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 H2O molecules on skin– when H2O vaporizes, removes heat

• dependent upon concentration gradient of H2O (vapor pressure)– during high humidity, H2O won’t evaporate

(100% humidity in air there is virtually no concentration gradient for H2O on skin

Insert table 12.1

Evaporation Example

• running 30 C and 100 % humidity, skin temp 34– vapor pressure on skin ~ 35 mmHg– vapor pressure in air ~ 32 mmHg

• running 10 C and 50 % humidity, skin temp 30– vapor pressure on skin ~ 32 mmHg– vapor pressure in air ~ 4 mmHg

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