERATURE

Extremetemperaturesinjuretissuedirectly.Aprotein'sbiologicalactivitydependsonthelocationofelectricalchargesinthemoleculeandonitsoverallconfiguration.Many physicochemicalprocessescanalteraprotein'sconfigurationandchargedistribution,andthuschangeitsactivity,withoutaffecting thesequenceofaminoacids.Suchalterationofaproteiniscalleddenaturation:andby inactivatingacell'sproteins,denaturationinjuresor killsthecell.Hightemperature candenatureproteins,andafamiliarillustrationof thiseffectisthecoagulationofthealbumininthewhiteof acookedegg.Iflivingtissueisheated,injuryoccursattemperatureshigherthanabout45 C ,whichisalsothetemperatureatwhichheatingtheskincausespain.The degreeofinjurydependsonboth temperatureanddurationoftheheating1 .

Asawater-basedsolution freezes,crystalsofpureice form.Thusallthedissolvedsubstancesareleftbehindintheliquidwhichhasno tyetfrozen,andwhichbecomesmoreandmoreconcentratedasmoreiceforms.Freezingdamagescellsthroughtw omechanisms.First,icecrystalsthemselvesprobablydisruptthecellmembranesmechanically.Second,theincreaseinsoluteconcentrationofthecytoplasmasice formsdenaturestheproteinsby removingtheirwaterofhydration,by increasing theionicstrengthofthecytoplasm,andby otherchangesinthephysicochemicalenvironmentinthecytoplasm.

Mammals,includinghumanbeings,arehomeotherms,or warm-blooded animals,andregulatetheirinternalbodytemperatureswithinanarrow bandnear37C(Fig.),inspiteofwidevariationsinenvironmentaltemperature.Tissuesandcellscan toleratetemperaturesfrom justabovefreezingtonearly 45 Carangefarwiderthanthelimitswithinwhichhomeothermsregulatebodytemperature.Whatbiologicaladvantagedohomeothermsgainby maintainingsuchastablebody temperature?

Temperatureisafundamentalphysicochemicalvariablethatprofoundly affectsmany biologicalprocesses,boththroughspecificeffectsonsuchspecializedfunctionsaselectricalpropertiesandfluidity ofcellmembranes,andthroughageneraleffectonmostchemicalreactionrates.Mostreactionratesvaryapproximatelyasan exponentialfunctionof temperature(T )withinthephysiologicalrange,andincreasingTby 10Cincreasesthereactionrateby afactorof2to3 .oranyreaction,theratioof thereactionratesattw otemperatures0C apartiscalledtheQ _ ,0forthatreaction,andtheeffectoftemperatureonreaction rateiscalledtheQ ,neffect.Theconceptof


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HumanAdaptationtoHotEnvironments(TMM)/20/97Q ]0isoftengeneralizedtoapplytoagroupofreactions thatare thoughtofascomprisingaphysiologicalprocessbecausetheyshareameasurableoveralleffectsuchas02consumption.heeffectofbody temperatureon metabolicprocessesisclinically importantincaringforpatientswithhighfeverswho are receivingfluidandnutritionintravenously,andanoft-usedrulestatesthateachC offeverincreasesapatient'sfluidandcalorieneeds3% 2.

BODY TEMPERATURES AND HEAT TRANSFER I N THE BODY

CoreTemperature

The bodyisdividedintoawarminternalcoreandanoutershellYFig.2) ,whosetemperatureisstronglyinfluencedby theenvironment.Althoughshelltemperatureisno tregulatedwithinnarrow limitstheway internalbody temperatureis,thermoregulatoryresponsesdo stronglyaffectthetemperatureof theshell,andespeciallyitsoutermostlayer,theskin.Theshell'sthicknessdependson theenvironmentandtheneedtoconservebodyheat.Inawarmenvironment,theshellmay belessthan mthick;bu tinasubjectconservingheatinacoldenvironment,itmay extendseveralcentimetersbelowtheskin.Theinternalbodytemperaturethatisregulatedisthetemperatureofthevitalorgansinsidetheheadandtrunkwhich,togetherwithavariableamountofothertissue,comprisethewarminternalcore.

Thoughheatisproducedthroughoutthebody,itislostonlyfrom tissuesincontactwiththeenvironment,mostlyskinandrespiratory passages.Sinceheatflowsfromwarmerregionstocoolerregions,thegreatestheatflowswithinthebodyare thosefrommajorsitesofheatproductiontotherestof thebody,andfromcoretoskin.Withinthebody,heatistransportedby twomeans:conduction through thetissues;andconvectionby theblood,theprocessby whichflowingbloodcarriesheatfromwarmertissuestocoolertissues.

Heatflowby conductionisproportionaltothethermalconductivity ofthetissues,thechangeoftemperaturewithdistanceinthedirectionofheatflow,andthearea(perpendiculartothedirectionofheatflow)throughwhichthe heatflows(seeSanteeandMatthew).AsTable hows,thetissuesare ratherpoorheatconductors.


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HumanAdaptationtoHotEnvironments(TMM)/20/97Heatflowby convectiondependsontherateofbloodflowandthetemperaturedifferencebetweenthe

tissueandthebloodsupplying thetissue.Becausethecapillarieshavethinwallsand,takentogether,alargetotalsurfacearea,thecapillarybedsare thesiteswhereheatexchangebetweentissueandbloodismostefficient.

Sincetheshellliesbetweenthecoreandtheenvironment,allheatleaving thebody viatheskinmustfirstpassthroughtheshell.Thustheshellinsulatesthecorefromtheenvironment.Inacoolsubjectskinbloodflowislow,sothatcore-to-skin heattransferisdominatedby conduction;andthesubcutaneousfa tlayeraddstotheinsulationvalueoftheshell,becauseitaddstothethicknessoftheshellandbecausefa thasaconductivityonlyabout0.4timesthatofdermisormuscle(Table1).Inawarmsubject,ontheotherhand,theshellisrelatively thin,andthusprovideslittleinsulation.Furthermoreawarmsubject'sskinbloodflow ishigh,sothatheatflowfromthecoretotheskinisdominatedby convection.Inthesecircumstancesthesubcutaneousfa tlayerwhichaffectsconductionbu tno tconvectionhaslittleeffecton heatflowfromcoretoskin.

Coretemperaturevariesslightlyfromonesitetoanotherdependingonsuchlocalfactorsasmetabolicrateandbloodsupply,andthetemperaturesofneighboringtissues.Howeverthenotionofasingleuniform coretemperatureisausefulapproximation, sincetemperatures atdifferentplacesinthecoreareallclosetothetemperatureof thecentralblood,andtendtochangetogether.Siteswherecoretemperatureismeasuredclinically includethemouth,thetympanicmembrane,therectum,andoccasionally theaxilla.Nositeisidealineveryrespect,andeachhascertaindisadvantagesandlimitations(Seetextbox).

[INSERTst(CLINICALMEASUREMENTOF TEMPERATURE) AND2nd(BRAINEMPERATURE)TEXT BOXESABOUTHERE.]

The valueof98.6F thatisoftengivenasthenormallevelofbodytemperaturemay suggestthatbodytemperatureisregulatedsoprecisely thatitisno tallowedtodeviateevenafew tenthsof adegree.Infact,98.6FissimplytheFahrenheitequivalentof37C;and,asFig. ndicates,bodytemperaturedoesvary.Theeffectsofheavyexerciseandfeverarequitefamiliar.Inadditionvariationamongindividualsandsuchfactors4astimeofday(Fig.3 ),phaseofthemenstrualcycle5'6,andacclimatizationtoheatcan causedifferencesofuptoabout C incoretemperatureinhealthysubjectsatrest.The thermoregulatory systemreceivesinformationaboutthelevelofcore


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HumanAdaptationtoHotEnvironments(TMM)/20/97temperatureprovidedby temperature-sensitive neuronsandnerveendingsintheabdominalviscera,greatveins,spinalcord,andespecially thebrain78.Laterinthechapterwe discusshow thethermoregulatory systemprocessesthisinformation,andusesittomaintaincoretemperaturewithinanarrow range.

SkinTemperature

Skintemperatureisimportantinheatexchangeandthermoregulatorycontrol.Mostheatisexchangedbetweenthebodyandtheenvironmentattheskinsurface.Skintemperatureismuchmorevariablethancoretemperature,andisaffectedby thermoregulatoryresponsessuchasskinbloodflow andsweatsecretion,by thetemperaturesofunderlyingtissues,andby environmentalfactorssuchasair temperature,airmovement,andthermalradiation.Skintemperature,inturn,isoneofthemajorfactorsdetermining heatexchangewiththeenvironment.For thesereasons,skintemperatureprovidesthethermoregulatorysystem withimportantinformationabouttheneedtoconserveorlosebodyheat.Manybarenerveendingsjustundertheskinarevery sensitivetotemperature.Dependingontherelationofdischargeratetotemperature,they are classifiedaseitherwarmor coldreceptors 7 - 9(Fig.4).rom therelativedensitiesofcold-andwarm-sensitivespotsinhumanskin'0,coldreceptorsappeartobe roughlytentimesasnumerousaswarm receptorssince,asarule,asinglecoldorwarmfiberinnervatesasinglecold-orwarm-sensitivespot".Withheating oftheskin,warmreceptorsrespond withatransientburstofactivity,whilecoldreceptorsrespondwithatransientsuppression;andthereversehappenswithcooling.Thesetransientresponsesatthebeginningofheatingorcoolinggivethecentralintegratoralmostimmediateinformationaboutchangesinskintemperature,andmayexplain,forexample,theintense,briefsensationofbeing chilledthatoccursduringaplungeintocoldwater.

Skintemperatureusuallyisno tuniformoverthebodysurface,sothatameanskintemperature(T^)isfrequently calculatedfromskintemperaturesmeasuredatseveralselectedsites,usuallyweightingthetemperaturemeasuredateachsiteaccordingtothefractionofbodysurfaceareathatitrepresents.Itwouldbeprohibitivelyinvasiveanddifficulttomeasureshelltemperaturedirectly.Instead,therefore,skintemperaturealsoiscommonlyusedalongwithcoretemperaturetocalculateameanbodytemperatureandtoestimatechangesintheamountofheatstoredinthebody.


