D5 - Excessive Heat

• • Exercise increases metabolic rate by 20-25 times, and could increase core temperature by 1’C every six minutes if heat loss did not take place.
  • This would result in death from hypothermia if exercise continued.
  • Therefore, with the added stress of a hot environment, an exercising athlete has to maximise heat loss in order to perform optimally and to avoid hypothermia.
• The hypothalamus acts as a thermostat and initiates the responses that protect the body from overheating. 

  It receives information about the temperature of the body via two sources:

  • Indirectly from the thermoreceptors in the skin
  • Directly by changes in blood temperature by thermoreceptors in the blood vessels
• Methods of Heat Loss in a Hot Environment

  • Heat loss through radiation is not possible if the environment is hotter than the person exercising .
  • Therefore, there are only three forms of heat loss available to a person exercising in a hot environment.
• Conductive Heat Loss occurs by the peripheral blood vessels dilating and bringing blood close to the skin’s surface. This results in the rosy-coloured skin associated with hot athletes. The heat from the blood warms the air molecules around the person and any cooler surfaces that come into contact with the skin. Conductive heat loss works in conjunction with convective heat loss.
• Convective Heat Loss occurs much more rapidly if there is increased air flow around the body, e.g. if it is windy or a fan is being used. If there is little air movement, the air next to the skin is warmed and acts as a layer of insulation that minimises further convective heat loss. However, if the warmed air surrounding the body is frequently changed due to increased air currents, heat loss through convection will continue to remove excess body heat.
• Evaporation Heat Loss provides the main source of heat dissipation. As the sweat evaporates, it cools down the skin surface. This has the effect of cooling the blood as it travels through the blood vessels that are close to the skin surface. In order for evaporative heat loss to occur maximally, the person must be hydrated and have normal levels of salt and electrolytes in their body.
• Evaporation Heat Loss:

  Children are much more prone to overheating than adults. This is partly because they do not have a fully developed sweating mechanism and also because they have a much higher surface area to volume ratio. This fact means that they will gain (or lose) heat much more quickly than adults. Therefore, when children are taking part in sports or exercise on a hot day, they should be given lots of rest periods and drinks and where possible kept out of direct sunlight.
• Evaporation Heat Loss:

   Women usually have lower sweat rates than men, therefore, on average, males are able to lose more heat through evaporation heat loss than females have a higher body surface area to volume ratio than males, they are able to lose more heat through radiation. Research has shown that these variations in heat loss between the sexes evens out, so that there is no real difference in the ability to dissipate heat between males and females.
• Evaporation Heat Loss:

   The circulatory system is virtually important in ensuring that these three methods of heat loss can occur. Not only does the blood have to supply the muscles with oxygen and nutrients, it also plays a major part in thermoregulation. The blood is redirected to the periphery by dilation of peripheral blood vessels. In extreme conditions, 15%-25% of the cardiac output is directed to the skin. As a result of these two cardiovascular demands, the heart rate is higher when exercising in the heat than in normal conditions.
• Hyperthermia:

  If the body is unable to lose the excess heat generated from exercise and/or from the environment, the person will suffer from hyperthermia.

  There are three major forms of hyperthermia:

  • Heat cramps
  • Heat exhaustion
  • Heat stroke
• Hyperthermia:

  • Heat cramps are muscle spasms caused by heavy sweating. Although heat cramps can be quite painful, they usually do not result in permanent damage.
  • Heat exhaustion is more serious than heat cramps. 
  • It occurs primarily because of dehydration and loss of important minerals.
  • In order to lose body heat, the surface blood vessels and capillaries dilate to cool the blood.
• Hyperthermia:

  • However, when the body is dehydrated during heat exhaustion, the blood volume is reduced so there is not enough blood to supply both the muscles and the skin with their required blood supply.
  • This results in the peripheral dilated blood vessels constricting which significantly reduces heat loss.
  • This can be observed by looking at the face of an athlete suffering from heat exhaustion - it will suddenly change from a red, rosy appearance to much paler colour or white.
• Hyperthermia:

  • If a person ignores the symptoms of heat exhaustion and continues to exercise, they will suffer from heat stroke, which is a life-threatening condition and has a high death rate.
  • It occurs because the body has depleted its supply of water and salt, and results in the person’s body temperature rising to deadly levels.
  • If the core temperature of the body reaches 43’C or more, the proteins start to break down and change their structure permanently.
  • The same principle applies to the body’s proteins, such as the enzymes and hormones.
• Dehydration:

  • Heart rate is also elevated because of the slight to severe dehydration that often occurs while exercising in the heat.
  • If the person is dehydrated, then plasma volume is decreased.
  • A decreased volume will lead to a decreased stroke volume.
  • Therefore, as we know:
  • Cardiac output = heart rate x stroke volume
  • Q (1 per minute) = HR (bpm) x SV (ml)
  • In order for cardiac output to remain the same, heart rate has to increase to make up for the decreased stroke volume.
• Adaptation to excessive heat on the body during sport and exercise performance:

  • Adaptation to excessive heat takes around 14 days for full acclimatisation; however, in the first 1-5 days the body will start to adapt
• Increased sweat production -  The body is able to produce at a faster rate, so more sweat is produced to have a greater cooling effect on the body through the evaporation of the sweat.
• Reduced electrolyte concentration in sweat - The body starts to converse the electrolytes sodium and chloride so not as much is lost in sweat, which helps with water retention.
• Increased blood plasma volume - Plasma volume expands, which means there is more water in the blood that can be used to produce sweat to cool the person down.
• Earlier onset of sweating - The body is able to produce sweat at a lower core temperature, which helps the body to start cooling down sooner.
• Aerobic performance:

  • Aerobic performance is the main type of performance negatively affected by excessive heat because of dehydration.
  • Dehydration causes a significant loss of performance.
  • This is because dehydration will cause a loss of blood plasma affecting blood flow and the ability to sweat.
  • When we sweat it is predominantly blood plasma that is lost and thus cardiac output (the amount of the blood leaving the heart per minute) is reduced.
  • Therefore, rehydration is important to improve the circulation of the blood and the body’s ability to control temperature.
• Aerobic performance:

  • The effects of adaptation to heat help a person to produce more sweat to help to cool them down through evaporation.
  • Any athlete competing in a hot environment will need to go through a full acclimation process to ensure their body is able to cope with the demands of exercising in the heat.
• Anaerobic performance

  • Anaerobic performance will only last for short periods of time and not rely to the same extent on the volume of blood in the body as aerobic exercise.
  • Therefore, athletes competing in anaerobic sporting events are not affected to the same degree as those competing in aerobic events.