ThermoregulationThermoregulation

The maintenance of a The maintenance of a particular temperature in a particular temperature in a

living body.living body.

Overview of Overview of ThermoregulationThermoregulation

•Mechanisms of ThermoregulationMechanisms of Thermoregulation

•Exercise in Heat StressExercise in Heat Stress

•Heat IllnessHeat Illness

•Exercise in Cold StressExercise in Cold Stress

Thermal BalanceThermal Balance

• Core temperature (TCore temperature (TCOCO) is in dynamic ) is in dynamic equilibrium as a result of balance equilibrium 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 skin and represents an average of skin and internal temperatures.internal temperatures.

Hypothalamus Regulation of Hypothalamus Regulation of TemperatureTemperature

• Hypothalamus acts Hypothalamus acts as “thermostat” as “thermostat” that makes that makes thermoregulatory thermoregulatory adjustments to adjustments to deviations from deviations from temperature norm temperature norm in the brain (37in the brain (37 C C ± 1± 1 C or C or 98.698.6±1.8±1.8 F ). F ).

Hypothalamus Regulation of Hypothalamus Regulation of TemperatureTemperature

• Mechanisms are Mechanisms are activated in two activated in two ways:ways:– Thermal receptors Thermal receptors

in skin provide input in skin provide input to central commandto central command

– Direct stimulation of Direct stimulation of hypothalamus hypothalamus through changes in through changes in blood temperature blood temperature perfusing areaperfusing area

Thermoregulation in ColdThermoregulation in Cold

• Vascular adjustments: constrict Vascular adjustments: constrict peripheral blood vessels.peripheral blood vessels.

• Muscular activity: exercise energy Muscular activity: exercise energy metabolism and shivering.metabolism and shivering.

• Hormonal output: epinephrine and Hormonal output: epinephrine and norepinephrine increase basal heat norepinephrine increase basal heat production; prolonged cold – production; prolonged cold – thyroxin.thyroxin.

Thermoregulation in HeatThermoregulation in Heat

Thermoregulation in HeatThermoregulation in Heat

• Heat Loss by Radiation (~ 10%)Heat Loss by Radiation (~ 10%)– Objects emit electromagnetic heat Objects emit electromagnetic heat

waves without molecular contact with waves without molecular contact with warmer objects.warmer objects.

– When temperature of things in When temperature of things in environment exceeds the skin environment exceeds the skin temperature, radiant heat energy is temperature, radiant heat energy is absorbed from the surroundings.absorbed from the surroundings.

ThermoregulationThermoregulation

Thermoregulation in HeatThermoregulation in Heat

• Heat Loss by Conduction Heat Loss by Conduction – Direct transfer of heat through a liquid, Direct transfer of heat through a liquid,

solid, or gas from one molecule to solid, or gas from one molecule to another.another.

– A small amount of body heat moves by A small amount of body heat moves by conduction directly through deep tissues conduction directly through deep tissues to cooler surface. Heat loss involves the to cooler surface. Heat loss involves the warming of air molecules and cooler warming of air molecules and cooler surfaces in contact with the skin.surfaces in contact with the skin.

– The rate of conductive heat loss The rate of conductive heat loss depends on thermal gradient.depends on thermal gradient.

Thermoregulation in HeatThermoregulation in Heat

• Heat Loss by Convection Heat Loss by Convection (+ conduction (+ conduction 35%)35%)– Effectiveness depends on how rapidly the Effectiveness depends on how rapidly the

air (or water) adjacent to the body is air (or water) adjacent to the body is exchanged.exchanged.

– Air currents at 4 mph are about twice as Air currents at 4 mph are about twice as effective for cooling air currents at 1 mph.effective for cooling air currents at 1 mph.

Thermoregulation in HeatThermoregulation in Heat

• Heat Loss by Evaporation (~ 55%)Heat Loss by Evaporation (~ 55%)– Heat transferred as water is vaporized from Heat transferred as water is vaporized from

respiratory passages and skin surfaces.respiratory passages and skin surfaces.– For each liter of water vaporized, 580 kcal For each liter of water vaporized, 580 kcal

transferred to the environment.transferred to the environment.– When sweat comes in contact with the skin, When sweat comes in contact with the skin,

a cooling effect occurs as sweat evaporates.a cooling effect occurs as sweat evaporates.– The cooled skin serves to cool the blood.The cooled skin serves to cool the blood.

