11 Exercise in Hot and Cold Environments: Thermoregulation chapter

Learning Objectives Find out how the body gets rid of excess body heat to maintain homeostasis at rest and during exercise Discover how the body adapts to exercise in a hot environment Learn why humidity is important when exercising in the heat (continued)

Learning Objectives (continued) Differentiate between heat cramps, heat exhaustion, and heatstroke Learn how the body minimizes excessive heat loss during exposure to cold Find out the dangers of exercising in the cold

Temperature Conversions In physiology, temperatures are expressed in degrees Centigrade –To go from °F to °C: subtract 32 °, then divide by 1.8 –To go from °C to °F: multiply by 1.8, then add 32 °

To Maintain a Steady-State Core Temperature, the Body Must Balance Heat Gain With Heat Loss

Modes of Heat Transfer Conduction: the transfer of heat from one solid material to another though direct molecular contact Convection: moving heat from one place to another by the motion of gas or liquid across a heated surface Radiation: heat is given off in the form of infrared rays Evaporation: heat is lost when fluid evaporates

Removal of Heat From the Skin

Thermograms of the Body Showing Variations in Radiant Heat From Department of Health and Human Performance, Auburn University, Alabama. Courtesy of John Eric Smith, Joe Molloy, and David D. Pascoe. By permission of David Pascoe.

Evaporation Primary avenue for heat dissipation during exercise Evaporation accounts for 80% of heat lost during exercise vs % at rest As body core temperature increases, sweat production increases Sweat must evaporate to provide cooling

Heat Balance Equation M – W ± R ± C ± K – E = S M = Metabolism W = Work R = Radiation C = Convection K = Conduction E = Evaporation S = Storage

Humidity Water vapor pressure of the air plays a major role in evaporative heat loss High humidity (regardless of temperature) limits evaporation of sweat Low humidity is ideal for sweat evaporation

The Complex Interaction Between the Body’s Mechanisms for Heat Balance and Environmental Conditions Adapted, by permission, from C.V. Gisolfi and C.B. Wenger, 1984, “Temperature regulation during exercise: Old concepts, new ideas,” Exercise and Sport Sciences Reviews 12:

Heat Balance Key Points Humans are homeothermic; they regulate an internal temperature between 36.1 to 37.8 °C (97.0 to °F) Body heat is transferred by conduction, convection, radiation, and evaporation At rest most heat is lost via radiation During exercise, evaporation is the most important avenue of heat loss Higher humidity decreases the capacity to lose heat by evaporation

Internal Body Temperature Can exceed 40 °C (104 °F) during exercise May be 42 °C (107.6 °F) in active muscles Small increases can make muscles’ energy systems more efficient Temperatures above 40 °C can affect the nervous system and reduce the ability to unload excess heat

A Simplified Overview of the Role of the Hypothalamus in Controlling Body Temperature During Hyperthemia

A Simplified Overview of the Role of the Hypothalamus in Controlling Body Temperature During Hypothermia

Thermoregulatory Control of Heat Exchange Preoptic-anterior hypothalamus Central and peripheral thermoreceptors Effectors that alter body temperature – Sweat glands – Smooth muscle around arterioles – Skeletal muscles – Endocrine glands

Thermoregulatory Control of Heat Exchange Key Points The preoptic-anterior hypothalamus houses the thermoregulatory center, acting as a thermostat, monitoring temperature and accelerating heat loss or heat production as needed Peripheral thermoreceptors in the skin and central thermoreceptors in the brain transmit information about the body’s internal temperature Central thermoreceptors are far more sensitive to temperature change than peripheral thermoreceptors Effectors stimulated by the hypothalamus can alter the body temperature

Thermoregulatory Effector Organs Key Points Smooth muscle in the skin arterioles can cause these vessels to dilate to direct blood to the skin for heat transfer or to constrict to retain heat deep in the body Increased skeletal muscle activity increases temperature by increasing metabolic heat production Metabolic heat production can also be increased by actions of hormones such as thyroxine and the catecholamines Increased sweat gland activity decreases the temperature by increasing evaporative heat loss

