2Objectives Define the term homeotherm. Present an overview of heat balance during exercise.Discuss the concept of “core temperature.”List the principal means of involuntarily increasing heat production.Define four processes by which the body can lose heat during exercise.Discuss the role of hypothalamus as the body’s thermostat.
3ObjectivesExplain the thermal events that occur during exercise in both a cool/moderate and hot/humid environment.List physiological adaptations that occur during acclimatization to heat.Describe the physiological responses to a cold environment.Discuss the physiological changes that occur in response to cold acclimatization.
4Outline Overview of Heat Balance During Exercise Temperature Measurement During ExerciseOverview of Heat Production/Heat LossHeat ProductionHeat LossHeat Storage in the Body During ExerciseBody’s Thermostat— HypothalamusShift in the Hypothalamic Thermostat Set Point Due to FeverHeat Index—A Measure of How Hot It FeelsThermal Events During ExerciseExercise in the HeatSweat Rates During ExerciseExercise Performance in a Hot EnvironmentGender and Age Differences in ThermoregulationHeat AcclimatizationLoss of AcclimatizationExercise in a Cold EnvironmentCold Acclimatization
5An Overview of Heat Balance Overview of Heat Balance During ExerciseAn Overview of Heat BalanceHumans are homeothermsMaintain constant body core temperatureHeat loss must match heat gainNormal core temperature is 37°CAbove 45°CMay destroy proteins and enzymes and lead to deathBelow 34°CMay cause slowed metabolism and arrhythmiasThermal gradient from body core to skin surfaceIdeal gradient is ~4°CIn extreme cold, may be 20°C
6An Overview of Heat Balance Overview of Heat Balance During ExerciseAn Overview of Heat BalanceFigure 12.1
7Overview of Heat Balance During Exercise In SummaryHomeotherms are animals that maintain a rather constant body core temperature. In order to maintain a constant core temperature, heat loss must match heat gain.Temperature varies a great deal within the body. In general, there is a thermal gradient from the deep bode temperature (core temperature) to the shell (skin) temperature.
8Temperature Measurement During Exercise Deep-body (core) temperatureMeasured at rectum, ear, or esophagusUsually in laboratoryIngestible temperature sensor telemetry systemIn athletes during practice sessionsSkin temperatureThermistors at various locationsCalculate mean skin temperatureTskin = (Tforehead + Tchest + Tforearm + Tthigh + Tcalf + Tabdomen + Tback) ÷ 7
9Temperature Measurement During Exercise In SummaryMeasurements of deep-body temperature can be accomplished via mercury thermometers, or devices known as thermocouples or thermistors. Common sites of measurement include the rectum, the ear (tympanic temperature), and the esophagus.Skin temperature can be measured by placing temperature sensors (thermistors) on the skin at various locations.
10Heat Production Voluntary Exercise Involuntary Shivering Overview of Heat Production/Heat LossHeat ProductionVoluntaryExercise70–80% energy expenditure appears as heatInvoluntaryShiveringIncreases heat production by ~5xAction of hormonesThyroxineCatecholaminesCalled nonshivering thermogenesis
11Overview of Heat Production/Heat Loss Figure 12.2
12Heat Loss Radiation Transfer of heat via infrared rays Overview of Heat Production/Heat LossHeat LossRadiationTransfer of heat via infrared rays60% heat loss at restCan be a method of heat gainConductionHeat loss due to contact with another surfaceConvectionHeat transferred to air or waterExample: a fan pushing air past skin
13Overview of Heat Production/Heat Loss EvaporationHeat from skin converts water (sweat) to water vaporRequires vapor pressure gradient between skin and airEvaporation rate depends on:Temperature and relative humidityConvective currents around the bodyAmount of skin surface exposedBody loses 0.58 kcal heat/L sweat evaporated1 L sweat results in heat loss of 580 kcal25% heat loss at restMost important means of heat loss during exercise
14Temperature, Relative Humidity, and Vapor Pressure Overview of Heat Production/Heat LossTemperature, Relative Humidity, and Vapor Pressure
