thermo regulat

7/25/2019 Thermo Regulat

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ThermoregulationThermoregulation

The maintenance of aThe maintenance of a

particular temperature in aparticular temperature in aliving body.living body.


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Overview ofOverview of

ThermoregulationThermoregulationMechanisms of ThermoregulationMechanisms of Thermoregulation

Exercise in Heat StressExercise in Heat Stress

Heat llnessHeat llness

Exercise in !old StressExercise in !old Stress


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Thermal "alanceThermal "alance

!ore temperature #T!ore temperature #T!O!O$ is in dynamic$ is in dynamice%uilibrium as a result of balancee%uilibrium as a result of balance

between heat gain and heat loss.between heat gain and heat loss. Mean body temperature #TMean body temperature #Tbodybody$$

represents an average of s&in andrepresents an average of s&in and

internal temperatures.internal temperatures.


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Hypothalamus 'egulation ofHypothalamus 'egulation of

TemperatureTemperature

Hypothalamus actsHypothalamus actsas (thermostat)as (thermostat)that ma&esthat ma&esthermoregulatorythermoregulatoryad*ustments toad*ustments todeviations fromdeviations fromtemperature normtemperature norm

in the brain #+,in the brain #+,!!- - ! or! or/0.1/0.1-.0-.02 $.2 $.


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Hypothalamus 'egulation ofHypothalamus 'egulation of

TemperatureTemperature Mechanisms areMechanisms are

activated in twoactivated in twoways3ways3

4 Thermal receptors inThermal receptors ins&in provide input tos&in provide input tocentral commandcentral command

4 5irect stimulation of5irect stimulation ofhypothalamushypothalamusthrough changes inthrough changes inblood temperatureblood temperatureperfusing areaperfusing area


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Thermoregulation in !oldThermoregulation in !old

6ascular ad*ustments3 constrict6ascular ad*ustments3 constrictperipheral blood vessels.peripheral blood vessels.

Muscular activity3 exercise energyMuscular activity3 exercise energymetabolism and shivering.metabolism and shivering.

Hormonal output3 epinephrine andHormonal output3 epinephrine and

norepinephrine increase basal heatnorepinephrine increase basal heatproduction7 prolonged cold 4 thyroxin.production7 prolonged cold 4 thyroxin.


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Thermoregulation in HeatThermoregulation in Heat


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Heat 8oss by 'adiation #9 :;$Heat 8oss by 'adiation #9 :;$4 Ob*ects emit electromagnetic heatOb*ects emit electromagnetic heat

waves without molecular contact withwaves without molecular contact withwarmer ob*ects.warmer ob*ects.

4


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ThermoregulationThermoregulation


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Heat 8oss by !onductionHeat 8oss by !onduction4 5irect transfer of heat through a li%uid=5irect transfer of heat through a li%uid=

solid= or gas from one molecule tosolid= or gas from one molecule to

another.another.

4 > small amount of body heat moves by> small amount of body heat moves by

conduction directly through deep tissuesconduction directly through deep tissues

to cooler surface. Heat loss involves theto cooler surface. Heat loss involves the

warming of air molecules and coolerwarming of air molecules and cooler

surfaces in contact with the s&in.surfaces in contact with the s&in.

4The rate of conductive heat loss dependsThe rate of conductive heat loss depends

on thermal gradient.on thermal gradient.


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Heat 8oss by !onvectionHeat 8oss by !onvection #? conduction#? conduction+@;$+@;$

4 EAectiveness depends on how rapidly theEAectiveness depends on how rapidly theair #or water$ ad*acent to the body isair #or water$ ad*acent to the body is

exchanged.exchanged.

4 >ir currents at B mph are about twice as>ir currents at B mph are about twice as

eAective for cooling air currents at mph.eAective for cooling air currents at mph.


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Heat 8oss by Evaporation #9 @@;$Heat 8oss by Evaporation #9 @@;$4 Heat transferred as water is vaporiCed fromHeat transferred as water is vaporiCed from

respiratory passages and s&in surfaces.respiratory passages and s&in surfaces.4 2or each liter of water vaporiCed= @0: &cal2or each liter of water vaporiCed= @0: &cal

transferred to the environment.transferred to the environment.

4


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Heat 8oss at High >mbientHeat 8oss at High >mbient

TemperaturesTemperatures

EAectiveness of heat loss viaEAectiveness of heat loss viaconduction= convection= and radiationconduction= convection= and radiation

decreases.decreases.


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Heat 8oss in High HumidityHeat 8oss in High Humidity

Total sweat vaporiCed from s&in dependsTotal sweat vaporiCed from s&in dependson3on3

4 Surface area exposed to environmentSurface area exposed to environment4Temperature and humidity of ambient airTemperature and humidity of ambient air

4 !onvective air currents about the body!onvective air currents about the body

Most important factor is relative humidity.Most important factor is relative humidity.


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ntegration of HeatDntegration of HeatD

5issipating Mechanisms5issipating Mechanisms !irculation.!irculation.

Supercial venousSupercial venous

and arterial bloodand arterial bloodvessels dilate tovessels dilate to

divert warm blooddivert warm blood

to the body shell.to the body shell.