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HumanAdaptationtoHotEnvironments(TMM)/20/97BALANCEBETWEENHEATPRODUCTIONAND HEATLOSS

Allanimalsexchangeenergywiththeenvironment.Someenergyisexchangedasmechanicalwork,bu tmostisexchangedasheat,by conduction,convection,andradiation;andaslatentheatthroughevaporationor (rarely)condensationofwater(Fig.5).If thesumofenergy productionandenergy gainfromtheenvironmentdoesnotequalenergyloss,theextraheatis"stored"in,orlostfrom,thebody.Thisissummarizedintheheatbalanceequation

M=E+ R + C + K+W+S 1) whereMismetabolicrate;Eisrateofheatlossby evaporation;R andCare ratesofheatlossby radiationandconvection,respectively;Kistherateofheatlossby conduction(onlytosolidobjectsinpractice,asexplained later);W israteofenergylossasmechanicalwork;andSisrateofheatstorageinthebody,whichtakestheformofchangesintissuetemperatures'213.

Misalwayspositive,bu ttheothertermsineq. may beeitherpositiveornegative.E,R ,C,K,andW are positiveifthey representenergylossesfromthebody,andnegativeif they representenergygains.WhenS = 0,thebodyisinheatbalanceandbodytemperatureneitherrisesnorfalls.Whenthebodyisnotinheatbalance,itsmeantissuetemperatureincreasesifSispositive,anddecreasesif Si snegative.Thiscommonly occursonashort-term basisandlastsonlyuntilthebodyrespondstochangesinitstemperature withthermoregulatoryresponsessufficienttorestorebalance;bu tif thethermalstressistoogreatfo rthethermoregulatory systemtorestorebalance,thebodywillcontinuetogainorloseheat,untileitherthestressdiminishessothatthethermoregulatorysystemcanagainrestorethebalance,ordeathoccurs.[INSERT 3 rdTEXTBOX(HEAT UNITS)ABOUTHERE.]HeatProduction

Metabolicenergyisrequiredforactivetransportviamembranepumps,formuscularwork,andfo rchemicalreactionssuchasformationofglycogenfromglucoseandproteinsfromaminoacids,whoseproductscontainmoreenergythanthematerials thatenteredintothereaction.Mostof theenergyusedintheseprocessesis


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HumanAdaptationtoHotEnvironments(TMM)/20/97transformedintoheatwithinthebody.The transformationmaybealmostimmediate,aswithenergyusedinactivetransportorwithheatproducedasaby-productofmuscularcontraction.Inotherprocessestheconversionofenergy toheatisdelayed,aswhentheenergy thatwasusedtoformglycogenorproteinisreleasedasheatwhentheglycogenisconvertedbackintoglucose,ortheproteinbackintoaminoacids.

MetabolicRateandSitesofHeatProductionat Rest

Metabolicrateatrestvarieswithbodysize,andisapproximately proportionaltobodysurfacearea.Inafasting youngmanitisabout45W/m2(Fig.6)(81W or70kcal/hfo r1.8m2bodysurfacearea(Table3 ), correspondingtoan02consumptionofabout24 0ml/min.)Atrestthetrunkvisceraandbrainaccountfo rabout70% ofenergyproduction,eventhoughthey comprise only about3 6% ofthebodymass(Table2) .Al ltheheatrequiredtomaintainheatbalanceatcomfortableenvironmentaltemperaturesissupplied asaby-productofmetabolicprocessesthatserveotherfunctions,thoughinthecoldsupplementalheatproduction may be elicitedto maintainheatbalance.

Factorsotherthanbodysizethataffectmetabolismatrestincludesexandage(Fig.6) ,hormones,anddigestion.Anon-pregnantwoman'smetabolicrateis5 to0% lowerthanthatof amanofthesameageandsurfacearea,probablybecausethefemalebodyincludesahigherproportionoffat,atissuewithalow metabolicrate.(Howeverthegrowingfetus'senergyrequirementsincreaseapregnantwoman'smeasuredmetabolicrate.)

Catecholaminesandthyroxinearethehormoneswiththelargesteffectonmetabolicrate.Catecholaminesstimulatemanyenzymesystems,thusincreasingcellularmetabolism;andhypermetabolismoccursinsomecasesofpheochromocytoma, asecretingtumorof theadrenalmedulla.Thyroxinemagnifiesthemetabolicresponsetocatecholaminesandstimulatesoxidationinthemitochondria.Hyperthyroidismmaydoublethemetabolicrateinseverecases,althoughanincreaseto45% abovenormalismoretypical;andmetabolic rateistypically 25% below normalinhypothyroidism,bu tmaybe45% below normalwithtotallackofthyroxine.

Metabolicrateatrestincreasesafteramealasaresultof thethermiceffectoffood(or"specificdynamicaction",theolderterm).heincreasevariesaccording tothecompositionofthemealandthephysiologicalstate,


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HumanAdaptationtoHotEnvironments(TMM)/20/97includingthelevelofnutrition,of thesubject14.nawell-nourishedsubjecttheincreaseistypically 10-20%.he effectlastsseveralhoursandappearstobeassociatedwithprocessingtheproductsofdigestionby theliver.

Measurementof MetabolicRate

Heatexchangewiththeenvironmentcanbemeasureddirectly withahumancalorimeter15,aspeciallyconstructedinsulatedchamberthatallowsheattoleaveonlyintheair ventilatingthechamberor,often,inwaterflowingthroughaheatexchangerinthechamber.Fromaccuratemeasurementsof theflowofair andwater,andtheirtemperaturesasthey enterandleavethechamber,onecancomputethesubject'sheatlossby conduction,convectionandradiation;andfrommeasurementsof themoisturecontentofair enteringandleavingthechamberonecanalsodetermineheatlossby evaporation.Directcalorimetrv .asthistechniqueiscalled,issimpleinconcept,butdifficultandcostlyinpractice.Thereforemetabolicrateisoftenestimatedby indirectcalorimetrv16basedon measurementsof02consumption,sincevirtuallyallenergyavailabletothebodydependsultimately on reactionsthatconsume02.Consumptionofoneliterof02isassociated withreleaseof21 .lk J(5.05kcal)if thefueliscarbohydrate,9.8kJ(4.74kcal)ifthefuelisfat,and8.6kJ(4.46kcal)if thefuelisprotein.For metabolismofamixeddiet,anaveragevalueof20.2kJ(4.83kcal)perliterof02isoftenusedTable3 ).TheratioofC02produced to02consumedinthetissues,calledtherespiratoryquotient(RQ),is.0fo roxidationofcarbohydrate,0.71or oxidationoffat,and0.80for oxidationofprotein.Inasteady statewhereC02isexhaledatthesameratethatitisproducedinthetissues,RQ isequaltotherespiratoryexchangeratio,R;andtheaccuracy ofindirectcalorimetrycanbeimprovedby alsodeterminingR,andeitherestimatingtheamountofproteinoxidizedusually smallcomparedtofat andcarbohydrateor calculatingitfromurinary nitrogenexcretion.

SkeletalMuscleMetabolismandMuscularWork

Evenduringverymildexercisethemusclesare thechiefsourceofmetabolic heat,andduring heavy exercisetheymay accountforupto90% (Table2) .Ahealthy bu tsedentary youngman performingmoderately intenseexercisemayincreasehismetabolicrateto600W (incontrasttoabout80W atrest);andatrainedathlete


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HumanAdaptationtoHotEnvironments(TMM)/20/97performingintenseexercise,to400W ormore.Exercisingmusclesmay be nearlyC warmerthanthecore,becauseoftheirhighmetabolicrate.Bloodiswarmedasitperfusesthesemuscles,andtheblood,inturn,warmstherestofthebodyandraisescoretemperature.Likeenginesthatburnfossilfuels,musclesconvertmostof theenergy inthefuelsthattheyconsumeintoheatratherthanmechanicalwork.WhenADPisphosphorylatedtoform ATP,58% of theenergyreleasedfromthefuelisconvertedintoheat,andonlyabout42% iscapturedintheAT Pthatisformed.ThenwhenATPishydrolyzedduringamusclecontraction,someoftheenergyintheATPisconvertedintoheatrather thanintomechanicalwork.The efficiencyofthisprocessvariesenormously,andiszeroinisometriccontraction,inwhichamuscle'slengthdoesno tchangewhileitdevelopstension,sothatthemuscledoesnoworkeventhoughitconsumesmetabolicenergy.Finallysomemechanicalworkisconvertedby frictionintoheatwithinthebodyas,forexample,happenstothemechanicalworkdoneby theheartinpumping blood.Atbest,nomorethanonequarterofthemetabolicenergyreleasedduringexerciseisconvertedintomechanical work"outsidethebody,andtheremainingthreequartersor moreisconvertedintoheatwithinthebody17 .

[INSERT4thTEXT BOX (MILITARY TASKS)ABOUT HERE.]

HeatExchangewiththeEnvironment

Convection,radiation,andevaporationare thedominantmeansofheatexchangewiththeenvironment(SeealsoSanteeandMatthew).oth theskinandtherespiratorypassagesexchangeheatwiththeenvironmentby convectionandevaporation,butonlytheskinexchangesheatby radiation.nsomeanimalspecies,pantingisan importantthermoregulatoryresponse,whichcanproducehighratesofheatloss.nhumans,however,respiration usuallyaccountsforonly aminorfractionoftotalheatexchangeandisnotpredominantlyunderthermoregulatory control,althoughhyperthermicsubjectsmayhyperventilate.ThereadershouldrefertoSanteeandMatthewfo rmoredetaileddiscussionofallaspectsofheatexchangethanisprovidedinthissection.)

