CHAPTER 10 CHAPTER 10 THERMOREGULATIONAND EXERCISE THERMOREGULATIONAND EXERCISE
w Learn why humidity, wind, and cloud cover are important factors when exercising in the heat. (continued) Learning Objectives w Find out how the body gets rid of excess body heat to maintain homeostasis at rest and during exercise. w Discover how the body adapts to exercise in a hot environment.
Learning Objectives w Differentiate heat cramps from heat exhaustion from heat stroke. w Learn how the body minimizes excessive heat loss during exposure to cold. w Find out the dangers of cold-water immersion. w Discover how to exercise safely in the cold.
Modes of Heat Transfer Conduction—direct molecular contact Convection—motion of gas or liquid across heated surface Radiation—infrared rays Evaporation—as fluid evaporates, heat is lost
Evaporation w As body temperature rises, sweat production increases. w Sweat reaches the skin and evaporates. w Evaporation accounts for 80% of heat lost during exercise.
Humidity w Plays a major role in heat loss w Affects our perception of thermal stress w When high (regardless of temperature), limits evaporation of sweat
Key Points Heat Balance w Humans maintain a constant internal temperature of 36.1 to 37.8 °C (97.0 to 100.0 °F). w Body heat is transferred by conduction, convection, radiation, and evaporation. w During exercise, evaporation is the main means of heat loss; during rest, radiation is. w Higher humidity reduces potential evaporation and thus affects heat loss.
Internal Body Temperature w Can exceed 40 °C (104 °F) during exercise w May be 42 °C (107.6 °F) in active muscles w Small increases can make muscles' energy systems more efficient w Above 40 °C can affect the nervous system and reduce the ability to unload excess heat
Effectors w Sweat glands w Smooth muscle around arterioles w Skeletal muscles w Endocrine glands Regulators of Heat Exchange Hypothalamus Central and peripheral thermoreceptors
Mean body temperature (Tbody) is the weighted average of w Skin temperature (Tskin) w Core temperature (Tr) Body Temperature Assessments Tbody = (0.4 ´ Tskin) + (0.6 ´ Tr)
Heat Content w Total calories of heat contained in body tissues w Average specific heat of body tissues is 0.83kcal ´ kg–1´ °C–1 w Heat content = 0.83 (Body weight ´ Tbody)
Rate of Heat Exchange w Heat produced by average body at rest is 1.25 to 1.5 kcal per minute. w Heat produced during exercise can exceed 15 kcal per minute. w This heat must be dissipated by the body’s thermoregulatory systems.
w Sweat gland activity increases to lower body temperature by evaporative heat loss. (continued) Key Points Control of Heat Exchange w The hypothalamus monitors the body’s temperature and speeds up heat loss or heat production as needed. w Peripheral thermoreceptors in the skin relay information back to the hypothalamus. w Central thermoreceptors in the hypothalamus transmit information about internal body temperature.
Key Points Control of Heat Exchange w Smooth muscles in the arterioles dilate or constrict to allow the blood to dissipate or retain heat. w Skeletal muscle activity increases temperature by increasing metabolic heat. w Metabolic heat production can also be increased by actions of hormones. w Mean body temperature is a weighted average of skin and core temperatures.
Cardiovascular Exercise in the Heat w Active muscles and skin compete for blood supply. w Stroke volume decreases. w Heart rate gradually increases to compensate for lower SV (cardiac drift).
Metabolic Responses to Exercise in the Heat w Body temperature increases. w Oxygen uptake increases. w Glycogen depletion is hastened. w Muscle lactate levels increase.
w High volumes of sweat cause – Blood volume to decrease, – Loss of minerals and electrolytes, and – Release of aldosterone and ADH and water reabsorption in kidneys. Body Fluid and Exercise in the Heat w Sweating increases.
Variables of Environment Heat Load w Air temperature w Humidity w Air velocity w Amount of thermal radiation
Wet Bulb Globe Temperature w Simultaneously accounts for conduction, convection, evaporation, and radiation. w Dry bulb measures air temperature (TDB). w Wet bulb measures temperature as water evaporates from it (TWB). w Black globe absorbs radiated heat (TG). w WBGT = 0.1TDB + 0.7TWB + 0.2TG
Treatment of Heat Disorders Heat cramps—move to cooler location and administer fluids or saline solution Heat exhaustion—move to cooler environment, elevate feet; give saline if conscious or intravenous saline if unconscious Heat stroke—rapidly cool body in cold water, ice bath or with wet towels; seek medical attention
Preventing Hyperthermia w Avoid exercising in humid conditions above a WBGT index of 28 °C (82.4 °F). w Schedule practices or events in early morning or at night. w Wear light-weight, light-colored, loosely-woven clothing. w Drink plenty of fluids. w Know the symptoms of heat stress.
Key Points Heat Stress w WBGT measures air temperature and accounts for heat exchange via conduction, convection, evaporation, and radiation. w Heat cramps appear to be caused by loss of fluids and minerals due to sweating. w Heat exhaustion results from the cardiovascular system being unable to meet the needs of muscles and skin due to lower blood volume (from sweating). w Heat stroke is caused by failure of the body's thermoregulatory system.
Heat Acclimatization w Ability to get rid of excess heat improves w Sweating becomes more efficient w Blood flow to skin is reduced; more blood is available to muscles w Blood volume increases w Heart rate increase is less (than nonacclimatized) w Stroke volume increases w Muscle glycogen usage decreases
Did You Know…? You can achieve heat acclimatization by exercising in the heat for 1 hour or more each day for 5 to 10 days. Cardiovascular adaptations occur within the first 3 to 5 days while changes in sweating mechanisms may take up to 10 days. Reduce exercise intensity to 60% to 70% the first few days before resuming more intense workouts.
Key Points Heat Acclimatization w Repeated exposure to heat stress during exercise improves your ability to get rid of excess heat. w Sweating increases in well-exposed areas to promote heat loss. w Stroke volume increases to aid in delivery of more blood to the muscles and skin. w Muscle glycogen use is reduced to delay onset of fatigue. w Amount of heat acclimatization depends on environmental conditions and duration of exposure and intensity of exercise.
How Does the Body Conserve Heat? Shivering—rapid involuntary cycle of contraction and relaxation of muscles Nonshivering thermogenesis—stimulation of metabolism Peripheral vasoconstriction—reduces blood flow to skin
Factors That Affect Body Heat Loss w Body size and composition w Air temperature w Wind chill w Water immersion
Responses to Exercise in the Cold w Muscles weaken and fatigue occurs more rapidly w Susceptibility to hypothermia increases w Exercise-induced FFA mobilization is impaired due to vasoconstriction of subcutaneous blood vessels
Health Risks of Exercise in the Cold w Ability to regulate body temperature is lost if Tbody drops below 34.5 °C (94.1°F). w Hypothermia causes heart rate to drop, which reduces cardiac output. w Vasoconstriction in the skin reduces blood flow to skin, eventually causing frostbite.