Temperature Regulation When studying temperature regulation the body can be divided into two regions: the core and the shell. Core (internal organs) –temperature highly regulated (high gain) –traditional values: 37°C, 98.6°F –  1°F within normal range –small variations with time of day, exercise and environmental temperature Fig G

Shell (skin and limbs) –temperature not highly regulated (low gain) –varies with changes in environmental temperature Cold Environment –body heat conserved for the core –shell allowed to become cold Warm environment –excess heat leaves body via the shell Fig G

Physics of Heat heat = molecular motion –cold = absence of heat downhill movement –from region of higher temperature to region of lower temperature Flow of heat = temperature gradient X conductance

Mechanisms of Heat Transfer 1) Radiation, especially infrared 2) Conduction: heat transfer by direct contact conductors – high conductance insulators – low conductance (high resistance)  Convection:  conduction due to movement of a liquid or gas From Guyton, Medical Physiology

Mechanisms of Heat Transfer 3) Evaporation –0.58 Cal required to evaporate 1g of H 2 O from the surface of the skin. heat of vaporization = 0.54 Cal 1 Cal = 1 kcal 1 Cal (“nutritional” calorie) = 1000 cal (“physics” calorie) –When environmental temperature > body temperature, evaporation is the only mechanism to cool the body.

Roles of the Skin insulation Fig. 6.1 evaporative cooling vasodilation and vasoconstriction

Roles of the Skin Radiation Conduction Convection Modulated by vasodilation and vasoconstriction From Guyton, Medical Physiology

Roles of the Skin evaporative cooling maximum rate of perspiration = 2-3 L H 2 O/hr Na +, Cl -, K +, urea, and lactic acid are also excreted in sweat. Fig. 6.1

Central Control of Body Temperature Controlled Variable –temperature of body core Set Point –37 o C Receptors –central thermoreceptors: in hypothalamus of brain –peripheral thermoreceptors: in skin (mostly affect behavior, very little role in regulating core temperature) Control Center –hypothalamus Effectors –dermal blood vessels –skeletal muscle –endocrine glands –brown fat –sweat glands

Hypothalamic Control of Body Temperature When core body temperature drops below the set point, the following mechanisms are activated to increase body temperature:  heat loss vasoconstriction [piloerection]  heat production shivering muscle contraction generates heat hormone secretion e.g., thyroxine, epinephrine (adrenaline) activation of brown fat e.g., in human infants mitochondria generate heat instead of ATP

Hypothalamic Control of Body Temperature When core body temperature rises above the set point, the following mechanisms are activated to decrease body temperature:  heat loss vasodilation (inhibit vasoconstriction) sweating  heat production  voluntary activity  fuel intake  thyroxine secretion