Chapter42-HeatandColdPHYSIOLOGICALRESPONSESTOTHETHERMALENVIRONMENTW.LarryKenneyHumanslivetheirentireliveswithinaverysmall,fiercelyprotectedrangeofinternalbodytemperatures.Themaximaltolerancelimitsforlivingcellsrangefromabout0°C(icecrystalformation)toabout45°C(thermalcoagulationofintracellularproteins);however,humanscantolerateinternaltemperaturesbelow35°Corabove41°Cforonlyverybriefperiodsoftime.Tomaintaininternaltemperaturewithintheselimits,peoplehavedevelopedveryeffectiveandinsomeinstancesspecializedphysiologicalresponsestoacutethermalstresses.Theseresponses—designedtofacilitatetheconservation,productionoreliminationofbodyheat—involvethefinelycontrolledcoordinationofseveralbodysystems.HumanThermalBalanceByfar,thelargestsourceofheatimpartedtothebodyresultsfrommetabolicheatproduction(M).Evenatpeakmechanicalefficiency,75to80%oftheenergyinvolvedinmuscularworkisliberatedasheat.Atrest,ametabolicrateof300mlO2perminutecreatesaheatloadofapproximately100Watts.Duringsteady-stateworkatanoxygenconsumptionof1l/min,approximately350Wofheataregenerated—lessanyenergyassociatedwithexternalwork(W).Evenatsuchamildtomoderateworkintensity,bodycoretemperaturewouldriseapproximatelyonedegreecentigradeevery15minwereitnotforanefficientmeansofheatdissipation.Infact,veryfitindividualscanproduceheatinexcessof1,200Wfor1to3hourswithoutheatinjury(GisolfiandWenger1984).Heatcanalsobegainedfromtheenvironmentviaradiation(R)andconvection(C)iftheglobetemperature(ameasureofradiantheat)andair(dry-bulb)temperature,respectively,exceedskintemperature.TheseavenuesofheatgainaretypicallysmallrelativetoM,andactuallybecomeavenuesofheatlosswhentheskin-to-airthermalgradientisreversed.Thefinalavenueforheatloss—evaporation(E)—isalsotypicallythemostimportant,sincethelatentheatofvaporizationofsweatishigh—approximately680W-h/lofsweatevaporated.Theserelationsarediscussedelsewhereinthischapter.Undercooltothermoneutralconditions,heatgainisbalancedbyheatloss,noheatisstored,andbodytemperatureequilibrates;thatis:M-W±R±C-E=0However,inmoresevereexposuretoheat:M-W±R±C>Eandheatisstored.Inparticular,heavywork(highenergyexpenditurewhichincreasesM-W),excessivelyhighairtemperatures(whichincreaseR+C),highhumidity(whichlimitsE)andthewearingofthickorrelativelyimpermeableclothing(whichcreatesabarriertoeffectiveevaporationofsweat)createsuchascenario.Finally,ifexerciseisprolongedorhydrationinadequate,Emaybeoutstrippedbythelimitedabilityofthebodytosecretesweat(1to2l/hforshortperiods).BodyTemperatureandItsControlForpurposesofdescribingphysiologicalresponsestoheatandcold,thebodyisdividedintotwocomponents—the“core”andthe“shell”.Coretemperature(Tc)representsinternalordeepbodytemperature,andcanbemeasuredorally,rectallyor,inlaboratorysettings,intheoesophagusoronthetympanicmembrane(eardrum).Thetemperatureoftheshellisrepresentedbymeanskintemperature(Tsk).Theaveragetemperatureofthebody(Tb)atanytimeisaweightedbalancebetweenthesetemperatures,thatisTb=kTc+(1–k)Tskwheretheweightingfactorkvariesfromabout0.67to0.90.Whenconfrontedwithchallengestothermalneutrality(heatorcoldstresses),thebodystrivestocontrolTcthroughphysiologicaladjustments,andTcprovidesthemajorfeedbacktothebraintocoordinatethiscontrol.Whilethelocalandmeanskintemperatureareimportantforprovidingsensoryinput,Tskvariesgreatlywithambienttemperature,averagingabout33°Catthermoneutr...
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