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Exercise Thermoregulation, Fluid Balance, and Rehydration

Exercise Thermoregulation, Fluid Balance, and Rehydration. Chapter 10 Part 1. Temperature Regulation. Body core temperature regulation Critical for: Cellular structures Metabolic pathways Too high - hyperthermia(5 o C) Protein structure of cells destroyed Too low – hypothermia (10 o C)

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Exercise Thermoregulation, Fluid Balance, and Rehydration

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  1. Exercise Thermoregulation, Fluid Balance, and Rehydration Chapter 10 Part 1

  2. Temperature Regulation • Body core temperature regulation • Critical for: • Cellular structures • Metabolic pathways • Too high - hyperthermia(5oC) • Protein structure of cells destroyed • Too low – hypothermia (10oC) • Slowed metabolism • Cardiac arrhythmias

  3. Temperature Regulation • Thermal gradients • Determine rate and direction of heat transfer • Heat transfer from higher to lower temperature • Core to shell (skin) • Typical gradient when ambient temp is less than body temp • Skin to core • If ambient temperature is Greater than body temp

  4. Temperature Regulation • Net gain of body heat • Heat loss is less than heat production • Body temperature rises • Positive thermal balance • Net loss of body heat • Heat loss exceeds heat production • Body temperature decreases • Negative thermal balance

  5. Temperature Regulation • Homeotherms (mammals) • Internal body temperature is kept fairly constant • Humans are unable to tolerate wide changes • Hour to hour, day to day variations < 1o C • Normal temperature: 37o C • Normal range 36.1 – 37.8o C (97 – 100o F) • Conversion: Tf = (Tc * 1.8) + 32 • So, if it is 37o C, Tf = (37 * 1.8) + 32 • 66.6 + 32 = 98.6o F • Heavy exercise, illness, extreme conditions of heat and cold – can push an individual outside this range

  6. Heat Balance • Greater extremes • Early morning (<36oC) • Heavy exercise >40oC • Muscles >42oC or 107.6oF • Body temperature (core temperature) reflects the balance between heat production and heat loss

  7. Body Temperature • Core body temperature: deep tissues • Skeletal muscles and organs • Shell body temperature: peripheral • Skin • Initiate response to external temperature • Thermoreceptors relay information to • Hypothalamus • Cerebral cortex

  8. Hypothalamic Regulation • Hypothalamus (37  1oC) • Central coordinating center • Acts as a thermostat • Heat gain • Heat loss

  9. Thermal Balance

  10. Heat-Regulating Mechanisms • Activated by: • Temperature changes in blood • Skin heats up/cools down changes temperature of blood • Directly stimulate hypothalamus • Thermal receptors in the skin • Provide input to modulate hypothalamic activity. • Receptors (temperature regulation) • Skin • Subcutaneous tissue.

  11. Heat Transfer

  12. Thermal Regulation

  13. Radiation • Objects “radiate” heat • Body temperature is warmer than the environment. • Radiant heat energy transfers to solid cooler objects around us. • Body temperature is cooler than the environment. • Radiant heat energy transfers to us.

  14. Conduction • Transfers heat directly through a liquid, solid, or gas from one molecule to another • We conduct heat to cooler air, water or objects • Warmer air, water or objects conduct heat to us

  15. Convection • Air, water movement • So, if cool/hot air is travelling over the body (e.g. wind) or cool/hot water, we will be cooled/heated

  16. Evaporation • Major physiologic defense against overheating • Respiratory water loss • Sweat • Heat stress • 2-4 million sweat (eccrine) glands • Secrete large quantities of hypotonic saline solution. • *Cooling occurs when sweat evaporates from the skin surface. • Has to evaporate

  17. Environmental Temperature • Increased ambient temperature • Reduces heat loss by conduction, convection, and radiation. • This heat loss stops when ambient temp exceeds body temp • At that point, the only way to dissipate heat is • Sweat evaporation from the skin • depends upon: • Surface exposed to the environment • relative humidity of ambient air • Convective air currents around the body

  18. High Humidity

  19. Heat-Dissipating Mechanisms • Circulation • Cardiac output (HR x SV) • Rest – 15-20% to skeletal muscles • Heavy exercise – 80-85% to skeletal muscles • “Workhorse” to maintain thermal balance • 15-25% of cardiac output – periphery • If cardiac output falls, skin blood flow falls and ability to dissipate heat is reduced • So, when exercising in the heat • Cardiac output is directed more to periphery • Less to muscle • Cause of fatigue

  20. Cardiac Drift

  21. Heat-Dissipating Mechanisms • Sweating: rate dependent upon: • Climate conditions • Exercise intensity • Clothing • Conditioning • Evaporation • Major cooling mechanism

  22. Fluid conservingMechanisms • Hormones • Antidiuretic hormone - ADH (Vasopressin) • Posterior Pituitary • Increases water reabsorption from kidneys • Conserves water and maintains BP • Aldosterone • Adrenal cortex • Increases reabsorption of sodium

  23. Heat-related Deaths • Since 1936 (National Safety Council) • 30,000 people have died from heat related illnesses. • On the average, 384 people die each year from heat stroke. • Associated with normal hot temperatures.  • In the event of a heat wave, the number of deaths increase tremendously. • Occurrence of heat related injuries • Often with the elderly;  • People who are not in good physical condition; • or not acclimatized to the heat • Or heat waves

  24. Exercise in the Heat • Increased heat load • Increased sweating • Increased Fluid loss • Decreased plasma volume • Decreased cardiac output • Decreased cutaneous circulation

  25. Distribution of Cardiac Output • Thermoneutral environment vs hot environment

  26. Water Loss • Dehydration • Sweat loss 0.5 – 1.5 L/h – moderate exercise • Maximal daily sweat rates-10-15 L • Highest sweat rates 2 to 3 L/hr • Can be sustained for only a few hours • Considerable water loss • Both intracellular and extracellular compartments • The risk of heat illness • Increasesin a dehydrated state.

  27. Sweat Rates • 1 to 3% loss in body weight through dehydration can adversely affect performance and body’s physiologic function • Marathon runners • Loss of 5 L • 6-10% of body weight

  28. Core Temperature and Sweat Rate During Exercise in Heat/Humidity Fig 12.10

  29. Exercise in the Heat • Compromised BF to muscles • Increased use of glycogen • Increased production of LA • Hastens fatigue

  30. Dehydration and Exercise Performance

  31. Dehydration and Exercise Performance

  32. Dehydration and Exercise Performance • Any degree of dehydration impairs • Capacity of circulatory system • Temperature-regulating mechanisms • Dehydration - 2% body mass • Impairs physical work capacity and physiologic function • Predisposes to heat injury when exercising in a hot environment.

  33. Dehydration and Exercise Performance • The risk for dehydration increases during vigorous cold-weather exercise. • Less moisture in cold • Respiratory fluid loss is increased • Excessive clothing • Creates a heated microclimate under the clothing

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