15A Closer Look 12.1 Calculation of Heat Loss via Evaporation Overview of Heat Production/Heat LossA Closer Look 12.1 Calculation of Heat Loss via EvaporationEvaporation of 1,000 ml of sweat results in 580 kcal of heat lossExample:20 min cycling at VO2 = 2.0 L•min–1 (10 kcal•min–1)20% efficient = 80% energy lost as heatTotal energy expenditure20 min x 10 kcal/min = 200 kcalTotal heat produced200 kcal x 0.80 = 160 kcalEvaporation to prevent heat gain160 kcal ÷ 580 kcal/L = L
16Heat Exchange Mechanisms During Exercise Overview of Heat Production/Heat LossHeat Exchange Mechanisms During ExerciseFigure 12.3
17Heat Storage in the Body During Exercise Overview of Heat Production/Heat LossHeat Storage in the Body During ExerciseHeat produced that is not lost is stored in body tissuesWill raise body temperatureAmount of heat required to raise body temperatureSpecific heat of human body is 0.83 kcal/kgBody heat gain during exercise = heat produced – heat lossHeat required to raise body temp 1°C = specific heat x body mass
18Overview of Heat Production/Heat Loss A Closer Look 12.2 Calculation of Body Temperature Increase During Exercise60-kg runner, 40 min at VO2 = 3.0 L•min–1 (15 kcal•min–1)20% efficient, can lose 60% of heat producedPre-exercise body temperature = 37°CTotal energy expenditure40 min x 15 kcal/min = 600 kcalTotal heat produced600 kcal x 0.80 = 480 kcalTotal heat stored480 kcal x 0.40 = 192 kcalAmount of heat storage to increase body temperature by 1°C0.83 kcal/kg x 60 kg = 49.8 kcalIncrease in body temperature during exercise192 kcal/49.8 kcal/°C = 3.86°CPost-exercise body temperature37°C °C = 40.86°C
19Overview of Heat Production/Heat Loss In SummaryMuscular exercise can result in large amounts of heat production. Since the body is at most 20% to 30% efficient, 70% to 80% of the energy expended during exercise is released as heat.Body heat can be lost by evaporation, convection, conduction, and radiation. During exercise in a cool environment, evaporation is the primary avenue for heat loss.
20Overview of Heat Production/Heat Loss In SummaryThe rate of evaporation from the skin is dependent upon three factors: (1) temperature and relative humidity, (2) convective currents around the body, and (3) the amount of skin exposed to the environment.Body heat storage is the difference between heat production and heat loss.The amount of heat required to elevate body temperature by 1°C is termed the specific heat of the body.
21The Body’s Thermostat—Hypothalamus Anterior hypothalamusResponds to increased core temperatureCommencement of sweatingIncreased evaporative heat lossIncreased skin blood flowAllows increased heat lossPosterior hypothalamusResponds to decreased core temperatureShivering and increased norepinephrine releaseIncreased heat productionDecreased skin blood flowDecreased heat loss
22Physiological Responses to “Heat Load” The Body’s Thermostat—HypothalamusPhysiological Responses to “Heat Load”Figure 12.4
23Physiological Responses to Cold Stress The Body’s Thermostat—HypothalamusPhysiological Responses to Cold StressFigure 12.5
24Shift in Hypothalamic Set Point Due to Fever The Body’s Thermostat—HypothalamusShift in Hypothalamic Set Point Due to FeverFeverIncreased body temperature above normalDue to pyrogensProteins or toxins from bacteriaChange in set point of hypothalamus
25In Summary The body’s thermostat is located in the hypothalamus. The Body’s Thermostat—HypothalamusIn SummaryThe body’s thermostat is located in the hypothalamus.The anterior hypothalamus is responsible for reacting to increases in core temperature, while the posterior hypothalamus governs the body’s responses to a decrease in temperature.An increase in core temperature results in the anterior hypothalamus initiating a series of physiological actions aimed at increasing heat loss. These actions include: (1) the commencement of sweating and (2) an increase in skin blood flow.
26The Body’s Thermostat—Hypothalamus In SummaryCold exposure results in the posterior hypothalamus promoting physiological changes that increase body heat production (shivering) and reduce heat loss (cutaneous vasoconstriction).