Heat Loss at High Ambient Heat Loss at High Ambient TemperaturesTemperatures

• Effectiveness of heat loss via Effectiveness of heat loss via conduction, convection, and radiation conduction, convection, and radiation decreases.decreases.

• When ambient temperature exceeds When ambient temperature exceeds body temperature, heat is gained.body temperature, heat is gained.

• The only effective mechanism is The only effective mechanism is evaporation of sweat and respiratory evaporation of sweat and respiratory tract vaporization of water.tract vaporization of water.

Heat Loss in High HumidityHeat Loss in High Humidity

• Total sweat vaporized from skin depends Total sweat vaporized from skin depends on:on:– Surface area exposed to environmentSurface area exposed to environment– Temperature and humidity of ambient airTemperature and humidity of ambient air– Convective air currents about the bodyConvective air currents about the body

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

• When relative humidity is high, the When relative humidity is high, the ambient water vapor pressure approaches ambient water vapor pressure approaches that of the moist skin and evaporation is that of the moist skin and evaporation is impeded.impeded.

Integration of Heat-Integration of Heat-Dissipating MechanismsDissipating Mechanisms

• Circulation. Circulation. Superficial venous Superficial venous and arterial blood and arterial blood vessels dilate to vessels dilate to divert warm blood divert warm blood to the body shell.to the body shell.

Integration of Heat-Dissipating Integration of Heat-Dissipating MechanismsMechanisms

• Evaporation. Evaporation. Sweating begins Sweating begins within 1.5 s after start within 1.5 s after start of vigorous exercise.of vigorous exercise.

• Hormonal Hormonal adjustments. Certain adjustments. Certain hormonal hormonal adjustments are adjustments are initiated in heat stress initiated in heat stress as body attempts to as body attempts to conserve fluids and conserve fluids and sodium.sodium.

Hormones in Heat StressHormones in Heat Stress

• Antidiuretic Antidiuretic hormone (ADH) is hormone (ADH) is released to increase released to increase water re-absorption water re-absorption from kidneys.from kidneys.

• Aldosterone is Aldosterone is released to increase released to increase the re-absorption of the re-absorption of sodium.sodium.

Effects of Effects of ClothingClothing

Cold Weather Cold Weather Clothing provide an Clothing provide an air barrier to air barrier to prevent convection prevent convection and conduction.and conduction.

• Layers provide Layers provide more trapped airmore trapped air

• Allow water vapor Allow water vapor to escapeto escape

Warm Weather Clothing Warm Weather Clothing loose fitting to permit loose fitting to permit free convection.free convection.

• The less surface The less surface covered the more covered the more evaporative cooling.evaporative cooling.

• Clothing should be Clothing should be loosely woven to loosely woven to allow skin to breathe.allow skin to breathe.

Exercise in Heat StressExercise in Heat Stress

• Circulatory Adjustments.Circulatory Adjustments.– Cardiovascular drift – fluid loss reduces plasma Cardiovascular drift – fluid loss reduces plasma

volume (about 10% of fluid lost comes from volume (about 10% of fluid lost comes from plasma. About 50% comes from intracellular water).plasma. About 50% comes from intracellular water).

– Visceral vascular constriction and skin & muscle Visceral vascular constriction and skin & muscle vascular dilation.vascular dilation.

– Maintaining blood pressure. Circulatory regulation Maintaining blood pressure. Circulatory regulation and maintenance of muscle blood flow take and maintenance of muscle blood flow take precedence over temperature regulation often at precedence over temperature regulation often at the expense of spiraling core temperature during the expense of spiraling core temperature during exercise in heat.exercise in heat.

Exercise in Heat StressExercise in Heat Stress

• Core temperatureCore temperature– More than likely a More than likely a

modest increase modest increase in core in core temperature temperature reflects favorable reflects favorable internal internal adjustments.adjustments.

Water Loss in the HeatWater Loss in the Heat

• Magnitude of Fluid Loss in Exercise.Magnitude of Fluid Loss in Exercise.• Consequences of Dehydration.Consequences of Dehydration.

