Heat production and dissipation
When resting, an adult man uses approximately 3.0 ml of O2 per kg of body weight, per minute, which corresponds to an energy consumption of 1 W/kg, that is entirely dissipated as heat.
During exercise, things become more dramatic. For example, a marathon runner whose consumption of O2 is 60 ml per kg of body weight, per minute, has a 20 W/kg energy consumption. If the heat the runner produces is not removed, their average body temperature would increase by 20.9°C an hour!
It is a well known fact that body temperature remains constant, both when we are resting (37°C), and exercising. This shows that all of the heat produced is removed to the external environment.
An increase in body temperature due to an increase in O2 consumption, which occurs during exercise, will need to be counterbalanced by increasing heat dispersion. In turn, this can be achieved by increasing the skin temperature, which promotes heat dispersion. Furthermore, the increase in body temperature triggers sweating, whose purpose is to deposit a film of liquid, which when evaporated from the skin's surface, cools the body down.
Training improves tolerance to intense muscular work, especially through its effects on sweating. In fact, training:
1- leads to a more homogeneous production of sweat across the entire body surface, instead of just on limited areas of the skin (for example, the forehead) which often occurs in unfit people;
2- increases the production of sweat at the same body temperature;
3- reduces the concentration of salts, such as sodium and chloride ions, in your sweat.
Heat loss occurs only when the sweat evaporates, not when it is produced. Moreover, reducing your excess salt levels, will allow you to have a better physiological balance during exercise.
Fluid balance, dehydration, heat stroke
During prolonged exercise in hot temperatures (such as in summer), you can sweat up to 2 litres of fluid an hour. It is therefore evident that, without an adequate water supply, it is easy to become dehydrated whilst exercising for extended periods of time. Dehydration can have two negative effects: on the one hand, it reduces your ability to carry out intense work; and on the other, it promotes the onset of heat stroke. Being less able to carry out work is due to the fact that dehydration leads to a reduced blood mass and an increase in blood viscosity. These factors burden the load that the heart must bear to maintain cardiac output. It therefore decreases its maximum aerobic working capacity to 60% of the value observed under ideal temperature conditions. To avoid becoming further dehydrated, the body puts various mechanisms in place in order to reduce water loss, such as substantially reducing sweat production. However, this can lead to hyperthermia and subsequently, heat stroke. Signs of hyperthermia begin to manifest when you have lost approximately 3% of your body weight in water, so this limit should never be exceeded. The level of dehydration, which is observed hyperthermia and which runs a clear risk of heat stroke, corresponds to a loss of fluid which is equal to or more than 5% of your body weight. Also, you should not overlook the fact that people with higher body masses are more at risk of dehydration and heat stroke than people with smaller body masses.