THERMOREGULATION

1. THERMOREGULATION

2. Temperature “Set Point?” • Poikilothermic “cold-blooded” • Ectothermy • Heterothermic (hybernation) • Homeothermic “warm blooded” • Endothermy

5. 98.6o F ? • Equivalent to 37o C • True Range is 36.1o Cto 37.8o C (97o-100o F) • Common causes of variability • Thermic effect of food • Low morning temp (circadian) • Mentrual (follicular increase) • Fitness, acclimation, genetics etc. • With all factors controlled, men and women have equal capacity to regulate temperature. • If all factors controlled, would all individuals have the same resting temperature?

7. Thermoregulation Overview Inputs Sensors Integrators Primary Secondary Outputs Outputs System maintains body temperature

8. BODY HEAT GAINED AND LOST (Inputs)

9. 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

11. HEAT REMOVAL FROM THE SKIN

12. 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 (Ventilation, diffusion through skin). 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.

13. Sweat • Sodium Chloride • Water • Urea • Lactic acid • Potassium chloride • Calcium NaCl is partially reabsorbed while passing through sweat duct (more so with acclimation, less so with high sweat rates)

14. Sweating • Eccrine sweat glands in non-primate mammals are activated by SNS (cholinergic) for the purpose of improving traction but main thermoregulatory sweat gland in humans • Apocrine sweat glands associated with hair follicle in specific regions of the body, emotional sweating, odor

15. MECHANISMS FOR HEAT BALANCE

16. Humidity and Wind w Plays a major role in heat loss w Affects our perception of thermal stress • When humidity is 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 • High wind optimizes this gradient

17. Heat Balance Internal (core)Internal (shell)External ambient Temp Metabolic heat (HP) conduction (HL) convection (HL) (Vasodilation, skin) Radiation Evaporation (sweat) wind, humidity High Temp ∆T Low temp

18. 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) Rectal, Tympanic, Esophageal, vs. stomach temps

19. 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

20. 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 and low wind reduces potential evaporation and thus attenuates heat loss.

21. Thermoregulation Overview Inputs Sensors Integrators Primary Secondary Outputs Outputs Inputs (Heat Balance) • Ambient temperature (gradient) • Wind, humidity • Conduction • Convection • Radiation • Evaporation

22. Sensors (Thermoreceptors) • Peripheral (Mainly in skin) • Cold (primary) • Warm (Secondary) • Central (Mainly in preoptic/hypothalamus) • Cold (Secondary) • Warm (Primary)

23. Regulators of Heat Exchange • Hypothalamus – “Thermostat” or integrator • Preoptic posterior hypothalamus responsible for heat production • Fraction of cold sensitive preoptic neurons are glucosensitive • Fraction of warm sensitive preoptic neurons are sensitive to osmolality

24. Primary Sympathetic Controllers Arterioles Sweat glands Adipose Motor Skeletal Muscle Hormonal Thyroxin Catecholes Leptin Q10 effect* Primary to Secondary Outputs Secondary • Sympathetic Effectors • Vasoconstriction/dilation • sweating • BAT (UCP) • Motor • Shivering • Hormonal • Non-shivering Thermogenesis • Non-shivering Thermogenesis • Non-shivering Thermogenesis • Q10 effect * An increase in temperature increases metabolic rate

25. 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

26. Example: 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.

29. Engineers Model

30. Reciprocal Cross-Inhibition (RCI)

31. W W PW W C PC

34. Sweat Production • First increase the number sweat glands activated • Second, increase the rate of sweat production per gland

35. Thermoregulation Overview Inputs Sensors Integrators Primary Secondary Outputs Outputs Heat Gain Peripheral/central HypothalamusControllersEffectors Heat Lossthermoreceptors Arterioles Vasocontriction/dilation Ambient Sweat glands sweating Evaporation Adipose BAT (UCP) radiation Skeletal muscle shivering conduction Hormones nonshiv. Thermogen. convection Q10 effect nonshiv. Thermogen Wind/humidity