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 18^oF (10^oC)

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

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

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

Thermal Balance

Mean Body Temperature

T_body = (0.6 x T_core) + (0.4 x T_skin)

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