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
  • As body core temperature increases, sweat production increases, sweat turns into vapor when it reaches skin and cooling occurs
  • 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)

Thermoregulatory Control

The body's thermoregulatory responses are very effective for normal ranges of body and air temperature. Core body temperature at rest is regulated at approximately 37 degrees C (98.6 degrees F). If the internal (core) body temperature exceeds 40 degrees C, physiological function will be inhibited. Humans are able to briefly withstand core temperatures below 35 degrees or above 41 degrees.

• Body's thermoregulatory centre - preoptic anterior hypothalamus (POAH)
  • acts as a thermostat - monitors temperature and accelerating heat loss or heat generation when necessary
• Sensory thermoreceptors provides information to POAH
  • skin - peripheral thermoreceptors
  • brain and spinal cord - central thermoreceptors
    • central thermoreceptors are more sensitive to temperature changes than peripheral thermoreceptors
• When POAH senses deviation in body temperature (below or above normal), it sends signals through the sympathetic nervous system to the following thermoregulatory effectors:
  • skin arterioles
    • POAH sends signal via the sympathetic nervous system to smooth muscle in arteriole walls resulting in either vasodilation or vasoconstriction
      • vasoconstriction to minimise heat loss
      • vasodilation to enhance heat loss
  • eccrine sweat glands
    • sufficient rise in core and skin temperature - POAH sends signal via the sympathetic nervous system to eccrine sweat glands, leading to active secretion of sweat. Read more on the Physiology of Sweat
  • endocrine glands
    • the effect of various hormones results in increased cellular metabolic rate, and increased metabolism leads to an increase in heat production
  • skeletal muscle
    • increased skeletal muscle activity increase body temperature by increasing metabolic heat production
    • muscle actions can be voluntary or involuntary (for example when a person is shivering because of being cold)

Physiological Responses to Exercise in the Heat

Cardiovascular Response

Independent from the environment, exercise results in an increased heat load due to muscle work and an increased metabolism. The POAH sends signals via the sympathetic nervous system to the skin arterioles to vasodilate to increase conductive and evaporative heat loss. Vasoconstriction occurs to shunt of blood to non-essential tissue such as the gut, liver and kidneys. Cardiac output further increases via an increase in heart rate and changes in contractility to maintain the cardiac output to working muscle. Due to sweating, blood volume decreases and the ability to increase stroke volume is also limited due to peripheral blood pooling. Heart rate increases to compensate for the decrease in stroke volume (phenomenon known as cardiovascular drift).

Fluid Balance

Hot environmental temperatures and strenuous physical activity increase skin and core temperatures. The POAH controls the sweat glands. Sweat forms in the coiled section of the sweat gland and initially the electrolyte composition is the same as that of blood as plasma is the source of sweat formation. The filtrate of plasma moves through the duct of the sweat gland and sodium and chloride are reabsorbed back into the tissue and then into the blood. The sweat that is released onto the skin has thus less electrolytes than plasma. Light sweating - filtrate travels slowly through duct thus there is more time for reabsorption of sodium and chloride, sweat is very diluted

Heavy sweating - less time for reabsorption of sodium and chloride as filtrate moves quicker through sweat duct and the sodium and chloride content of sweat is higher

There is a difference in the electrolyte concentration of sweat in trained versus untrained individuals. With training and heat acclimation more sodium and chloride is reabsorbed and sweat is more dilute. Concentrations of potassium, calcium and magnesium stays more or less the same.

Exercise in hot and humid conditions can result in a loss of 2.5 % t0 3.2 % of body weight through sweat. High rates of sweating reduces blood volume. This leads to an increased heart rate and eventually leading to a decrease in cardiac output and this affects athletic performance.

Read this article:

ACSM. Exercise and Fluid Replacement

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