Cardiovascular Function in the Heat The circulatory system has to continue to transport blood to the working muscle and increase blood flow to the skin for thermoregulation –Increased cardiac output –Redistribution of blood flow from inactive organs (gut, liver, and kidneys)

What Limits Exercise in the Heat? At some point the cardiovascular system can no longer compensate for the increased demands of endurance activity and thermoregulation –Heart rate approaches maximum –Muscle blood flow is maintained Critical temperature theory: the brain will send signals to stop exercise when some critical temperature is reached, usually between 40 and 41 °C

Rectal Temperature and Cardiovascular Responses in Thermoneutral and Hot Environments Adapted, by permission, from L.B. Rowell, 1974, “Human cardiovascular adjustments to heat stress,” Physiological Reviews 54:

Body Fluid Balance: Sweating Anatomy of an eccrine sweat gland that is innervated by a sympathetic cholinergic nerve

Exercise in the Heat Key Points Skin competes with the active muscles for more of the limited cardiac output Muscle blood flow is well maintained unless severe dehydration occurs Blood flow is redistributed from nonessential regions to the skin to aid in heat dissipation Although cardiac output may remain constant or decrease slightly, stroke volume may decline, resulting in a gradual upward drift in heart rate (continued)

Exercise in the Heat (continued) Key Points Sweating increases during exercise in the heat, which can quickly lead to dehydration and electrolyte loss Aldosterone and ADH release are increased, causing sodium and water retention, which can expand the plasma volume

Variables of Environmental Heat Load Air temperature Humidity Air velocity Amount of thermal radiation

Wet-Bulb Globe Temperature Simultaneously accounts for conduction, convection, evaporation, and radiation Dry bulb measures air temperature (T db ) Wet bulb measures temperature as water evaporates from it (T wb ) Black globe absorbs radiated heat (T g ) WBGT = 0.1T db + 0.7T wb + 0.2T g

Wet-Bulb Globe Temperature

Heat-Related Disorders Heat cramps Heat exhaustion Heatstroke

Warning Signs and Symptoms of Heat Cramps, Heat Exhaustion, and Heatstroke Adapted by permission of All Sport, Inc.

Heat Cramps Symptoms: Severe and painful cramping of the large muscle groups Causes: Sodium losses and dehydration Prevention: Proper hydration and liberally salt food Treatment: Move to cooler location and administer fluids or saline solution

Heat Exhaustion Symptoms: Extreme fatigue, dizziness, nausea, vomiting, fainting, rapid pulse Causes: Cardiovascular system’s inability to adequately meet the body’s needs as it becomes dehydrated, blood volume decreases, and/or rarely sodium depletion Treatment: Move to cooler environment, elevate feet; give oral saline if conscious or intravenous saline if unconscious

Heat Stroke Symptoms: Increase in internal body temperature (>40 °C), cessation of active sweating, rapid pulse and respiration, and confusion, disorientation, or unconsciousness Cause: Failure of the body’s thermoregulatory system Treatment: Rapidly cool body in cold water or ice bath or with wet towels; seek medical attention

Preventing Hyperthermia Recognize the symptoms of heat illness Avoid exercising in humid conditions above a WBGT of 28 °C (82.4 °F) Schedule practices or events in early morning or at night Wear lightweight, light-colored, loosely woven clothing Make fluids readily availably to encourage fluid consumption to match sweat losses

Effects of Fluid Intake on Core Body Temperature Archives of Environmental Health, “Fluid ingestion during distance running,” D.L. Costill, 1: , Adapted with permission of the Helen Dwight Reid Educational Foundation. Published by Heldref Publications, th Street NW, Washington, DC Copyright © 1970.

Heat Stress Key Points WBGT measures air temperature and accounts for heat exchange via conduction, convection, evaporation, and radiation Heat cramps are caused by loss of fluids and sodium due to excessive sweating Heat exhaustion results from the cardiovascular system’s failure to meet the needs of the active muscles and skin due to dehydration and low blood volume Heat exhaustion can deteriorate into heat stroke Heatstroke is caused by failure of the body’s thermoregulatory system, which can be fatal

Heat Acclimation How can athletes prepare for prolonged activity in the heat? Does repeated exercise in the heat make us better able to tolerate thermal stress?