– plasma volume plasma volume peripheral blood peripheral blood flow & flow & sweat ratesweat rate

• Water ReplacementWater Replacement– Primary aim of fluid replacement during Primary aim of fluid replacement during

exercise is to maintain plasma volumeexercise is to maintain plasma volume– The most effective defense against heat The most effective defense against heat

stress is adequate hydrationstress is adequate hydration

• Electrolyte Replacement.Electrolyte Replacement.

Acclimatization to HeatAcclimatization to HeatAcclimatization refers to physiological Acclimatization refers to physiological

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

2 – 4 hours daily heat exposure produce 2 – 4 hours daily heat exposure produce complete acclimatization 5-10 days. complete acclimatization 5-10 days.

o Rectal temperature• HR∆ Sweat rate

Factors that Improve Heat Factors that Improve Heat Tolerance: AcclimatizationTolerance: Acclimatization

Improved cutaneous blood Improved cutaneous blood flowflow

Transports metabolic heat Transports metabolic heat from deep tissues to body’s from deep tissues to body’s shellshell

Effective distribution of Effective distribution of cardiac outputcardiac output

Appropriate circulation to Appropriate circulation to skin & muscles to meet skin & muscles to meet demands.demands.

Lowered threshold for start Lowered threshold for start of sweatingof sweating

Evaporative cooling begins Evaporative cooling begins early in exercise.early in exercise.

More effective distribution More effective distribution of sweat over skin surfaceof sweat over skin surface

Optimum use of surface for Optimum use of surface for effective evaporative effective evaporative cooling.cooling.

Increased rate of sweatingIncreased rate of sweating Maximize evaporative Maximize evaporative cooling.cooling.

Decreased salt Decreased salt concentration of sweatconcentration of sweat

Dilute sweat preserves Dilute sweat preserves electrolyte in fluids.electrolyte in fluids.

Factors that Improve Heat Factors that Improve Heat ToleranceTolerance

• Fitness LevelFitness Level

• Age (see FYI)Age (see FYI)Aging delays the onset of sweating and Aging delays the onset of sweating and

blunts the magnitude of sweating blunts the magnitude of sweating responseresponse

• GenderGender

• Body fatnessBody fatness

Evaluating Heat StressEvaluating Heat Stress

• Prevention remains Prevention remains most effective way most effective way to manage heat-to manage heat-stress injuriesstress injuries

• Wet bulb-globe Wet bulb-globe temperaturetemperature relies relies on ambient on ambient temperature, relative temperature, relative humidity, and humidity, and radiant heat.radiant heat.

• Heat stress indexHeat stress index

Heat IllnessHeat Illness

Heat IllnessHeat Illness

Prevention of Heat Prevention of Heat IllnessIllness• Allow adequate time for acclimatization.Allow adequate time for acclimatization.

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

• Limit/defer exercise if heat stress index is Limit/defer exercise if heat stress index is in high risk zone.in high risk zone.

• Hydrate properly prior to exercise and Hydrate properly prior to exercise and replace fluid loss during and after replace fluid loss during and after exercise.exercise.

• Wear clothing that is light in color and Wear clothing that is light in color and loose fitting.loose fitting.

Exercise in the ColdExercise in the Cold

• Cold strainCold strain– Exposure to cold produces physiological & psychological Exposure to cold produces physiological & psychological

challengeschallenges– Body fat differences effect physiological function in coldBody fat differences effect physiological function in cold

• Acclimatization to the ColdAcclimatization to the ColdHumans adapt more successfully to chronic heat than cold Humans adapt more successfully to chronic heat than cold

exposure.exposure.

Exercise in the Exercise in the ColdCold

• Evaluating Evaluating Environmental Cold Environmental Cold StressStress– Wind chill indexWind chill index

• Respiratory tract in ColdRespiratory tract in Cold– Cold air does not damage Cold air does not damage

respiratory passages.respiratory passages.– Air warms to between 80° Air warms to between 80°

F to 90° F as it reaches F to 90° F as it reaches bronchi.bronchi.

– Humidification of inspired Humidification of inspired cold air produces water & cold air produces water & heat loss from respiratory heat loss from respiratory tract.tract.