Exercise and Thermal stress Thermoregulation Heat balance Heat

Exercise and Thermal stress • • Thermoregulation Heat balance Heat injuries Hydration

Thermal Balance Core temperature is in dynamic equilibrium

Thermal Balance Body Heat Content • Radiation • Muscular activity • Conduction • Environmental heat • Convection • Evaporation Heat Gain Heat Loss 37± 0. 5 o. C

Hypothalamic Regulation of Temperature • Thermoregulatory center (thermostat) • Set point ~ 37 o. C

Hypothalamic Regulation of Temperature

Cold stress Vascular Response: • Peripheral vascular adjustments – constriction • Skin temperature • Insulation by skin and subcutaneous fat Muscular activity • Shivering • Physical activity (can sustain core temperature in air temperature as low as (-30 o. C) • Metabolic rate (VO 2) is higher in cold stress • Hormonal output • Increased heat production is due partially to the action of epinephrine, norepinephrine and thyroxin

Heat Stress • Body heat may be lost by radiation, conduction, convection and evaporation Radiation- קרינה Conduction- הולכה Convection- הסעה Evaporation- נידוף

Thermodynamics 30 o. C 35 o. C

Heat Loss in High Ambient Temperature • When ambient temperature exceeds body temperature, heat is actually gained by the mechanisms of thermal transfer (conduction, convection, radiation) • When ambient temperature exceeds body temperature, heat dissipation occurs only by sweat evaporation • The rate of sweating increases directly with the ambient temperature • A very small contribution to cooling provided by the vaporization of water from the respiratory system

Heat Loss in High Humidity • 1) 2) 3) The total sweat vaporized from the skin depends on three factors: The body surface exposed to the environment The humidity of the ambient air The convective air currents about the body • The relative humidity (RH) is the most important factor that determines the effectiveness of evaporative heat loss • RH (%)= amount water in the ambient air total quantity of moisture that can be carried in the air X 100 • Hot, dry desert climate are more comfortable than cooler but more humid tropical climates • Sweat per se does not cool the skin. Only evaporation cool the skin

Physiological Integration of Heat Dissipation Mechanisms Circulation (peripheral vasodilation) Metabolic Heat Dissipation Evaporation Environmental Heat Hormonal Adjustment: - ADH - Aldosterone

Chimney Effect Heat

Thermoregulation During Exercise 1. Competitive Demands Metabolic heat must be transported by the blood from the deep tissues to the periphery Muscles that require oxygen 2. During exercise, evaporation becomes the predominant avenue of heat loss. 3. Excessive sweating leads to fluid loss and a reduction in plasma volume. This may cause circulatory failure and increase the rate of elevation in body temperature.

Circulatory Adjustments Constriction and Dilation: • In the heat adequate cutaneous and muscle blood flow are achieved at the expense of other tissues (viscera) that can temporarily compromise their blood supply • During exercise in the heat skin blood flow may increase to 20% of cardiac output compared to only 5% of cardiac output during rest in similar conditions Maintenance of Blood Pressure • Vasoconstriction in the viscera serves to increase total vascular resistance. In this way, arterial blood pressure is maintained during work in the heat (peripheral vasodilation) Aerobic vs. Anaerobic Metabolism • The work in the heat is accomplished with a greater dependence on anaerobic metabolism than in cooler environment. This may result in early fatigue during exercise in the heat.

Core Temperature • Q 10 effect • May increase to as high as 40 o. C in champion distance runners with no ill effects • May point to a heat stroke when having the required clinical symptoms

Body Morphometry • Surface Area/Body Mass • Fat- The specific heat of fat is greater than for muscle tissue. Therefore, excess fat increases the insulatory quality of the body shell and prevents conduction of heat to the periphery

Heat Stroke • Exertional • Classic

Heat Acclimatization and the Effect of Thermoregulation During Exercise in the Heat • • • The physiologic adaptive changes that improve heat tolerance Two weeks of regular daily exercise in hot conditions Improved cutaneous blood flow Increased plasma volume Effective distribution of cardiac output Lowered sweating threshold Increased sweat output More effective distribution of sweat over skin surface Lowered salt concentration in sweat Lowered heart rate Lowered body temperature Subjective feeling

Hyperthermia vs Fever Antipyretics Metabolic Heat Thermoregulatory set point (-) Endotoxins Bacteria Disease