Transcript Temperature Regulation - Eastern Michigan University

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
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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
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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