Effects of Heat Acclimation Improvement in the ability to get rid of excess body heat Decreased body core temperature for a given submaximal work rate Sweating increases and becomes more dilute Blood flow to skin is reduced; more blood is available to muscles Blood volume increases Attenuated heart rate Stroke volume is maintained

Effects of Heat Acclimation on Body Core Temperature and Heart Rate During a 90 Minute Run Adapted, by permission, from D.S. King et al., 1984, “Muscle metabolism during exercise in the heat in unacclimatized and acclimatized humans,” Journal of Applied Physiology 59:

Changes in Rectal Temperature, Heart Rate and Sweat Loss During Heat Acclimation

Achieving Heat Acclimation You can achieve heat acclimation by exercising in the heat for 1 hour or more each day for 9 to 14 days. Cardiovascular changes generally occur first, starting with—and supported by—plasma volume expansion during the first 1-3 days. Changes in the sweating mechanisms generally take longer, up to 10 days or more.

Heat Acclimation Key Points Repeated exposure to heat stress during exercise improves your ability to get rid of excess heat Sweating starts earlier and sweat rate increases especially in well-exposed areas Skin temperature decreases facilitating heat transfer by increasing the thermal gradient Core temperature and heart rate during exercise are reduced, whereas stroke volume increases Requires exercise in a hot environment Amount and rate of heat acclimation depends on training status, duration of exposure, and rate of internal heat production

Exercise in the Cold: Body Heat Conservation Peripheral vasoconstriction reduces blood flow to skin Nonshivering thermogenesis is stimulation of metabolism by the sympathetic nervous system Shivering involves rapid involuntary cycle of contraction and relaxation of muscles to increase heat production

Summary of Human Thermoregulatory Mechanisms

Factors That Affect Body Heat Loss Body size and composition Air temperature Windchill Water immersion

Windchill Equivalent Temperature Chart

Heat Loss in Cold Water Water has a thermal conductivity about 26 times greater than air Heat loss is 4 times faster in water Heat transfer is accelerated if the water is moving around the individual

Exercise in the Cold Key Points Peripheral vasoconstriction decreases the transfer of core heat to the skin and the environment Nonshivering thermogenesis increases metabolic heat production through the actions of the sympathetic nervous system Shivering increases metabolic heat production Increased surface area, decreased muscle mass, and decreased subcutaneous fat facilitate the loss of body heat to the environment Wind increases heat loss by convection Immersion in cold water tremendously increases heat loss through convection

Physiological Responses to Exercise in the Cold When muscle is cooled, it is less able to produce force, and fatigue can occur more rapidly During prolonged exercise in the cold people may become more susceptible to hypothermia – Energy supplies diminish – Metabolic heat production decreases Exercise triggers the release of catecholamines, which increase the mobilization and use of FFA for fuel. But in the cold, vasoconstriction impairs circulation to subcutaneous fat tissue

Hypothermia Ability of the hypothalamus to regulate body temperature is lost if body core temperature drops below 34.5 °C (94.1 °F) Hypothermia causes heart rate to drop, from impaired cardiac conduction through the SA node Decreased respiratory rate and volume

The Warming of Inspired Air as it Moves Through the Respiratory Tract

Frostbite Exposed skin can freeze quickly This can lead to gangrene and loss of tissue

Health Risks During Exercise in the Cold Key Points The hypothalamus begins to lose its ability to regulate body temperature below 34.5 °C (94.1 °F) Hypothermia critically affects the heart’s SA node, decreasing heart rate and cardiac output Breathing cold air does not freeze the respiratory passages or lungs when ventilation is low Exposure to extreme cold decreases respiratory rate and volume Frostbite occurs as a consequence of the body’s attempts to prevent heat loss and can lead to tissue necrosis