27Thermal Events During Exercise As exercise intensity increases:Heat production increasesLinear increase in body temperatureCore temperature proportional to active muscle massHigher net heat lossLower convective and radiant heat lossHigher evaporative heat lossAs ambient temperature increases:Heat production remains constant
28Changes in Metabolic Energy Production and Heat Loss During Exercise Thermal Events During ExerciseChanges in Metabolic Energy Production and Heat Loss During ExerciseFigure 12.6
29Body Temperature During Arm and Leg Exercise Thermal Events During ExerciseBody Temperature During Arm and Leg ExerciseFigure 12.7
30Heat Exchange During Exercise at Different Environmental Temperatures Thermal Events During ExerciseHeat Exchange During Exercise at Different Environmental TemperaturesFigure 12.8
31Thermal Events During Exercise Increase in body temperature is directly related to exercise intensityBody heat load increases with intensityMechanisms of heat loss during exerciseEvaporationMost important means of heat lossConvectionSmall contributionRadiationSmall role in total heat loss
32Heat Exchange At Rest and During Exercise Thermal Events During ExerciseHeat Exchange At Rest and During ExerciseFigure 12.9
33Thermal Events During Exercise In SummaryDuring constant intensity exercise, the increase in body temperature is directly related to the exercise intensity.Body heat production increases in proportion to exercise intensity.
34Heat Index Measure of body’s perception of how hot it feels Heat Index—A Measure of How Hot It FeelsHeat IndexMeasure of body’s perception of how hot it feelsRelative humidity added to air temperatureExample:Air temperature = 80°F, relative humidity = 80%Heat index = 89°FHigh relative humidity reduces evaporative heat lossLowers heat lossIncreases body temperature
35Relationship of Relative Humidity (%), Temperature, and Heat Index Heat Index—A Measure of How Hot It FeelsRelationship of Relative Humidity (%), Temperature, and Heat IndexFigure 12.10
36Exercise in the Heat Inability to lose heat Higher core temperature Risk of hyperthermia and heat injuryHigher sweat rateMay be as high as 4–5 L/hourRisk of dehydration
37Exercise in the HeatCore Temperature and Sweat Rate During Exercise in a Hot/Humid EnvironmentFigure 12.11
38Exercise in the HeatClinical Applications 12.1 Exercise-Related Heat Injuries Can Be PreventedGuidelinesExercise during the coolest part of the dayMinimize exercise intensity and duration on hot/humid daysExpose a maximal surface area of skin for evaporationProvide frequent rests/cool-down breaks with equipment removalAvoid dehydration with frequent water breaksRest/cool-down breaks should be in the shade and offer circulating, cool air
39The Winning Edge 12.1 Prevention of Dehydration During Exercise Exercise in the HeatThe Winning Edge 12.1 Prevention of Dehydration During ExerciseDehydration of 1–2% body weight can impair performanceGuidelinesHydrate prior to performance400–800 ml fluid within three hours prior to exerciseConsume 150–300 ml fluid every 15–20 minVolume adjusted based on environmental conditionsEnsure adequate rehydrationConsume equivalent of 150% weight loss1 kg body weight = 1.5 L fluid replacementMonitor urine colorSports drinks are superior to water for rehydration
40Exercise Performance in a Hot Environment Exercise in the HeatExercise Performance in a Hot EnvironmentCan result in muscle fatigue and impaired performanceReduced mental drive for motor performanceReduced muscle blood flowAccelerated glycogen metabolismIncreased lactic acid productionIncreased free radical production
41Research Focus 12.1 Exercise in the Heat Accelerates Muscle Fatigue Rapid onset of muscle fatigue in hot/humid environmentsHeat-related muscle fatigue due to:High brain temperature reduces neuromuscular driveReduction in motor unit recruitmentAccelerated muscle glycogen metabolism and hypoglycemiaControversialIncreased free radical productionDamage to muscle contractile protein
42Gender and Age Differences in Thermoregulation Exercise in the HeatGender and Age Differences in ThermoregulationWomen less heat tolerant than menLower sweat ratesHigher percent body fatAge itself does not limit ability to thermoregulateDecreased thermotolerance with age due to:Deconditioning with ageLack of heat acclimatization
