Thermoregulation and Exercise in the Heat and Cold

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Introduction

Exercise in the heat or cold can influence the body's ability to regulate body temperature. Although these processes are effective and well-functioning in normal conditions, in extreme hot or cold conditions thermoregulation (regulation of body temperature around a physiological set point) can be inadequate. The human body is able to adapt to these environmental thermal conditions with continued exposure over time. Acclimation refers to the body's short-term adaptations (days/weeks) to these environmental stressors, whereas acclimatisation refers to long-term adaptations (months/ years) to environmental stressors.

Regulation of Body Temperature

• Humans are homeothermic - this means that the internal body temperature is physiologically regulated and kept nearly constant despite environmental temperature changes.
• Fluctuations in body temperature are usually no more than 1.0 degrees Celsius (1.8 degrees F)
• Body temperature is a reflection of balance between heat production and heat loss. If this balance is disturbed, this will result in body temperature changes.
• Normal baseline range of body temperature (36.1 to 37.8 degrees C (97.0 - 100.0 degrees F)
  • Situations that will cause a change in normal baseline temperatures:
    • Fever due to illness
    • Extreme conditions of heat or cold
    • Prolonged heavy exercise

Transfer of Heat between Body and Environment

• Metabolic heat is produced by active tissues
• Heat moves from the body core (deep within the body) to the body shell (skin) via blood
• When heat reaches the skin it can be dissipated by conduction, convection, radiation and evaporation.
• Maintaining a constant internal body temperature is dependent on the body's ability to balance heat gain (from metabolic processes and the environment) with heat loss. Figure 1. demonstrates this balance: ADD IMAGE
  • Heat gain: metabolic heat and environmental heat (conduction, convection, radiation)
  • Heat loss: radiation, conduction, convection, evaporation
• Conduction = heat transfer from one solid material to another through direct contact ADD IMAGE
  • Example: sitting on cold or hot metal bleachers watching a sports event
• Convection = heat transfer by movement of gas or liquid across a surface
  • During movement such as playing sports the air around us passes over the skin and heat is exchanged with air molecules
  • Greater movement of air results in greater heat exchange
  • Major daily thermoregulatory factor
• Radiation = heat transfer in the form of infrared rays
  • The body can give off or receive radiant heat
  • Major daily thermoregulatory factor
• Evaporation = heat loss via phase change from liquid to gas
  • Primary way for heat loss during exercise
  • Sweat must evaporate to have cooling effect, dripping sweat provides little or no cooling effect

Challenges with the Transfer of Body Heat

• Humidity and heat loss
  • High humidity limits evaporation of sweat as the air contains many H2O molecules
  • Low humidity provides ideal opportunity for evaporation
• Prolonged sweat evaporation without adequate fluid intake can cause dehydration
• Temperature of skin greater than the temperature of the environment - heat loss through conduction, convection and radiation and evaporation
• Temperature of the environment greater than skin temperature - heat can only be removed through evaporation
• In hot humid situations, evaporation of sweat is limited and convection occurs from the environment to the skin (thus heat is gained)

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