Thermoregulation and Exercise

Thermoregulation and
 the Body’s Limits

•    Normal body temperature fluctuates throughout the day:

–   Can tolerate a drop in deep body temperature of 18oF (10oC)

–   Can only tolerate an increase of 9oF (5oC)


•    Oral temperature is 1oF (.56oC) less than rectal temperature

–   With exercise, this difference increases since air circulates via the oral and nasal passages

Thermal Balance

Mean Body Temperature

Tbody =  (0.6 x Tcore) + (0.4 x Tskin)


This equation gives the average body temperature at any given time such that:

        - 60% is accounted for by the core

        - 40% is accounted for by the skin

What regulates body temperature?

•    Hypothalamus

–   Contains the central coordinating center for temperature regulation

–   Receives input from:

•   Thermal receptors in the skin provide information

•   Temperature of the blood (as it flows by hypothalamus) provides information

–   Anterior hypothalamus – stimulates heat loss

–   Posterior hypothalamus – stimulates heat conservation

Mechanisms of temperature regulation

•     When it is hot: (need for heat loss)

–    There is vasodilation of subcutaneous blood vessels; more sweating  ( heat loss)

–    There is decreased muscle activity; decreased secretion of thyroxine and epinephrine ( heat production)


•     When it is cold: (need for heat retention)

–    There is vasoconstriction of skin blood vessels; also curling up to stay warm ( heat loss)

–    Shivering and increased voluntary muscle activity; increased secretion of thyroxine and epinephrine ( heat production)

Heat Loss Mechanisms

•    Radiation – emission of electromagnetic heat waves

•    Conduction – direct transfer of heat through a liquid, solid, or gas (direct contact)

•    Convection – transfer of heat via air currents over surface of skin

•    Evaporation – vaporization of water from respiratory passages or surface of skin (2-4 million sweat glands)

Factors affecting heat loss

•    Increased ambient temperature

–   Reduces effectiveness of heat loss particularly by radiation, conduction, and convection

•    Increased relative humidity

–   Reduces effectiveness of heat loss by evaporation

•    Decreased wind velocity

–   Reduces both convective and evaporative effectiveness

•    Reduced surface exposed to environment

–   Reduces effectiveness of all heat loss mechanisms

Environmental Stress During Exercise in the Heat

•    2 circulatory adjustments:

–   Must increase O2 delivery to muscles to sustain energy metabolism

–   Must increase blood flow to skin to dissipate heat; however, this takes away blood flow to muscle

Cardiovascular Response to Exercise in the Heat

•    Cardiac Output

–   Stays the same during submaximal effort

•  Heart rate increases

•  Stroke volume decreases

–   Decreases at maximal exercise

•  Heart rate increases but not enough to offset decrease in stroke volume

Cardiovascular Need vs Temperature Regulation Need

•    Maintenance of blood flow to muscle takes precedence over regulation of temperature in the body

–   Can lead to increase heat build-up in core, hence the problem of heat illness during exercise

Core Temperature response to Exercise

•    Core temperature will increase as exercise intensity increases

–   Trained and acclimatized individual has greater tolerance to increased body heat

Water Loss in the Heat

•    For an acclimatized person:

–   Sweat loss peaks at ~ 3 L/hr during intense exercise in the heat

•   This would average out to be 12 L (or 26 lbs) in a day

•   A high school wrestler could lose 9-13% of their weight prior to weigh-in (primarily due to water restriction and excessive sweating from exercise)

–   Difficult to regain this amount lost within a 20-hr period; only about 8 lbs.
–   Places wrestler in a dehydrated state

Consequences of Dehydration

•    Fluid loss equivalent to 1% of body mass

–   Increases core temperature

•    Fluid loss equivalent to 5% of body mass

–   Increases core temperature and heart rate

–   Decreases sweating rate, VO2 max, and aerobic capacity

Water Replacement

•    On average:

–   Consume 250 ml (8.5 oz) of fluid every 10-15 minutes during a prolonged workout or competition of moderate intensity

•  If working at a high level, should consume fluid at higher amounts every 10 minutes

•    Electrolyte replacement can be helpful

•  Helps maintain plasma [sodium], sustains thirst drive, promotes retention of ingested fluid, and more rapidly restores lost plasma volume

