C H A P T E R 10

1. C H A P T E R 10 EXERCISE IN HOT AND COLD ENVIRONMENTS: THERMOREGULATION

2. w Differentiate heat cramps from heat exhaustion from heat stroke. (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. w Learn why humidity, wind, and cloud cover are important factors when exercising in the heat.

3. Learning Objectives 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.

4. BODY HEAT GAINED AND LOST

5. Modes of Heat Transfer Radiation—infrared rays given off by any molecular mass as a function of the temperature of the mass Conduction—through direct molecular contact heat is transferred down a heat gradient (ΔH) from a hotter medium (like skin) to a cooler medium (like air or water) Convection is movement of the air or water (e.g., from a fan), which facilitates conduction by removing the warmed air or water next to the skin so a greater ΔH is maintained Evaporation—as fluid evaporates, heat is lost (580 kcal/L of water evaporated) Convection facilitates evaporation by removing the humidified air next to the skin so a greater difference in water vapor pressure can be maintained

6. 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, but only for about 20% at rest. wInsensible (non-sweating) water loss removes about 10% of heat. w Dehydration is a potential problem with sweating. It is essential that water be replaced when exercising, particularly in a hot humid environment when one is sweating profusely.

7. HEAT REMOVAL FROM THE SKIN

8. Rest Exercise Mechanism of heat loss % total kcal/min % total kcal/min Conduction and convection 20 0.3 15 2.2 Radiation 60 0.9 5 0.8 Evaporation 20 0.3 80 12.0 Total 100 1.5 100 15.0 Estimated Caloric Heat Loss at Rest and During Prolonged Exercise

9. Humidity w Plays a major role in heat loss w Affects our perception of thermal stress w When high (regardless of temperature), humidity limits evaporation of sweat because it lowers the water vapor pressure difference between the sweat on the skin and the water in the surrounding air

10. Key Points Heat Balance: Homeothermy w Humans maintain a relatively constant internal temperature of 36.1 to 37.8 °C (97.0 to 100.0 °F) during normal rest. w Body heat is transferred by radiation, conduction, and evaporation, the latter two facilitated by convection. w During exercise, evaporation is the main means of heat loss; during rest, radiation is. w Higher humidity reduces potential evaporation and thus attenuates heat loss.

11. 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 by increasing the rates of enzyme reactions. w Above 40°C can adversely affect the nervous system and reduce the ability to unload excess heat (leading to heat-related disorders)

12. Effectors w Sweat glands w Smooth muscle around arterioles in the skin w Skeletal muscles w Endocrine glands (e.g., adrenal medulla, thyroid) Regulators of Heat Exchange Hypothalamus – “Thermostat” Central (in the hypothalamus) and peripheral (primarily in the skin) thermoreceptors

13. HYPOTHALAMUS AND HYPERTHERMIA • Hypothalamus integrates the input comparing it with the “set point” then initiates cooling mechanisms • Skin arterioles vasodilate under control of the sympathetic nerves, allowing elevated blood flow in the skin, thus heating the skin • Sweat glands increase sweat secretion onto the surface of the skin under control of the sympathetic nerves, allowing increased evaporative heat loss • Elevated “core temperature” sensed by thermoreceptors in the hypothalamus

14. HYPOTHALAMUS AND HYPOTHERMIA • 1. Hypothalamus integrates the input from skin and hypothalamic thermoreceptors comparing it with the “set point” then initiates warming mechanisms • 2. Skin arterioles vasoconstrict under control of the sympathetic nerves, decreasing blood flow in the skin, thus cooling the skin and decreasing the difference in temperature between skin and air • Skeletal muscles are activated to shiver, thus increasing heat production • With prolonged exposure to the cold, thyroid hormone and catecholamine secretion increase, which also stimulates muscle metabolism

15. 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 to avoid dangerously high body temperatures.