43Heat Acclimatization Requires exercise in hot environment Exercise in the HeatHeat AcclimatizationRequires exercise in hot environmentAdaptations occur within 7–14 daysIncreased plasma volumeEarlier onset of sweatingHigher sweat rateReduced sodium chloride loss in sweatReduced skin blood flowIncreased cellular heat shock proteinsAcclimatization lost within a few days of inactivity
44Research Focus 12.2 Can Sweat Clothing Promote Heat Acclimatization? Exercise in the HeatResearch Focus 12.2 Can Sweat Clothing Promote Heat Acclimatization?Training in cool climates using sweat clothingAlternative to traveling to warmer climate to acclimatizeRaises body temperatureDoes it work?Yes, but not as effective as training in hot/humid environmentIncreases sweat rate and body water lossNot effective for fat lossRisk of hyperthermia and heat injury
45Research Focus 12.3 Heat Acclimatization and Heat Shock Proteins Exercise in the HeatResearch Focus 12.3 Heat Acclimatization and Heat Shock ProteinsHeat acclimatization reduces the risk of heat injuryIn response to exposure of heat stressRelated to synthesis of heat shock proteinsProtect cells from thermal injuryStabilizing and refolding damaged proteins
46Primary Adaptations of Heat Acclimatization Exercise in the HeatPrimary Adaptations of Heat Acclimatization
47Exercise in the HeatIn SummaryDuring prolonged exercise in a moderate environment, core temperature will increase gradually above the normal resting value and will reach a plateau at approximately thirty to forty-five minutes.During exercise in a hot/humid environment, core temperature does not reach a plateau, but will continue to rise. Long-term exercise in this type of environment increases the risk of heat injury.Heat acclimatization results in: (1) an increase in plasma volume, (2) an earlier onset of sweating, (3) a higher sweat rate, (4) a reduction in the amount of electrolytes lost in sweat, (5) a reduction in skin blood flow, and (6) increased levels of heat shock protein in tissues.
48Exercise in a Cold Environment Enhanced heat lossReduces chance of heat injuryMay result in hypothermiaLoss of judgment and risk of further cold injuryCold acclimatizationResults in lower skin temperature at which shivering beginsIncreased nonshivering thermogenesisMaintain higher hand and foot temperatureImproved peripheral blood flowImproved ability to sleep in the coldDue to reduced shiveringAdaptations begin in one week
49Exercise in a Cold Environment In SummaryExercise in a cold environment enhances an athlete’s ability to lose heat and therefore greatly reduces the chance of heat injury.Cold acclimatization results in three physiological adaptations: (1) improved ability to sleep in cold environments, (2) increased nonshivering thermogenesis, and (3) a higher intermittent blood flow to the hands and feet. The overall goal of these adaptations is to increase heat production and maintain core temperature, which will make the individual more comfortable during cold exposure.
50Study QuestionsDefine the following terms: (1) homeotherm, (2) hyperthermia, and (3) hypothermia.Why does a significant increase in core temperature represent a threat to life?Explain the comment that the term body temperature is a misnomer.How is body temperature measured during exercise?Briefly discuss the role of the hypothalamus in temperature regulation. How do the anterior hypothalamus and posterior hypothalamus differ in function?
51Study QuestionsList and define the four mechanisms of heat loss. Which of these avenues plays the most important part during exercise in a hot/dry environment?Discuss the two general categories of heat production in people.What hormones are involved in biochemical heat production?Briefly outline the thermal events that occur during prolonged exercise in a moderate environment. Include in your discussion information about changes in core temperature, skin blood flow, sweating, and skin temperature.
52Study QuestionsCalculate the amount of evaporation that must occur to remove 400 kcal of heat from the body.How much heat would be removed from the skin if 520 ml of sweat evaporated during a thirty-minute period?List and discuss the physiological adaptations that occur during heat acclimatization.How might exercise in a cold environment affect dexterity in such skills as throwing and catching?Discuss the physiological changes that occur in response to chronic exposure to cold.