Convectionistransferofheatduetomovementofafluid,eitherliquidorgas.nthermalphysiology thefluidisusuallyair orwaterintheenvironment,orbloodinsidethebody,asdiscussedearlier.luidsconductheatinthesameway assolidsdo,andaperfectlystillfluidtransfersheatonlyby conduction.inceair andwaterareno t


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HumanAdaptationtoHotEnvironments(TMM)/20/97goodconductorsofheat(Table),perfectlystillair orwaterisno tveryeffectiveinheattransfer.luids,however,are rarelyperfectlystill,andevennearlyimperceptiblemovementproducesenoughconvectiontocausealargeincreaseintherateofheattransfer.husalthoughconductionplaysaroleinheattransferby afluid,convectionso dominatestheoverallheattransferthatwerefertotheentireprocessasconvection.Theconductionterm( A T )ineq. 1sthereforerestrictedtoheatflowbetweenthebodyandothersolidobjects,andusually representsonlyasmallpartof thetotalheatexchangewiththeenvironment.

Convectiveheatexchangebetweentheskinandtheenvironmentisproportionaltothedifferencebetweenskinandambientairtemperatures,asexpressedby theequation

C=hc-A-k-Ta) 2) whereAisthebodysurfacearea,X kandT aare meanskinandambienttemperatures,andhcistheconvectiveheattransfercoefficient.hcincludestheeffectsofallthefactorsbesidestemperatureandsurfaceareathataffectconvectiveheatexchange (SeeSanteeandMatthew).orthewholebody,themostimportantofthesefactorsisair movement,andconvectiveheatexchange(andthushc)variesapproximately asthesquarerootof theair speed(Fig.7)unlessair movementisveryslight.

Everysurfaceemitsenergyaselectromagnetic radiationwithapoweroutputthatdependsonitsarea,itstemperature,anditsemissivitv(e),anumberbetween0and1thatdependson thenatureofthesurfaceandthewavelengthoftheradiation.heemissivityofanysurfaceisidenticaltoitsabsorptivity ,i.e.,thefractionofincomingradiantenergythatthesurfaceabsorbsratherthanreflects.(Forthispurposetheterm"surface"hasabroadermeaningthanusual,sothat,forexample,aflameandtheskyaresurfaces.)uchradiation,calledthermalradiation,hasacharacteristicdistributionofenergyasafunctionofwavelength,whichdependsonthetemperatureof thesurface.orasurfacethatisno tho tenough toglowthisradiationisintheinfraredpartof thespectrum,andatordinarytissueandenvironmentaltemperatures virtuallyalloftheemittedenergyisatwavelengthslongerthan3 microns.ostsurfacesexceptpolished metalshaveemissivities near nthisrange,andthusbothemitandabsorbradiationatnearly thetheoreticalmaximumefficiency.sasurface'stemperatureincreases,however,theaveragewavelengthofitsthermalradiationdecreases,andmostoftheenergyinsolarradiationisinthenearinfraredandvisiblerange,fo rwhichlightsurfaceshavelowerabsorptivitiesthandarkones.


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HumanAdaptationtoHotEnvironments(TMM)/20/970Iftw osurfacesexchangeheatby thermalradiation,radiationtravelsinbothdirections;bu tsinceeach

surfaceemitsradiationwithanintensitythatdependsonitstemperature,thene theatflowisfrom thewarmertothecoolerbody.adiativeheatexchangebetweentw osurfacesis,strictly,proportionaltothedifferencebetweenthefourthpowersofthesurfaces'absolutetemperatures.oweverif thedifferencebetweenT^andthetemperatureoftheradiantenvironment(T r)ismuchsmaller thantheabsolutetemperatureof theskin,R isnearly proportionalto(T*_Tr)-Somepartsof thebodysurface(e.g.,innersurfacesofthethighsandarms)exchangeheatby radiation withotherpartsof thebodysurface,sothatthebodyexchangesheatwiththeenvironmentasif ithadanareasmallerthanitsactualsurfacearea.Thissmallerareaiscalledtheeffective radiatingsurfacearea(A r),anddependson theposture,beingclosesttotheactualsurfaceareaina"spreadeagle" posture,andleastinsomeonecurledup .Radiativeheatexchangecanbe representedby theequation

/? =h r-e sk -A r-(Vr ) 3) wherehristheradiantheattransfercoefficient,6.43W/(m2-C)at28C;andeskistheemissivityof theskin.

Whenagramofwaterisconvertedintovaporat30C ,itabsorbs2425J(0.58kcal)Table3 ),thelatentheatofevaporation ,intheprocess.Whentheenvironmentishotterthantheskinasitusuallyiswhentheenvironmentiswarmerthan36Cevaporationisthe body'sonlyway toloseheat,andmustdissipateno tonlytheheatproducedby thebody'smetabolism,bu talsoanyheatgainedfromtheenvironmentby R andC.Mostwaterevaporatedintheheatcomesfromsweat;buteveninthecold,waterdiffusesthroughtheskinandevaporates.Evaporationofthiswateriscalledinsensibleperspiration918,andoccursindependentlyofthesweatglands. isnearlyalwayspositive(representinglossofheatfromthebody);bu titisnegativeinunusualcircumstances,suchasinasteamroom,wherewatervaporcondensing on theskingivesup heattothebody.

Evaporativeheatlossfromtheskinisproportionaltothedifferencebetweenthewatervaporpressureattheskinsurfaceandthewatervaporpressureintheambientair.Theserelationsaresummarizedinthefollowingequation:

=he-A-(P,k-P.) 4)wherePskisthewatervaporpressureattheskinsurface,P aistheambientwatervaporpressure,andheistheevaporativeheattransfercoefficient.


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HumanAdaptationtoHotEnvironments(TMM)/20/971 Sincewatervapor,likeheat,iscarriedawayby movingair,airmovementandotherfactorsaffectEandheinjustthesameway thattheyaffectC andh c.If theskinsurfaceiscompletely wet,thewatervaporpressureattheskinsurfaceisthesaturationwatervaporpressure(Fig.8) atskintemperature,andevaporativeheatlossis max ,themaximumpossiblefo rtheprevailing skintemperatureandenvironmentalconditions.Thissituationisdescribedineq.5:

^max=ne'A"(Psl


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HumanAdaptationtoHotEnvironments(TMM)/20/972 toskintemperaturethanistheenvironmentoutsidetheclothing.Furthermore,sincethebodyisasourceofwatervapor,theair insidetheclothingismorehumidthanoutside.The conditionsinsidethismicroenvironmentairtemperature,watervaporpressure,andtemperatureof theinnersurfaceoftheclothingarewhatdetermineheatgainorheatlossby unexposedskin.Theseconditionsinturnaredetermined by theconditionsoutsidetheclothing,thepropertiesoftheclothing,andtherateatwhichthebody releasesheatandmoistureintothismicroenvironment.Thereforethelevelofphysicalactivity determinesboththeappropriatelevelofclothingfortheenvironmentalconditions,andthedegreeofheatstrainthatresultsfromwearingclothingthatistoowarmfo rtheconditions,asprotectiveclothing oftenis.

Althoughclothing reducesheatexchangebetweencoveredskinandthe environment,ithaslittleeffecton heatexchangeofexposedskin.Thereforeespecially whenthe clothingisheavyandmostoftheskiniscoveredexposedskinmay accountforafractionof thebody'sheatlossthatfarexceedstheexposedfractionofthebody'ssurface.Thusinthecold,theheadmay accountforhalfoftheheatlossfromthebody 20;andinsomeoneexercisingwhilewearingNBC protectiveclothing withoutgasmaskandhood,donningthemaskandhoodwhilecontinuing toexercisemayleadtoadramaticincreaseinheatstrain21.

HeatStorage

Heatstorageisachangeinthebody'sheatcontent.Therateofheatstorageisthedifferencebetweenheatproduction/gain andheatloss(Eq.),andcanbedeterminedfromsimultaneousmeasurementsofmetabolismby indirectcalorimetryandheatgainorlossby directcalorimetry.Sinceheatstorageinthetissueschangestheirtemperature,theamountofheatstoredistheproductofbodymass,thebody'smeanspecificheat,andasuitablemeanbody temperature(T b).Thebody'smeanspecific heatdependsonitscomposition,especially theproportionoffat,andisabout3.39kJ/(kg-C)[0.81kcaI/(kg-C)](Table3 )forabodycompositionof16% bone,0% fat,and74% leansofttissue.EmpiricalrelationsofT btocoretemperature(T c)andT k ,determinedincalorimetricstudies,dependonambienttemperature,withTbvaryingfrom0.67-Tc+ 0.33 'Tkinthecoldto0.9-Tc+0.1-T sk intheheat9.TheshiftfromcoldtoheatintherelativeweightingofTcandX kreflectstheaccompanying changeinthethicknessof theshell(Fig.2) .


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HumanAdaptationtoHotEnvironments(TMM)/20/973 HEATDISSIPATION

Figure9showsrectalandmeanskintemperatures,heatlosses,andcalculatedshellconductancesfornuderestingmenandwomenattheendof2-hourexposuresinacalorimetertoambienttemperaturesfrom 23 to36C.Shellconductancerepresentsthesum ofheattransferby tw oparallelmodes,i.e.conductionthroughthetissuesoftheshell,andconvectionby theblood;anditiscalculatedby dividing heatlossthroughtheskin(HF sk)i.e.,totalheatlosslessheatlossthroughtherespiratory tractbythedifferencebetweencoreandmeanskintemperatures,asfollows:

C= HF sk/(T c- k) 7) whereCisshellconductance,andT cand\arecoreandmeanskintemperatures.

Atambienttemperatures below 28Cthesesubjects'conductanceisminimal,becausetheirskinbloodflow isquitelow.incetheminimumattainablelevelofconductance dependschiefly onthesubcutaneousfatlayer,thewomen'sthickerlayerallowsthemtoattainalowerconductancethanmen.tabout28Cconductancebeginstoincrease,andabove30Cconductance continuestoincreaseandsweatingbegins.or thesenudesubjects,therange28-30Cisthezoneofthermoneutralitv.i.e.,therangeofcomfortableenvironmentaltemperaturesinwhichthermalbalanceismaintainedwithouteithershiveringorsweating12.nthiszoneheatlossismatchedtoheatproductionby controllingconductance,andthusT sk ,R ,andC.

