C H A P T E R 10

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

16. 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.

17. 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.

18. BODY HEAT GAINED AND LOST

19. Modes of Heat Transfer Radiation—infrared rays given off by any 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

20. HEAT REMOVAL FROM THE SKIN

21. Front Back Before After Before After Before running outside at 30° C (75% humidity) After running outside at 30° C (75% humidity) THERMOGRAMS

22. 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. w Insensible (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.

23. 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

24. MECHANISMS FOR HEAT BALANCE

25. 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 because it lowers the water vapor pressure difference between the sweat on the skin and the water in the surrounding air

26. Key Points Heat Balance 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 conduction, radiation, and evaporation, and is 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.

27. 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)

28. 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

29. 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

30. 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 secretion increases, which also stimulates muscle metabolism

31. Thought Question What happens to the temperature set point when you catch the flu and what are the physiological effects? What happens when the fever “breaks” when the infection is over?

32. 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)

33. Heat Content w Total calories of heat contained in body tissues w Average specific heat of body tissues is 0.83 kcal ´ kg–1´ °C–1 w Heat content = 0.83 (Body weight ´ Tbody)

34. 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.

35. Cardiovascular Response to Exercise in the Heat w Active muscles and skin compete for blood supply. w Blood 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.

36. Metabolic Responses to Exercise in the Heat w Body temperature increases. w Oxygen uptake increases at a given submaximal level, but VO2max is reduced. w Glycogen depletion is hastened. w Muscle lactate levels increase.

37. Thought Question Why would the maximal oxygen consumption decrease when exercising in the heat?

38. 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 Fluids and Exercise in the Heat w Sweating increases.

39. EXERCISE IN HEAT AND COLD

40. ECCRINE SWEAT GLAND

41. Body Temperature Control When Tenvironment > Tbody there is a net transfer off heat from the environment to the body through conduction and radiation, which results in an increased body temperature Evaporation then is the only avenue of heat loss. If you are in an environment with high humidity, evaporation is limited. Thus, a 105 degree day in Amarillo is tolerable, whereas a 105 degree day in Houston is difficult. Evaporation requires sweating, and excessive sweating leads to dehydration, reduced plasma volume, and increased Tbody . You need more blood volume in skin to lose heat, but this compromises venous return and cardiac output.

42. 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.

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

44. Heat Load: Measured with Wet Bulb Globe Temperature w Simultaneously accounts for conduction, evaporation, and radiation. w Dry bulb measures air temperature (TDB). w Wet bulb measures temperature as water evaporates from it (TWB); convection lowers wet bulb temperature. w Black globe absorbs radiated heat (TG). w WBGT = 0.1TDB + 0.7TWB + 0.2TG

45. Thought Question When you exercise on a hot humid day, do you sweat more than when you exercise on a dry day of the same temperature? How do you explain the fact that in the high humidity the “sweat drips off you?”

46. WARNING SIGNS OF HEAT DISORDERS * *

47. 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.

48. 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

49. 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

50. 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.