Humans, being warm-blooded, have the innate ability to regulate the body temperature according to a fixed set point. That set point is usually 36.7 degrees Celsius for a normal healthy person.
The thermoregulatory system is controlled by the hypothalamus in the brain, which operates much like a thermostat. Whenever the temperature exceeds or goes below the certain set point, physiological processes kick in to bring the temperature back to normal. However, unlike the typical thermostats we use in instruments, the hypothalamus cannot "turn off" the heat. It only serves to prevent a build up or loss of heat.
The sensors in the thermoregulatory system are the hypothalamus and the skin receptors (peripheral receptors) while the effectors are the sweat glands, the skin capillaries, etc.
Thermoregulation in cold stress
When the external environmental temperature falls below the skin temperature, the cold receptors get stimulated. This causes the constriction of blood vessels at the periphery, redirecting blood back to the inner core, in order to conserve body heat. The hypothalamus also senses the low temperature of the blood that perfuses through it. This can cause further constriction of the blood vessels or trigger the onset of shivering to produce metabolic heat.
Thermoregulation in heat stress
When the warm receptors of the skin get stimulated due to an external temperature that is higher than the skin, vasodilatation of the peripheral blood vessels occur to allow for more heat loss through conduction, convection and radiation. The hypothalamus will also sense the increase in temperature of the blood. Another heat loss mechanism in the form of sweating will also kick in. Sweating per se does not promote heat loss. Rather, it is the evaporation of sweat.
After understanding the mechanism of loss through the body, it becomes apparent that high intensity physical work in impermeable clothing under warm environment poses a very real danger. An example would be fighter pilots and chemical and biological defense forces in the tropics. Without sufficient air circulation in the clothing, the sweat cannot evaporate. Vasodilatation is useless in this case as the external temperature is higher than the skin temperature. People operating under such conditions have a very high chance of suffering from heat illnesses.
To prepare people for these extreme conditions, acclimatisation has been suggested and it is widely practised. Acclimatisation has also been shown to be extremely effective in reducing the rates of heat sickness. However, the science behind it seems to be flawed and requires further research.
To explain the flaw', let us first think about an example whereby an athlete living in a temperate region undergoes training in a tropical climate. After repeated exposures to the hot environment, the thermoregulatory system of the body kicks in at a lower temperature than during first exposure. The body increases the capacity for sweating and the physical activity is carried out at a lower core temperature. More blood is also shunted from the core to the peripheral blood vessels. This mechanism is widely accepted as the reason why acclimatisation works.
However, this cannot be applied to people operating in impermeable suits under extreme temperatures. Increase in sweating capacity and vasodilatation is of no help to these people. It seems highly likely that acclimatization works in a different way in these professions. Also, psychology might play a large part in acclimatisation. As people get more familiar with wearing the impermeable suit, they would generally suffer from less mental stress. This might indirectly help them to cope with the heat.