Temperature Regulation Importance of Temperature Regulation core temps

Temperature Regulation

Importance of Temperature Regulation • core temps above 41 C result in protein destruction • core temps above 45 C can result in death • core temps below 34 C result in slowed metabolism and impaired cardiac function

Steady State Temp. Regulation

Overview of Heat Balance • balance maintained by matching heat loss to heat gain • during exercise heat production is great, so must increase heat loss • blood flow is primary means of controlling temperature

Temperature Measurement • core temperature is best estimated by rectal thermometers – also tympanic or esophageal • skin temperature can be estimated by sampling various surfaces and averaging the values • note that there may be a large gradient between the core and skin temps

Modes of Heat Production

Heat Production • Voluntary - exercise • involuntary - shivering, biochemical (thyroxine, sympathetic hormones) • 20 - 30% efficiency producing work (80 % heat production)

Involuntary Heat Production • shivering - can increase heat production 5 x resting values • non-shivering thermogenesis – thyroxine – catecholamines (sympathetic hormones epinephrine and norepinephrine)

Heat Loss • • radiation conduction convection evaporation

Radiation • loss of heat via infrared rays • no physical contact involved between two surfaces or molecules • at rest 60 % of heat loss can occur via radiation • need temperature gradient, therefore on hot day can gain heat via radiation as well (sun, pavement etc. )

Conduction • requires physical contact between two surfaces • eg. Sitting on a cool surface, metal or concrete • minor contributor in most cases

Convection • form of conduction specific to either air or water molecules • eg. Fan blowing cool air onto the skin – warm air molecules are moved away from the body as cooler ones come into contact which in turn become warmed and are moved away by cooler ones ad infinitum • when swimming in cool water convection primary means of cooling

Evaporation • 25 % of heat loss at rest • during exercise primary means of heat loss • heat transferred to H 2 O molecules on skin – when H 2 O vaporizes, removes heat • dependent upon concentration gradient of H 2 O (vapor pressure) – during high humidity, H 2 O won’t evaporate (100% humidity in air there is virtually no concentration gradient for H 2 O on skin

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Evaporation Example • running 30 C and 100 % humidity, skin temp 34 – vapor pressure on skin ~ 35 mm. Hg – vapor pressure in air ~ 32 mm. Hg • running 10 C and 50 % humidity, skin temp 30 – vapor pressure on skin ~ 32 mm. Hg – vapor pressure in air ~ 4 mm. Hg

Heat Exchange Mechanisms during Exercise

The Body’s Thermostat • Anterior hypothalamus - responds to increases in body heat • Posterior hypothalamus - responds to decreases in body heat • Thermostat set at 37 C

Increases in Temperature • Receptors on skin first sense changes – receptors also located in spinal cord and hypothalamus respond to core temp changes • Stimulates sweat glands - increases evaporation • Increases skin blood flow - vasodilation

Responses to Heat

Decreases in Temperature • Vasoconstriction of peripheral vessels – reduces heat loss to surroundings • Involuntary shivering – increases metabolic heat production • Piloerection - ineffective • Increased thyroxine production • Increased sympathetic hormone production – nonshivering thermogenesis

Responses to Cold

Changes in Heat Production and Loss during Exercise

Take Home Message • During exercise, evaporation is the most important method of heat loss • Elevated core temp during steady state exercise is not a new “set point”, but a balance between heat production and loss which is inadequate to reduce temp to set point

• Metabolic heat production increases in proportion to the exercise intensity • Convective and radiative heat loss do not increase with intensity as temp gradient between body and environment does not change significantly

Hyperthermia • Increased core temperature to the point that physiological functions are impaired • Contributing factors – dehydration – electrolyte loss – failure of cooling mechanisms to match heat production

Exercise in Hot/Humid vs. Cool Environment

Heat Acclimatization • • • Increased plasma volume Earlier onset of sweating Higher sweat rate Reduced electrolyte loss Reduced skin blood flow

Hypothermia • Reduced core temperature to the point that physiological functions are impaired • Swimming in cold water particularly dangerous • Water has higher convective potential than air (cold rainy day versus cold dry day) • Leaner individuals more susceptible

Cold Acclimatization • Increased nonshivering thermogenesis • Higher intermittent blood flow to extremities • Improved ability to sleep in cold environments