Evaporation

I n Fig.9evaporativeheatlossisnearlyindependentofambienttemperaturebelow30C ,andis9-10W/m2.hiscorresponds toevaporationofabout13-15gm/(m 2-h),ofwhichabouthalfislostthroughbreathingandhalfasinsensibleperspiration.hisheatlossisnotunderthermoregulatory control.oachieveheatbalanceathigherambienttemperatures,thesubjectsinFig.9dependmoreandmoreonevaporationofsweat,whichinhumanscandissipatelargeamountsofheat.

Thereare twohistological typesofsweatglands,eccrineandapocrine.nhumansapocrineglandsarefoundmostlyintheaxilla,inguinalregion,perianalskin,andmammaryareolae,andlessconsistently onotherparts


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HumanAdaptationtoHotEnvironments(TMM)/20/974of thetrunkandtheface22 .ccrinesweati sessentiallyadiluteelectrolytesolution,bu tapocrinesweatalsocontains fattymaterial.ccrinesweatglandsare widelydistributedandare themoreimportanttypeinhumanthermoregulation,andfunctionally activeeccrineglandsnumberabout2,000,000to3,000,000 23 .heyare controlledthroughpostganglionicsympatheticnerveswhichreleaseacetylcholine

23ratherthannorepinephrine.healthymanunacclimatized toheatcansecreteupto.5litersofsweatperhour.lthoughthenumberoffunctionalsweatglandsisfixedbeforetheageofthree23 ,thesecretorycapacity of theindividualglandscanchange,especially withendurance exercise trainingandheatacclimatization;andaman wellacclimatizedtoheatcansecretemorethan2.5litersperhour24 - 25.Suchratescannotbemaintained,however,andthemaximumdailysweatoutputisprobablyabout1 5 liters26 .

Sodiumconcentrationofeccrinesweatrangesfromlessthan5to60mEq/L27(versus35to145mEq/Linplasma);bu tevenat60mEq/L,sweatisthemostdilutebodyfluid.To producesweatthatishypotonic toplasma,'theglandsreabsorb sodiumfromthesweatductby activetransport.ssweatrateincreases,therateatwhichtheglandsreabsorbsodiumincreasesmoreslowly,sothatsodiumconcentrationinthesweatincreases.

SkinCirculationandDry(ConvectiveandRadiative)HeatExchange

Heatproducedwithinthebodymustbedeliveredtotheskinsurfacetobeeliminated.Whenskinbloodflowisminimal,core-to-skinthermalconductance(i.e.,the conductanceoftheshell)istypically5-9 W/C pe rm2ofbodysurface(Fig.9) . leanrestingsubjectwithasurfaceareaof1.8m2,minimalwhole-bodyconductanceof16W/C[i.e.,8.9W/(C-m2)x.8m 2]andametabolicheatproductionof80W,requiresatemperaturedifferencebetweencoreandskinof5C (i.e.,80W +6W/C)toallowtheheatproducedinsidethebodytobeconductedtothesurface.nacoolenvironment,T skmay easilybelow enoughfo rthistooccur.However,inanambienttemperatureof3 3 C T i< istypicallyabout35C;andwithoutanincreaseinconductance,coretemperaturewouldneedtoriseto40 Cahighthoughnotyetdangerouslevelfortheheattobe conductedtotheskin.utif therateofheatproductionwereincreasedto480W by moderateexercise,thetemperaturedifferencebetweencoreandskinwouldhavetoriseto30Candcoretemperaturetowellbeyondlethallevelstoallowalltheheatproduced tobeconductedtotheskin.nsuchcircumstancesalargeincreaseinconductanceisneededfo rthebodytore-


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HumanAdaptationtoHotEnvironments(TMM)/20/975 establishthermalbalanceandcontinuetoregulateitstemperature;andthisisaccomplishedby increasingskinbloodflow.

RoleofSkinBloodFlowinHeatTransfer

If weassumethatbloodonitsway to theskinremainsatcoretemperatureuntilitreachestheskin,comestoskintemperatureasitpassesthroughtheskin,andthenstaysatskintemperatureuntilitreturnstothecore,wecancomputetherateofheatflow(HF b)duetoconvectionby thebloodas:

HF b=SkBF-(Tc-T sk)-3.85kJ/(L-C) 8) whereSkBF =rateofskinbloodflow,expressedinL/sratherthanthemoreusualL/min,tosimplifycomputingHF inW (i.e.,J/s);and3.85kJ/(L-C)[0.92kcal/(L-C)]= volumespecificheatofblood28Table3 ). Conductance duetoconvectionby theblood(C b)iscalculatedas:

Cb= HF b/(Tc-Tsk)= SkBF-3.85kJ/(L-C) 9) Ofcourse,heatcontinuestoflowby conductionthroughthetissuesoftheshell,sothattotalconductanceisthesumofconductanceduetoconvectionby thebloodplusthatduetoconductionthroughthetissues;andtotalheatflowisgivenby :

HF =(C b+C 0)-(T C-Tsk) 10 )inwhichC 0isthermalconductanceof the tissueswhenskinbloodflowisminimal,andthusisduepredominantlyto conduction throughthetissues.

The assumptionsonwhicheq.8dependsrepresenttheconditionsformaximumefficiency ofheattransfer by theblood,andaresomewhatartificial.npracticebloodexchangesheatalsowiththetissuesthroughwhichitpassesgoingtoandfromtheskin.eatisexchangedwiththeseothertissuesmosteasilywhenskinbloodflowislow,andinsuchcasesheatflow totheskinmay bemuchlessthatpredictedby eq .8.owever,eq.8isareasonableapproximationinawarmsubjectwithmoderatetohighskinbloodflow.tisno tpossibletomeasurewhole-bodySkBFdirectly,bu ti tisestimatedtoreachnearly8 L/minduringmaximalcutaneousvasodilation2930.Maximalcutaneousvasodilationdoesnotoccurduringheavyexercise31,butSkBFstillmay reachseverallitersaminuteduringheavyexerciseintheheat29 .IfSkBF=1.89L/min(0.0315L/s), thenaccording toeq.9skinblood


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HumanAdaptationtoHotEnvironments(TMM)/20/97f :flowcontributesabout1 2 1 W/C totheconductanceoftheshell.fconductionthroughthetissuescontributes16W/C,totalshellconductanceis37W/C;andifT c=38.5CandT sk=35C ,thenthiswillproduceacore-to-skinheattransferof480W,theheatproductioninourearlierexampleofmoderateexercise.Thusevenamoderaterateofskinbloodflow canhaveadramaticeffectonheattransfer.

Inapersonwhoisnotsweating,raisingskinbloodflowbringsskintemperaturenearerto bloodtemperature,andloweringskinbloodflowbringsskintemperaturenearer toambienttemperature.ntheseconditionsthebodycontrolsdry(convectiveandradiative)heatlossby varying skinbloodflow andthusskintemperature.ncesweatingbegins,skinbloodflowcontinuestoincreaseastheperson becomeswarmer,bu tnow thetendencyofanincreaseinskinbloodflowtowarm theskinisapproximatelybalancedby thetendencyofan increaseinsweatingtocooltheskin.hereforeaftersweatinghasbegun,furtherincreasesinskinbloodflow usuallycauselittlechangeinskintemperatureordry heatexchange,andserveprimarily todelivertotheskintheheatthatisbeingremovedby evaporationofsweat.kinbloodflow andsweatingthusworkintandemtodissipateheatundersuchconditions.

SympatheticControlofSkinCirculation

Bloodflowinhumanskinisunderdualvasomotorcontrol8'3032.Inmostof theskinthevasodilationthatoccursduringheatexposuredependsonsympatheticnervoussignalsthatcausethebloodvesselstodilate,andthisvasodilationcanbepreventedorreversedby regionalnerveblock33 .Sinceitdependson theactionofnervoussignals,suchvasodilationissometimesreferredtoasactivevasodilation.Activevasodilationoccursinalmostalltheskinexceptintheso-calledacralregionshands,feet,lips,ears,andnose34 .Intheskinareaswhereactivevasodilationoccurs,vasoconstrictoractivityisminimalatthermoneutraltemperatures;andasthebodyiswarmed,activevasodilationdoesnotbeginuntilclosetotheonsetofsweating3035.Thusskinbloodflowintheseareasisnotmuchaffectedbysmalltemperaturechangeswithinthethermoneutralrange34 .Theneurotransmitterorothervasoactivesubstanceresponsiblefo ractivevasodilationinhumanskinhasnotbeenidentified36 .However,sincesweatingandvasodilationoperateintandemintheheat,someinvestigatorshaveproposedthatthemechanismfo ractivevasodilationissomehowlinkedtotheactionofsweatglands3037.


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HumanAdaptationtoHotEnvironments(TMM)/20/977Reflexvasoconstriction,occurringinresponsetocoldandalsoaspartofcertainnon-thermalreflexessuch

asbaroreflexes,ismediatedprimarily throughadrenergicsympatheticfiberswhicharedistributedwidelyovermostof theskin36 .Reducingtheflow of impulsesinthesenervefibersallowsthebloodvesselstodilate.Intheacralregions3036andinthesuperficialveins3 0,vasoconstrictor fibersare thepredominantvasomotorinnervation,andthevasodilation thatoccursduringheatexposureislargelyaresultofthewithdrawalofvasoconstrictoractivity 34 .Bloodflowintheseskinregionsissensitive tosmalltemperaturechangeseveninthethermoneutralrange,andmay be responsiblefo r"finetuning"heatlosstomaintain heatbalanceinthisrange.

