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. 14-22 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. 14-21 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 gradientX conductance

Mechanisms of Heat Transfer

1) Radiation, especially infrared2) Conduction: heat transfer by direct contact

conductors – high conductanceinsulators – 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 H2O from the surface of the skin.

heat of vaporization = 0.54 Cal

1 Cal = 1 kcal1 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 SkinRadiationConductionConvection

Modulated by vasodilation and vasoconstriction

From Guyton,Medical Physiology

Roles of the Skin

evaporative cooling

maximum rate of perspiration= 2-3 L H2O/hr

Na+, Cl-, K+, urea, and lactic acidare also excreted in sweat.

Fig. 6.1

Central Control of Body Temperature

• Controlled Variable– temperature of body core

• Set Point– 37oC

• 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 secretione.g., thyroxine, epinephrine (adrenaline)

activation of brown fate.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