Factors that Improve Heat Tolerance

1. Acclimatization

–    Physiologic adaptation that improves tolerance to heat

–    2 to 4 hours of daily exposure to hot environment allows adjustment to heat within 10 days

–    Adjustments made include:

•   Improved skin blood flow

•   Improved blood distribution (helps maintain BP)

•   Sweat sooner (lowered sweat threshold)

•   Sweat more

•   Greater distribution of sweat over body surface (more involvement of sweat glands

•   Decreased loss of salt in sweat

–    Benefits of acclimatization can be lost within 2-3 weeks

Factors that Improve Heat Tolerance

2. Exercise Training

–   Increases sweating response (sweat sooner)

–   Greater plasma volume

–   Also, other responses noted in acclimatized individual

Factors that Improve Heat Tolerance

3.  Age

–   Not a major issue with heat tolerance in most physically matured individuals

–   Children

•   Have a lower sweat rate

•   Higher core temperature response to exercise

•   Have large number of sweat glands

•   Different sweat composition than adults

•   Recommendations

–   Exercise at a lower intensity
–   Hydrate appropriately
–   Allow more time to acclimatize

Factors that Improve Heat Tolerance

4.  Gender

–   Sweating response

•  Women have more heat-activated sweat glands

•  Men sweat sooner and more than women

•  Women have lessened chance of dehydration compared to men

–   Cooling rate

•  Women can cool faster due to larger surface area

Factors that Improve Heat Tolerance

5.  Body Fat Level

–   The greater the amount of body fat, the greater potential for heat problems

Exercise in the Cold

•    Body loses heat approximately 2- 4 times faster in water compared to air at the same temperature

•    The colder the temperature, the greater the O2 consumption

–   Due to additional cost of shivering to stay warm

•    Body fat is protective against the cold

–   Swimmers generally have more subcutaneous fat than other athletes not subjected to colder temps.

Acclimatization to the Cold

•    More difficult to adapt to cold than heat

•    With Acclimatization:

–   Increase heat production does accompany body heat loss

–   Individuals regulate at a lower core temp

–   Protective response to frostbite in periphery (blood flow enhanced to hands and feet)


Cold Air and Exercise

•     Can you freeze your lungs during exercise?

–    Not likely, incoming cold air is warmed up by the time it gets to bronchi

•     Significant water and heat loss can occur via the respiratory tract (expired air)

–    Must continue to remain hydrated during prolonged exercise in the cold

•     Only lose about 20% of body’s heat through the head

–    Must still keep it covered in cold temp

–    Also should wear light layers of clothes while exercising

Exercise at Altitude

•    With an increase in altitude, there is a decrease in PO2

–   This just means that O2 molecules are not as close together

•  For example: At sea level, the PO2 is 100 mmHg; but at the top of Mt. Everest (8,848 m), it is 28 mmHg so there is only 58% O2 saturation of arterial blood

–   There is still 20.9% O2 in the air at altitude

Acclimatization to Altitude

•    It takes approximately 2 weeks to adapt to an altitude of 2300 meters

–   For each additional 610 m in altitude, it takes an additional week (up to 4572 meters)

Physiologic Adjustments to Altitude

•    Immediate:

–   Hyperventilation

–   Fluid loss (due to respiratory fluid loss to cold, dry air)

–   Increased cardiac output

•  Primarily due to heart rate; stroke volume remains the same

Physiologic Adjustments to Altitude

•    Long-term:

–   Hyperventilation

–   Increased C.O. and HR

–   Increased pH (more alkaline)

–   Increased O2-carrying capacity

•   Occurs due to plasma volume & RBC’s

–   Enhanced cellular adaptations

•   concentration of capillaries in muscle

•   mitochondria

•   2,3- DPG (improves release of O2 from RBC to muscle)

Exercise Capacity at Altitude

•    Aerobic capacity

–   Decreases with an increase in altitude

•  Approximately 1.5 to 3.5% decrease in VO2 max per each 1,000 feet increase above 5,000 ft altitude

•    Cardiovascular function

–   Decrease in maximal cardiac output

•  Primarily due to stroke volume

–   Does not improve with stay at altitude

Does training at altitude improve aerobic performance at sea level?

•    No

•    The benefit of training at altitude is for improving performance at altitude!

General Rule

•    Acclimation to altitude takes time and must be progressive

–   Much like scuba diving at different depths