THERMOREGULATORYCONTROL

Incontroltheorythewordsregulationandregulatehavemeaningsdistinctfromthoseofcontrol. controlsystemactsto minimizechangesintheregulatedvariable(e.g.,coretemperature)thatareproducedby disturbancesfromoutsidethesystem(e.g.,exerciseorchangesintheenvironment)by makingchangesincertainothervariables (e.g.,sweatingrate,skinbloodflow,metabolicrate,andthermoregulatorybehavior),whicharecalledcontrolledvariables.Humanbeingshavetw odistinctsub-systemstoregulatebodytemperature:behavioralthermoregulationandphysiologicalthermoregulation.Physiologicalthermoregulation iscapableoffairlypreciseadjustmentsofheatbalance,bu tiseffectiveonlywithinarelativelynarrow rangeofenvironmentaltemperatures.Ontheotherhandbehavioralthermoregulation,throughtheuse ofshelterandspaceheatingandclothing,enableshumanstoliveinthemostextremeclimatesonearth;bu titdoesno tprovidefinecontrolofbodyheatbalance.

BehavioralThermoregulation

Behavioralthermoregulationisgovernedby thermalsensationandcomfort.Sensoryinformationaboutbodytemperaturesisanessentialpartofbothbehavioralandphysiologicalthermoregulation.hedistinguishing featureofbehavioralthermoregulationisthedirectionofconsciousefforttoreducediscomfort.Warmthandcoldontheskinarefeltaseithercomfortableor uncomfortable,dependingonwhethertheydecreaseorincreasethephysiologicalstrain 38 .husashowertemperaturethatfeelspleasantafterstrenuousexercisemay beuncomfortably


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HumanAdaptationtoHotEnvironments(TMM)/20/978 coldonachillymorning.ecauseoftherelationbetweendiscomfortandphysiologicalstrain,behavioralthermoregulation,by reducingdiscomfort,alsoactstominimizethephysiologicalburdenimposedby astressfulthermalenvironment.or thisreasonthezoneof thermoneutrality ischaracterizedby thermalcomfortaswellasby theabsenceofshiveringandsweating.

The processing of thermalinformationinbehavioralthermoregulationisno taswellunderstoodasinphysiologicalthermoregulation.owever,perceptions of thermalsensationandcomfortrespondmuchmorequickly thaneithercoretemperatureorphysiologicalthermoregulatoryresponsestochangesinenvironmentaltemperature3 9 - 40 ,andthusappeartoanticipatechangesinthebody'sthermalstate.Suchananticipatory featurepresumably reducestheneedfo rfrequentsmallbehavioraladjustments.

PhysiologicalThermoregulation

Physiologicalthermoregulationoperatesthroughgradedcontrolofheat-production andheat-loss responses.amiliarnon-livingcontrolsystems,suchasmostrefrigeratorsandheatingandair-conditioningsystems,operateatonlytw olevels,becausetheyactby turningadeviceon oroff.ncontrast,mostphysiologicalcontrolsystemsproducearesponsethatisgradedaccording tothedisturbanceintheregulatedvariable.nmanyphysiologicalsystemschangesinthecontrolledvariablesare proportional todisplacementsoftheregulatedvariablefromsomethreshold value,andsuchcontrolsystemsare calledproportionalcontrolsystems.

Thecontrolofheat-dissipating responsesisanexampleof aproportionalcontrolsystem9.igure0showshow reflexcontrolofsweating andskinbloodflow dependsonbodycoreandskintemperatures.Eachresponsehasacore-temperaturethreshold,atemperatureatwhichtheresponse startstoincrease;andthesethresholdsdependon meanskintemperature.Thusatanygivenskintemperature,thechangeineachresponseisproportionalto thechangeincoretemperature;andincreasingtheskintemperaturelowersthethresholdlevelofcoretemperatureandincreasestheresponseatany givencoretemperature.nhumansachangeof1 C incoretemperatureelicitsaboutninetimesasgreatathermoregulatoryresponseasa C changeinmeanskintemperature8.(Besidesitseffecton thereflexsignals,skintemperaturehasalocaleffectthatmodifiestheblood-vesselandsweat-glandresponses,asdiscussedlater.)


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HumanAdaptationtoHotEnvironments(TMM)/20/979IntegrationofThermalInformation

Thecentralnervoussystemintegratesthermalinformationfromcoreandskin.eceptorsinthebodycoreandtheskintransmitinformationabouttheirtemperatures throughafferentnervestothebrainstem,andespeciallythehypothalamus,wheremuchof theintegrationoftemperatureinformationoccurs 41.he sensitivityofthethermoregulatory responsestocoretemperatureallowsthethermoregulatory system toadjustheatproductionandheatlossto resistdisturbancesincoretemperature.heirsensitivity tomeanskintemperatureallowsthesystem to respondappropriately tomildheator coldexposurewithlittlechangeinbodycoretemperature,sothatenvironmentallyinducedchangesinbody heatcontentoccuralmostentirelyintheperipheraltissues,asshowninFigure2.For example,theskintemperatureofsomeonewhoentersaho tenvironmentrisesandmay elicitsweatingevenifthereisnochangeincoretemperature.Ontheotherhand,anincreaseinheatproductionduetoexerciseelicitstheappropriateheat-dissipating responsesthroughariseincoretemperature.

Coretemperaturereceptorsinvolvedinthecontrolof thermoregulatory responsesareconcentratedespeciallyinthehypothalamus 42 ,bu ttemperaturereceptorsinothercoresites,includingthespinalcordandmedulla,alsoparticipate42 .Theanteriorpreoptic areaofthehypothalamuscontainsmany neuronswhichincreasetheirfiringrateeitherinresponsetowarmingorinresponsetocooling,andtemperaturechangesinthisareaofonly afew tenthsof 1 C elicitchangesinthethermoregulatoryeffectorresponsesofexperimentalmammals.Thermalreceptorshavebeenreportedelsewhereinthecore,including the heart,pulmonary vessels,andspinalcord;bu tthethermoregulatory roleofcorethermalreceptorsoutsidethecentralnervoussystemisno tknown8.

Le tusconsiderwhathappenswhenadisturbancesay,anincreaseinmetabolicheatproductiondueto exerciseupsetsthethermalbalance.eatisstoredinthebody,andcoretemperaturerises.Thethermoregulatory controller receivesinformationaboutthesechangesfromthethermalreceptors,andrespondsby callingforthappropriateheat-dissipating responses.Coretemperaturecontinues torise,andtheseresponsescontinuetoincrease,untiltheyare sufficienttodissipateheatasfastasitisbeingproduced,thusrestoringheatbalanceandpreventingfurtherincreasesinbodytemperatures.Theriseincoretemperaturewhichelicitsheat-dissipating responsessufficienttore-establishthermalbalanceduringexerciseisanexampleofaloaderror9;aloaderrorischaracteristicofanyproportionalcontrolsystemthatisresistingtheeffectofsomeimposeddisturbance or"load".lthoughthe


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HumanAdaptationtoHotEnvironments(TMM)/20/970disturbanceinthisexamplewasexercise,parallelargumentsapplyif thedisturbanceisachangeintheenvironment,exceptthatmostof thetemperature changewillbeintheskinandshellratherthaninthecore.

RelationofEffector SignalstoThermoregulatorySetPoint

Bothsweating andskinbloodflowdependoncoreandskintemperaturesinthesameway,andchangesinthethresholdforsweatingare accompaniedby similarchangesinthethresholdforvasodilation4.Wemay thereforethinkofthecentralintegrator(Fig.1)asgeneratingon ethermalcommandsignalforthecontrolofbothsweating andskinbloodflow.Thissignalisbasedon theinformation aboutcoreandskintemperaturesthattheintegratorreceives,andon thethermoregulatorysetpoint4 .Wemay thinkofthesetpointasthetargetlevelofcoretemperature,or thesettingofthebody's"thermostat".Intheoperationofthethermoregulatorysystem,itisareference pointwhichdeterminesthethresholdsofallthethermoregulatoryresponses.

Non-thermal Influences onThermoregulatoryResponses

Eachthermoregulatoryresponse may beaffectedby otherinputsbesidesbodytemperaturesandfactorsthataffectthethermoregulatorysetpoint.on-thermalfactorsmay produceaburstofsweatingatthebeginningofexercise41'4 ,andtheinvolvementofsweatingandskinbloodflowinemotionalresponsesisfamiliartoeveryone.Of thethermoregulatoryresponsethatareimportantduringheatstress,skinbloodflowismostaffectedby non-thermalfactorsbecauseofitsinvolvementinreflexeswhichfunction tomaintaincardiacoutput,bloodpressure, andtissue02deliveryduring heatstress,posturalchanges,andhemorrhage,andsometimesduringexercise,especiallyintheheat.

PhysiologicalandPathologicalChanges totheThermoregulatorySe tPoint

Severalphysiologicalandpathologicalinfluenceschangethethermoregulatorysetpoint.everelevates coretemperature atrest,heatacclimatizationdecreasesit,andtimeofdayandphaseofthemenstrualcyclechange


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HumanAdaptationtoHotEnvironments(TMM)/20/971it inacyclicalfashion4 " 6.Coretemperatureatrestvarieswithtimeofdayinanapproximatelysinusoidalfashion,reachingaminimumatnight,severalhoursbeforeawaking,andamaximumwhichis0.5to C higherin thelateafternoonorevening(Fig.3 ).lthoughthispatterncoincideswithpatternsofactivity andeating,itisindependenton them,occurringevenduringbedrestandfasting.Thispatternisanexampleofacircadian rhythm,i.e.,arhythmicpatterninaphysiologicalfunctionwithaperiodofabouton eday.uringthemenstrualcyclecoretemperatureisatitslowestpointjustbeforeovulation,andoverthenextfew daysrises0.5toC andremainselevatedfo rmostofthelutealphase.achof thesefactorsfever,heatacclimatization, thecircadianrhythm,andthemenstrualcycleaffectscoretemperatureatrestby changingthethermoregulatory set point,thusproducing correspondingchangesinthethresholdsfo rallthethermoregulatory responses.