16. Homeostasis and Steady State Steady State: a constant internal environment that may be different than rest, for example during steady state exercise Powers and Howley, Exercise Physiology, 2004

17. Cardiovascular Response to Exercise in the Heat wActive muscles and skin compete for blood supply. wBlood pools in the skin, reducing venous return so that stroke volume decreases. w Heart rate gradually increases to compensate for lower SV (cardiovascular drift) so that cardiac output is maintained. However, maximal cardiac output (and therefore VO2max) is reduced in the heat because of the attenuated stroke volume.

18. Metabolic Responses to Exercise in the Heat w Body temperature increases, which stimulates enzyme reactions (Q10 effect: 2-fold increase for each 10 degrees). w Oxygen uptake increases at a given submaximal level, but VO2max is reduced. w Glycogen depletion is hastened. w Muscle lactate levels increase.

19. w High volumes of sweat cause – Blood (plasma) volume to decrease (cardiovascular drift), – Loss of minerals and electrolytes, and – Release of aldosterone and ADH counter fluid loss by increasing water reabsorption in kidneys. Body Fluids and Exercise in the Heat w Sweating increases.

20. EXERCISE IN HEAT AND COLD

21. Sweat Na+ Sweat Cl– Sweat K+Subjects(mmol/L) (mmol/L) (mmol/L) Untrained males 90 60 4 Trained males 35 30 4 Untrained females 105 98 4 Trained females 62 47 4 Data from the Human Performance Laboratory, Ball State University Sodium, Chloride, and Potassium Concentrations in the Sweat of Trained and Untrained Subjects During Exercise Thus, with training there is a decreased loss of electrolytes in the sweat, along with an increased sweat rate.

22. Variables Affecting Environment Heat Load w Air temperature w Humidity—desert versus tropics w Air velocity (convection)—still air versus moving air w Amount of thermal radiation—e.g., cloud cover, ground

23. Preventing Hyperthermia w Avoid exercising in hot and 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. wDrink plenty of fluids!! w Know the symptoms of heat stress.

24. Rectal temperature Symptoms 40-40.5°C (104-105°F) Cold sensation over stomach and back with piloerection (goose bumps) 40.5-41.1°C (105-106°F) Muscular weakness, disorientation, and loss of postural equilibrium 41.1-41.7°C (106-107°F) Diminished sweating, loss of consciousness and hypothalamic control >42.2°C (> 108°F) Death Subjective Symptoms Associated With Overheating

25. Treatment of Heat Disorders Heat cramps—move to cooler location and administer fluids or saline solution (0.9% NaCl in water) 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 wet towels; immediately seek medical attention

26. Key Points Heat Stress 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 (metabolism) and skin (cooling) due to lower blood volume (from sweating). w Heat stroke is caused by failure of the body's thermoregulatory system, i.e., neural dysfunction.

27. FLUID INTAKE AND EXERCISING IN THE HEAT

28. Heat Acclimatization w Ability to get rid of excess heat improves w Sweat sooner, sweat glands produce a greater volume of sweat, and the sweat is more dilute (less concentrated because of reduced electrolytes) w Reduced blood flow to skin; more available to muscle w Blood volume increases w Heart rate increase is less (than non-acclimatized) w Stroke volume increases w Muscle glycogen usage decreases; lower lactate production

29. Heat Acclimatization 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.

30. HEAT ACCLIMATIZATION

31. 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

32. 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.

33. How Does the Body Conserve Heat? Shivering thermogenesis—rapid involuntary cycle of contraction and relaxation of muscles Nonshivering thermogenesis—stimulation of metabolism, e.g. through increased thyroid hormone release (brown fat?) Peripheral vasoconstriction—reduces blood flow to skin, so effectively increases the layer of insulation

34. Factors That Affect Body Heat Loss w Body size and composition w Air temperature w Wind chill w Water immersion

35. WARMING OF INSPIRED AIR

36. Exercise Capacity in the Cold •  VO2 max is usually decreased ~6-8% in the cold environment. • Also running speed is slower in the cold environment  • Performance is actually improved in mild weather (cold for Texans) ~55 degrees.