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 & RBCs

   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