PeripheralModificationofSkinVascularandSweatGlandResponses

Theskinistheorganmostdirectlyaffectedby environmentaltemperature,andskintemperatureaffectsheatlossresponsesno tonlythroughthereflexactionsshowninFig.0bu talsothroughdirecteffectsontheeffectorsthemselves.Localtemperaturechangesactonskinbloodvesselsinatleasttwoways.irst,localcoolingpotentiates(andheatingweakens)theconstrictionofbloodvesselsinresponsetonervoussignalsandvasoconstrictor substances36 .econd,inskinregionswhereactivevasodilationoccurs,localheatingdilatesthebloodvessels(andlocalcoolingconstrictsthem)throughadirectactionthatisindependentofnervoussignals 45 - 46 .Thiseffectisespeciallystrongatskintemperaturesabove35 C 46 ;andwhentheskiniswarmerthantheblood,increasedbloodflowhelpstocooltheskinandprotectitfromheatinjury.heeffectsoflocaltemperatureon sweatglandsparallelthoseonbloodvessels,sothatlocalheatingmagnifies(andlocalcoolingreduces) thesweatingresponsetoreflexstimulationor toacetylcholine 37 ,andintenselocalheatingprovokessweatingdirectly,eveninsympathectomizedskin47 .uringprolonged(severalhours)heatexposurewithhighsweatoutput,sweatratesgraduallydiminishandthesweatglands'responsetolocallyapplied cholinergicdrugsisreducedalso.he reductionofsweat-glandresponsivenessissometimescalledsweat-gland"fatigue".Wettingtheskinmakesthestratumcorneumswell,mechanically obstructing thesweatductandcausing areductioninsweatsecretion,an effectcalledhidromeiosis48 .heglands'responsiveness canbeatleastpartlyrestoredif theskinisallowedtodry


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HumanAdaptationtoHotEnvironments(TMM)/20/972(e.g.,byincreasing airmovement49 ),bu tprolongedsweatingalsocauseshistologicalchanges,includingdepletionofglycogen,inthesweatglands50 .

THERMOREGULATORY RESPONSES DURINGEXERCISE

Exerciseincreasesheatproductionsothatitexceedsheatlossandcausescoretemperaturetorise.he increaseincoretemperature,inturn,elicitsheat-lossresponses,bu tcoretemperaturecontinuestoriseuntilheatlosshasincreased enoughtomatchheatproduction,sothatheatbalanceisrestoredandcoretemperatureandtheheat-lossresponsesreachnewsteady-statelevels.Sincetheheat-lossresponsesare proportional to theincreaseincoretemperature,theincreaseincoretemperatureatsteadystateisproportionalto therateofheatproduction,andthustothemetabolicrate.

Achangeinambienttemperature changesthelevelsofsweatingandskinbloodflowthatareneededto maintainany givenrateofheatdissipation.oweverthechangeinambienttemperatureisaccompaniedby askin-temperaturechangethatelicits,viabothdirectandreflexeffects,muchof therequired changeintheseresponses. Foranygivenrateofheatproduction,thereisarangeofenvironmentalconditions(sometimescalledthe"prescriptivezone"-seeSawkaandPandolf schapter)withinwhichambient-temperaturechangeselicitthenecessarychangesinheat-dissipatingresponsesalmostentirelythroughtheeffectsofskin-temperaturechanges,withvirtuallynoeffectoncoretemperatureatsteadystate51.(Thelimitsofthisrangeofconditionsdependontherateofheatproduction,andsuchindividualfactorsasskinsurfaceareaandstateofheatacclimatization.)Withinthisrange,coretemperaturereachedduringexerciseisnearlyindependentofambienttemperature;andfo rthisreasonitwasoncebelievedthattheincreaseincoretemperature duringexerciseiscausedby anincreaseinthethermoregulatory setpoint52,justasduringfever.sstatedpreviously,however,theincreaseincoretemperaturewithexerciseisanexampleofaloaderrorratherthananincreaseinset point.nFig.2notethesedifference betweenfeverandexercise:irst,althoughheatproductionmay increasesubstantially(throughshivering) whencoretemperatureisrisingearlyduringfever,itdoesno tneedtostayhightomaintainthefever,bu tinfactreturnsnearly topre-febrilelevelsoncethefeverisestablished;duringexercise,however,anincreaseinheatproductionno tonlycausestheelevationincoretemperature,butisnecessary tosustainit.econd,whilecoretemperatureis


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HumanAdaptationtoHotEnvironments(TMM)/20/973 risingduring fever,rateofheatlossis,if anything,lowerthanbeforethefeverbegan;bu tduringexercise,theheat-dissipatingresponsesandtherateofheatlossstarttoincreaseearlyandcontinueincreasingascoretemperaturerises.Althoughfeverinthischaptermeansspecifically anelevationincoretemperaturedu etopyrogensandoccurringinconnection withinfection orotherinflammatory process,someauthorsusefevermorelooselytomeananysignificant elevationofcoretemperature.)

ChallengeofExerciseintheHeattoCardiovascularHomeostasis

As pointedou tearlier,skinbloodflowincreasesduringexerciseinordertocarryalloftheheatthatisproducedto theskin.nawarmenvironment,wherethetemperaturedifferencebetweencoreandskinisrelativelysmall,thenecessaryincreaseinskinbloodflowmay beseverallitersperminute.

ImpairmentofCardiacFilling

Whiletheworkofsupplyingtheskinbloodflowrequiredfo rthermoregulationintheheatmay representaheavyburdenforapatientwithcardiovascular disease53 ,inhealthy subjectstheprimarycardiovascularburdenofheatstressresultsfromimpairmentofvenousreturn2930'54 .sskinbloodflowincreases,bloodpoolsinthelarge,dilatedcutaneousvascularbed,thusreducingcentralbloodvolumeandcardiacfilling(Fig.3 ).incestrokevolumeisdecreased,ahigherheartrateisrequiredtomaintaincardiacoutput.heseeffectsare aggravatedby adecreaseinplasmavolumeif thelargeamountsofsaltandwaterlostinthesweatare no treplaced.incethemaincationinsweatissodium,disproportionately muchofthebodywaterlostinsweatisattheexpenseofextracellularfluid,includingplasma,althoughthiseffectismitigatedif thesweatisdilute.

CompensatoryCardiovascularResponses

Severalreflexadjustmentshelptomaintaincardiacfilling,cardiacoutput,andarterialpressureduringexerciseandheatstress.he cutaneousveinsconstrictduringexercise;andsincemostof thevascularvolumeisin


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HumanAdaptationtoHotEnvironments(TMM)/20/974 theveins,constrictionmakesthecutaneousvascularbedlesscompliant,andreducesperipheralpooling. Splanchnicandrenalbloodfloware reducedinproportiontotheintensityoftheexerciseor heatstress.hisreductionofbloodflow hastwoeffects.irst,itallowsacorresponding diversionofcardiacoutputtoskinandexercising muscle.Second,sincethesplanchnic vascularbedsare verycompliant,adecreaseintheirbloodflow reducestheamountofbloodpooledinthem 29 '3 0(Fig.3 ),helpingtocompensate fordecreasesincentralbloodvolumecausedby reducedplasmavolumeandbloodpoolingintheskin.ecauseoftheessentialthermoregulatoryfunctionofskinbloodflow duringexerciseandheatstress,thebodypreferentiallycompromises splanchnic andrenalflow tomaintain cardiovascularhomeostasis55.boveacertainlevelofcardiovascularstrain,however,skinbloodflow tooiscompromised.

Despitethesecompensatoryresponses,heatstressmarkedlyincreasesthethermalandcardiovascularstrainthatexerciseproducesinsubjectsunacclimatized toheat.nFig.456,acomparisonofresponseson thefirstdayofexerciseintheheatwiththoseoncooldaysshowssomeeffectsofunaccustomedenvironmentalheatstressontheresponsestoexercise.nthefirstdayintheheat,heartrateduringexercisereachedalevelabout40beats/minhigherthaninthecoolenvironment,tohelpcompensatefortheeffectsof impairedcardiacfillingandtomaintaincardiacoutput;andrectaltemperatureduring exerciserose C higherthaninthecoolenvironment.thereffectsofexercise-heatstressmay includeheadache,nauseaandvomitingsecondary tosplanchnicvasoconstriction, dizziness,cramps,shortnessofbreath,dependentedema,andorthostatichypotension.

Duringprolongedexercise thereisagradual"drift"inseveralcardiovascular andthermoregulatory responses.hismayincludeacontinuousriseinheartrate,accompaniedby afallinstrokevolumeandreductionsinaortic,pulmonaryarterial,andrightventricularend-diastolic pressures".owellnamedthesechanges"cardiovascular drift",andthoughtofthemasappearingasearlyasafter1 5 minofexercise57 .HeandJohnson 57 -58 emphasized theroleofthermoregulatoryincreasesinskinbloodflowinproducingcardiovasculardrift.oweverlaterauthors(e.g.,59-61)havedescribed,aspartofthepictureofcardiovascular drift,anupwardcreepincoretemperature,whichmay beginonlyafteraperiodofapparentthermalsteadystate(e.g.,after30-60minofexercise).nsomeofthesestudies,mostbu tno tallofthechangesincardiovascularandthermoregulatory responsescouldbe preventedby replacingfluidlostinsweat,suggestingthatthesechangesweremostlysecondary tochangesinplasmavolumeandosmolality andplasmaduetosweating.therfactorsthatmay affect


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HumanAdaptationtoHotEnvironments(TMM)/20/975cardiovascularandthermoregulatoryfunctionduringprolongedexerciseincludechangesinmyocardialfunction (c.f.Tibbits,62);changesinbaroreceptorsensitivity or peripherala-adrenergicreceptorresponsiveness(seeRaven63);oranupwardadjustmentof thethermoregulatorysetpoint64 ,presumablyduetosomesortofinflammatoryresponse,andperhapselicitedby productsofmuscleinjury64 .heseeffectshaveno tbeeninvestigatedextensively,andverylittleisknownabouttheunderlying physiologicalorpathologicalmechanisms.Someoftheseeffectshavebeenreported onlyafterseveralhoursofexerciseornearexhaustion,andlittleisknownabouttheconditionsofexercise durationandintensity requiredtoproducethem,andtheirpersistenceaftertheendofexercise.lthoughtheirfunctionalsignificanceis,asyet,onlypoorly understood,thesechangesmay be importantinlimiting performanceduringprolongedstrenuousactivity,suchasforcedmarches.