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ntegration of HeatD5issipatingntegration of HeatD5issipating

MechanismsMechanisms

Evaporation. SweatingEvaporation. Sweatingbegins within .@ sbegins within .@ safter start of vigorousafter start of vigorous

exercise.exercise. HormonalHormonal

ad*ustments. !ertainad*ustments. !ertainhormonal ad*ustmentshormonal ad*ustmentsare initiated in heatare initiated in heatstress as bodystress as bodyattempts to conserveattempts to conserveFuids and sodium.Fuids and sodium.


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Hormones in Heat StressHormones in Heat Stress

>ntidiuretic>ntidiuretichormone #>5H$ ishormone #>5H$ is

released to increasereleased to increase

water reDabsorptionwater reDabsorption

from &idneys.from &idneys.

>ldosterone is>ldosterone is

released to increasereleased to increasethe reDabsorption ofthe reDabsorption of

sodium.sodium.


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EAects ofEAects of

!lothing!lothing!old llow water vapor

to escapeto escape


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!irculatory >d*ustments.!irculatory >d*ustments.4 !ardiovascular drift 4 Fuid loss reduces plasma!ardiovascular drift 4 Fuid loss reduces plasma

volume #about :; of Fuid lost comes from plasma.volume #about :; of Fuid lost comes from plasma.

>bout @:; comes from intracellular water$.>bout @:; comes from intracellular water$.4 6isceral vascular constriction and s&in G muscle6isceral vascular constriction and s&in G muscle

vascular dilation.vascular dilation.

4 Maintaining blood pressure. !irculatory regulationMaintaining blood pressure. !irculatory regulation

and maintenance of muscle blood Fow ta&eand maintenance of muscle blood Fow ta&eprecedence over temperature regulation often atprecedence over temperature regulation often at

the expense of spiraling core temperature duringthe expense of spiraling core temperature during

exercise in heat.exercise in heat.


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!ore!oretemperaturetemperature

4 More than li&elyMore than li&elya modesta modest

increase in coreincrease in core

temperaturetemperature

reFects favorablereFects favorableinternalinternal

ad*ustments.ad*ustments.


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>cclimatiCation to Heat>cclimatiCation to Heat

>cclimatiCation refers to physiological>cclimatiCation refers to physiological

changes that improve heat tolerance.changes that improve heat tolerance.

I 4 B hours daily heat exposure produceI 4 B hours daily heat exposure produce

complete acclimatiCation @D: days.complete acclimatiCation @D: days.

o Rectal temperature

HR

Sweat rate


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2actors that mprove Heat2actors that mprove Heat

Tolerance3 >cclimatiCationTolerance3 >cclimatiCation

mproved cutaneous bloodmproved cutaneous blood

FowFowTransports metabolic heatTransports metabolic heat

from deep tissues to bodyJsfrom deep tissues to bodyJs

shellshell

EAective distribution ofEAective distribution ofcardiac outputcardiac output

>ppropriate circulation to>ppropriate circulation tos&in G muscles to meets&in G muscles to meet

demands.demands.

8owered threshold for start8owered threshold for start

of sweatingof sweating

Evaporative cooling beginsEvaporative cooling begins

early in exercise.early in exercise.More eAective distributionMore eAective distribution

of sweat over s&in surfaceof sweat over s&in surfaceOptimum use of surface forOptimum use of surface for

eAective evaporativeeAective evaporative

cooling.cooling.

ncreased rate of sweatingncreased rate of sweating MaximiCe evaporativeMaximiCe evaporative


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2actors that mprove Heat2actors that mprove Heat

ToleranceTolerance

2itness 8evel2itness 8evel

>ge #see 2K$>ge #see 2K$>ging delays the onset of sweating and>ging delays the onset of sweating and

blunts the magnitude of sweatingblunts the magnitude of sweating

responseresponse

LenderLender "ody fatness"ody fatness


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Evaluating Heat StressEvaluating Heat Stress

revention remainsrevention remainsmost eAective way tomost eAective way tomanage heatDstressmanage heatDstress

in*uriesin*uries Wet bulb-globeWet bulb-globe

temperaturetemperaturerelies onrelies onambientambient

temperature= relativetemperature= relativehumidity= and radianthumidity= and radiantheat.heat.

Heat stress indexHeat stress index


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revention of Heatrevention of Heat

llnessllness >llow ade%uate time for acclimatiCation.>llow ade%uate time for acclimatiCation.

Exercise during cooler parts of day.Exercise during cooler parts of day.

8imitdefer exercise if heat stress index is8imitdefer exercise if heat stress index isin high ris& Cone.in high ris& Cone.

Hydrate properly prior to exercise andHydrate properly prior to exercise and

replace Fuid loss during and after exercise.replace Fuid loss during and after exercise.


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Exercise in the !oldExercise in the !old

!old strain!old strain

4 Exposure to cold produces physiological G psychologicalExposure to cold produces physiological G psychologicalchallengeschallenges

4 "ody fat diAerences eAect physiological function in cold"ody fat diAerences eAect physiological function in cold

>cclimatiCation to the !old>cclimatiCation to the !oldHumans adapt more successfully to chronic heat than cold exposure.Humans adapt more successfully to chronic heat than cold exposure.


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Exercise in theExercise in the

!old!old Evaluating EnvironmentalEvaluating Environmental!old Stress!old Stress

4 ir warms to between 0:N

2 to /:N 2 as it reaches2 to /:N 2 as it reaches

bronchi.bronchi.4 Humidication of inspiredHumidication of inspired

cold air produces water Gcold air produces water G

heat loss from respiratoryheat loss from respiratory

tract.tract.

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