FACTORS THAT ALTER HEAT TOLERANCE

HeatAcclimatization

Prolongedorrepeatedexposuretostressfulenvironmentalconditionselicitssignificantphysiological changes,calledacclimatization ,whichreducethephysiologicalstrainthatsuchconditionsproduce.(Thenearlysynonymoustermacclimationisoftenappliedtosuchchangesproducedinacontrolledexperimentalsetting 12.)Figure4illustratesthedevelopmentof thesechangesduringa10-day programofdailytreadmillwalksintheheat.Overthe10days,heartrateduringexercisedecreasedby about40beats/min,andrectalandmeanskintemperaturesduringexercisedecreasedmorethanC .Sinceskintemperatureislowerafterheatacclimatizationthanbefore,dry(non-evaporative)heatlossisless(or,if theenvironmentiswarmerthantheskin,dryheatgainisgreater).Tocompensatefo rthechangesindryheatexchange,evaporativeheatloss,andthussweating,increases.Thelowerheartrateandcoretemperatureandhighersweatrateduring exercise-heat stressare thethreeclassicalsignsofheatacclimatization.therchangesincludeanincreasedability tosustainsweatproductionduringprolongedexercise-heatstress,whichisessentialtoincreasing tolerancetime;decreased soluteconcentrationsinsweat;redistributionofsweatingfrom trunktolimbs;increasesintotalbodywaterandchangesinitsdistribution;metabolicandendocrinechanges;andotherpoorlyunderstoodchangesthatprotectagainstheatillness.Theoveralleffectofheat


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HumanAdaptationtoHotEnvironments(TMM)20/976acclimatizationonperformancecanbequitedramatic,sothatacclimatizedsubjectscaneasilycompleteexerciseintheheatwhichpreviously wasdifficultorimpossible(cf.65).

At anygivenair temperature,increasing thehumidity impedesevaporationofsweat(Eq.6).oallow sweattoevaporaterapidlyenoughtomaintainheatbalance,thewettedareaofskinmustincrease.Thedistributionofsweatingmay changetoallowmoreof theskinsurfaceareatobewetted,bu twetterskinalsofavorsdevelopmentofhidromeiosis,limiting tolerance timeby hamperingmaintenanceofhighsweatrates.Althoughheatacclimatizationinadryenvironmentconfersasubstantialadvantageinhumidheat6667,acclimatizationinhumidheatproducessomewhatdifferentphysiologicaladaptations,correspondingtothecharacteristicphysiologicalandbiophysicalchallengesofhumidheat.

AcquisitionandLoss

Adegreeofheatacclimatizationisproduced eitherby heatexposurealoneorby regularstrenuous exercise,whichraisescoretemperatureandprovokesheat-lossresponses.ndeed,thefirstsummerheatwaveproducesenoughheatacclimatizationthatafterafew daysmostpeoplenoticeanimprovementintheirfeelingsofenergyandgeneralwellbeing.However,theacclimatization responseisgreaterifheatexposureandexercisearecombined,causingagreaterriseof internaltemperatureandmoreprofusesweating.ptoapointthedegreeofacclimatizationacquiredisproportionaltothedailyheatstressandtheamountof sweatsecretedduring acclimatization 68,bu tfulldevelopmentofexercise-heatacclimatization doesno trequirecontinuousheatexposure.Continuous,daily100-minperiodsofheatexposurewithexerciseare widelyconsideredsufficienttoproducean optimalheatacclimatizationresponseindryheat.oweverthisnotionisbasedchieflyononestudy69 ,inwhichsubjects'responseswereevaluatedonly during00-minheatexposures,whichprovidelittleinformationabouttheirabilitytosustainheat-lossresponsesovertime.nadequateassessmentofheattolerancemay,infact,requireaexposurelastingseveralhours.orexample,StrydomandWilliams comparedresponsesof tw ogroupsofsubjectsduring fourhoursofexerciseinhumidheat.Althoughthegroups'responseswereindistinguishable duringthefirsthour,theresponsesof themoreheat-tolerant groupwereclearlydifferentfromthoseofthelessheat-tolerantgroupduringthethirdandfourthhours.


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HumanAdaptationtoHotEnvironments(TMM)/20/977Severalfactorsaffectthespeedofdevelopmentofheatacclimatization.However,mostof the

improvementinheartrate,skinandcoretemperatures,andsweatratetypicallyisachievedduringthefirstweekofdailyexerciseinaho tenvironment,although thereisnosharpendtotheimprovement7 1 .Heartrateshowsthemostrapidreduction

7274,mostofwhichoccursinfourtofivedays72 .Aftersevendays,thereductioninheartrateis

virtuallycompleteandmostof theimprovementinskinandcoretemperatureshasalsooccurred 73 - 75;andthethermoregulatoryimprovementsare generallybelievedtobecompleteafter10-14daysofexposure65.Theimprovedsweating response7275andeaseofwalking7375reportedduringheatacclimatizationmay takeamonthtodevelopfully,andresistancetoheatstrokemay takeuptoeightweeks76 .Experimental heatacclimation developsmorequicklyinwarmweather67,probablybecausesubjectsarealreadypartlyacclimatized.

Highaerobicfitnesshastensdevelopmentofacclimatization7377.erobic exerciseelevatescoretemperatureandelicitssweating eveninatemperateenvironment,andaerobictrainingprogramsinvolvingexercis'eat70% ofmaximaloxygenuptake(V o 2max)or more7879producechangesinthecontrolofsweatingsimilartothoseproducedby heatacclimatization.herehas,however,beenmuchdisagreement astowhetherorno taerobictraininginatemperateenvironmentinducestrueheatacclimatization.Inacriticalreviewoftheevidenceandargumentsonbothsidesof theissue,GisolfiandCohen80concludedthatexercisetrainingprogramslastingtw omonthsormoreinatemperateenvironmentproducesubstantialimprovementinexercise-heat tolerance.oweverexercisetrainingaloneno tbeenshowntoproduceamaximalstateofexercise-heattolerance.

Thebenefitsofacclimatizationarelessenedorundoneby sleeploss,infection,andalcoholabuse72 - 81,saltdepletion 72 ,anddehydration72 - 82 - 83 .ea tacclimatizationgraduallydisappears withoutperiodicheatexposure,althoughpartiallossesdu etoafew days'lapseareeasilymadeup 81.Theimprovementinheartrate,whichdevelopsmorerapidly,alsoislostmorerapidlythanare thethermoregulatoryimprovements69 - 77 - 84 - 85 .Howeverthereismuchvariabilityinhowlongacclimatizationpersists.nonestudy,forexample,acclimatizationalmostcompletely disappearedafter7dayswithoutheatexposure86 ;bu tinanotherstudy,approximately threequartersoftheimprovementinheartrateandrectaltemperaturewasretainedafter1 8 dayswithoutheatexposure 77.Physicallyfit subjectsretainheatacclimatizationlonger66 -77;andwarmweathermay67ormay not85favorpersistenceofacclimatization,althoughintermittentexposure tocoldseemsnottohastenthelossofheatacclimatization 87.


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HumanAdaptationto Hot Environments(TMM)8/20/978ChangesinThermoregulatoryResponses

Afteracclimatization,sweatingduringexercise startsearlierandthecoretemperaturethresholdfo rsweating islowered.cclimatizationalsoincreasesthesweatglands'responsetoagivenincrementincoretemperature andalsotheirmaximum sweatingcapacity.heselatterchangesreflectchangesintheindividualglandsrather thaninthenervoussignalsto theglands,sinceafteracclimatizationtheglandsalsoproducemoresweatwhenstimulatedwithmethacholine 88 '89 ,which mimicstheeffectofacetylcholine.

Inan unacclimatizedperson,sweatingismostprofuseon thetrunk;butduringacclimatizationin humidheat,thefraction ofsweatsecretedon thelimbsincreases9093,enablingan acclimatizedpersonto makebetteruseoftheskinsurfacefo revaporationandachievehigherratesofevaporativeheatloss.uringaheatstresslastingseveralhours,sweatratesthatwerehigh initially tendgraduallyto declineastheheatstresscontinues.houghseveralmechanismsmay contributeto thedecline,much of thedeclineisdueto hidromeiosis,associatedwithwetnessof theskin,andthedeclineismostpronouncedinhumidheat.fteracclimatizationto humidheat,thisdeclineofsweatrateoccursmoreslowly68(Fig.5),sothathighersweatratescanbe sustainedandtolerancetimeisprolonged.hi seffectofacclimatizationappearsto ac tdirectly on thesweatglandsthemselves,andduringacclimatizationtodryheatitcanbe producedselectively on on earm by keepingthat arminahumidmicroclimateinsideaplasticbag94 .

Sinceheatacclimatizationisan exampleofaset-pointchange4 - 95 ,thresholdsfo rsweatingandcutaneousvasodilationbotharereducedin suchaway thatvasodilation andtheonsetofsweating accompanyeachotherafteracclimatizationin thesameway asbefore 96,andheattransferfromcoretoskinismaintained at thelowerlevelsofcoreandskintemperaturethatprevailafteracclimatization.hesechangesby themselves say nothingabouttheeffectofacclimatizationon thelevelsofskinbloodflow reachedduringexercise-heat stress.nmanystudies(e.g.,56 - 97 ,especially thoseusingdry heat,heatacclimatizationwasfound to widenthecore-to-skintemperaturegradient,presumablyallowing heatbalance to be reached withalowerlevelofskinbloodflow andalessercardiovascularstrain.owever,suchawidening of thecore-to-skin temperaturegradientdoesnotalwaysaccompanyacclimatization(e.g.,73 ).


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HumanAdaptationtoHotEnvironments(TMM)/20/979Non-T/termoregulatoryCh anges

On thefirstdayofexerciseintheheat,heartratereachesmuchhigherlevelsthanintemperateconditions(Fig.4) ,andstrokevolumeislower.Thereafter,heartratedecreases(Fig.4)andstrokevolumeusually,bu tnotalways,increases.rthostatictolerancealsoimproveswithheatacclimatization95 .everalmechanismsparticipateinthesechanges,buttheirrelativecontributionsare no tknownandprobably vary.lasmavolumeatrestexpandsduringthefirstweekofacclimatizationandcontributestothereductioninheartrateandcirculatorystrain;howeverifacclimatization continues,plasmavolumeatrestreturnstowardcontrollevelsafteraweekortw o74 '98 " 100 ,whiletheimprovementsincardiovascular functionpersist.naddition,itislikely thatadecreaseinperipheralpoolingofbloodhelpstosupportcardiovascular functioninacclimatizedsubjects.Whenitoccurs,adecreaseinskinbloodflow (allowedby awidenedcore-to-skintemperature gradient)presumablydecreasesperipheral poolingofblood.Inaddition,anincreaseinvenoustonemightsubstantiallydecreasepooling ofblood,sincevenoconstriction canmobilizeup to25% of thebloodvolume98 .heinformationavailableaboutsuchchanges'01" 103 ,however,isvery limitedandisfa rfrombeingconclusive.

Heatacclimatizationincreasestotalbody water,bu tthereismuchvariabilitybothinthetotalincreaseandinitsdistributionamongthevariousfluidcompartments95 . uchoftheincreaseisaccountedfo rby anexpansionofplasmavolumeatrest,whichdevelopsrapidlyatfirstandcontinuesmoreslowlyforaboutaweek.Theresultingincreaseinbloodvolumerangesfromof1 2 to27% 1 04 .Themechanismsresponsiblefo rthisexpansionare unclear,butmayincludeanincreaseinextracellularfluidrangingfrom6 to6% 104duetosaltretention,andane tfluidshiftfrominterstitialspacetoplasma,duetoanincreaseinthemassofproteinintheplasma105 - 106.

Atthestartofacclimatization,secretionofadrenocorticotrophic hormone(ACTH)increasesinresponse tothecirculatory straincausedbyheatstress.he adrenalcortexrespondstoACTHby increasing secretionofcortisolandaldosterone.fsaltintakeisinsufficienttoreplacelossesinsweat,theresultingsodiumdepletionalsoactsviatherenin-angiotensin systemtoincreasealdosteronesecretion.ortisolandaldosteronebothcontributetosodiumretentionbythekidneyswithinafew hours,andby thesweatglandsafter to2days.xerciseandheatstressalsoelicitsecretionofaldosterone 107108throughtherenin-angiotensinsystem.Withinafewdaysthesodium-conservingeffectsofaldosteronesecretedviathispathway are sufficientto restoreandmaintainsodiumbalance,andACTHsecretionreturnstonormal.ependingonsodiumintake,thekidneysmay eventually"escape" the


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HumanAdaptationtoHo tEnvironments(TMM)/20/970effectsofaldosteroneandexcreteasmuchsodiumasneededtomaintainsodiumbalance.he sweatglands,however,do no tescape,butcontinuetoconservesodiumaslongasacclimatization persists.

Anunacclimatizedpersonmay secretesweatwithasodiumconcentration ashighas60mEq/L,correspondingto3.5gofNaClpe rliter,andcanloselargeamountsofsaltinthesweat(Fig.6).Withacclimatization,thesweatglandsconservesodiumby secreting sweatwithasodiumconcentration aslowas5 mEq/L27 .cclimatized men inwhom sodiumconservationismaximallydevelopedcansweatup to9literspe rday andstayinsaltbalanceon5 gramsofNaClpe rday109110.Maximaldevelopmentofsodium-conservingcapacity wasaccomplishedwithaprogramthatcombinedgradualreductionofdietarysodiumintakewithdailyexerciseintheheat.owevermostCaucasianswhoare no tsecretinglargevolumesofsweatandareinsaltbalancewithan intakeof10gNaClpe rday(a typicalintakeforawesterndiet)havehighsweatsodium concentrations'".fsuddenlyrequiredtosecretelargevolumesofsweat,they may undergoasubstantialne tlossofsodium beforetheirmechanismsforsodium conservationbecomefullyactive.hereforesubjectswho areexercisinginahot environmentandareeitherunacclimatized or notconsuminganormaldietshould receive10gramsofsupplementalsaltpe rdayunlesswaterisinshortsupply 1".Howeversaltsupplementsareno trecommendedfo racclimatizedsubjectsperformingheavy exerciseintheheatif they are eatinganormaldietandare notsaltdepleted.

The mineralocorticoids aldosteroneanddesoxycorticosterone havebeenadministered tosubjectsjustbeforeorduringheatacclimatization programs98104"2"3.Mineralocorticoidadministrationproducedsomeresponsescharacteristicofheatacclimatization,bu tneitherproducedastateequivalenttowhatthesubjectsattainedasaresultofundergoingheatacclimatization,norreducedthetimenecessary toreachthatstate.oweverbecauseof theway thesestudiesweredesigned,theirresultsdonotsupportdefiniteconclusionsabouttheroleofendogenousaldosteroneinheatacclimatization95.

EffectsonHeatDisorders

Theharmfuleffectsofheatstressoperatethrough cardiovascularstrain,fluidandelectrolyteloss,and,especiallyinheatstroke,tissueinjurywhosemechanismisnotwellunderstood bu tevidentlyinvolvesmorethansimplyhightissuetemperatures(seeHubbard,"4andGaffin,Hubbard,andWenger'schapterfo rfurtherdiscussion).


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HumanAdaptationtoHotEnvironments(TMM)/20/971Heatsyncopeisatemporarycirculatoryfailureduetopoolingofbloodintheperipheralveinsandthe

resultingdecreaseindiastolicfillingof theheart.lthoughalargeincreaseinthermoregulatory skinbloodflowisthedirectcauseof theperipheralpooling,aninadequatebaroreflexresponseisprobablyanimportantcontributingfactor.ea tacclimatization rapidly reducessusceptibility toheatsyncope,asexpectedfromtheimprovementinorthostatictolerance1 01"-"6,notedearlier.

Likeheatsyncope,heatexhaustionisthoughttoresultfromadecreaseindiastolicfilling.owever,dehydrationwithresulting hypovolemia hasamajorroleinthedevelopmentofheatexhaustion;andthebaroreflexresponsesusually arestrongenoughtopreventsyncope,andalsoaccountfo rmuchof thesymptomatology.ittleisknownabouttheeffectofacclimatization onsusceptibility toheatexhaustion.

Heatstrokeisthemostsevereheatdisorder,andwithoutpromptappropriatetreatment,mortalitymay be high.Victimsoftheexertionalform,inwhichahighrateofmetabolicheatproductionisaprimary pathogenic factor,typicallyare athletesor military personnelespecially recruits.uringthesecondWorldWar,theincidence offatalheatstrokewaslowaftereightweeksoftraining76,by whichtimetherecruitswerewellacclimatized.Muchoftheprotectiveeffectofacclimatization presumablyowestothermoregulatoryimprovement,bu tacclimatizationandphysicalconditioningmay alsoprotectinwaysthatare poorlyunderstood,sincesomelong-distance runnerstoleratehighcoretemperatureswithoutapparentilleffect(e.g., 117118).

Asmallproportionofapparently healthyindividualscannotacclimatizetoheat"9120.nSouthAfricangoldminingrecruits(thepopulationstudiedmostextensivelyinthisregard)individualswhodo no tacclimatizeare,onaverage,smaller,older,andlessaerobicallyfitthanthosewhodo 120.

PhysicalFitness,Age,Drugs,andDisease

Lowphysicalfitness,variousdiseases,andageingdecreaseheattoleranceandthesensitivityofthesweatingresponse. anydrugsinhibitsweating,mostprominently thoseusedfo r theiranticholinergiceffects,suchasatropineandscopolamine.ntramuscularinjectionof2mg atropine(thedoseinoneautoinjectorforacutetreatmentofexposuretonerveagent)inhibitssweatingsufficiently tocauseanoticeableimpairmentofthermoregulationduringwalkingindryheat121.omedrugsusedforotherpurposes,suchasglutethimide(asleep


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HumanAdaptationtoHotEnvironments(TMM)/20/972medicine),tricyclicantidepressants,andphenothiazines(tranquilizersandantipsychoticdrugs)alsohavesomeanticholinergic action;andallof these,plusseveralothers,havebeenassociated withheatstroke 122123. 30-mg oraldoseofpyridostigminebromide(thedosegiventhricedailyforpre-treatment againstnerveagent)reducedthermoregulatoryvasodilationduringmoderate exerciseinawarmenvironment124 ,andmay potentiallyimpairthermoregulationundermoresevereheat-stressconditions.

Neurologicaldiseasesinvolvingthethermoregulatory structuresinthebrainstemcanimpairthermoregulation.lthoughhypothermia may result,hyperthermiaismoreusual,andtypicallyisaccompaniedby lossofsweatingandthecircadianrhythm.Severalskindiseasesimpairsweatingsufficientlythatheatexposure,especially combinedwithexercise,may producedangerouslyhighbodytemperatures.Ichthyosisandanhidroticectodermaldysplasiacanprofoundlylimittheabilitytothermoregulateintheheat.Inaddition,heatrash(miliariarubra)125andevenmildsunburn126impairsweatingandmayreducetolerancetoexerciseintheheat.he thermoregulatory effectsofheatrashmay persistforaweekorlongeraftertheappearanceoftheskinhasreturnedtonormal125 .


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