Heat is constantly produced in the human organism as a result of the metabolism in the cells. The greatest quantity of heat is produced in the liver. In the muscles, part of the energy released is used for work and the other, bigger part, is transformed into heat. The heat released during muscle contractions is considerable in quantity because the muscles make up about half of the body mass. Heat is constantly produced internally and transported to the surface of the body from where it is released into the environment.  If heat is not released, it is impossible to achieve constant body temperature.  Thanks to the dynamic balance between the produced and released heat there is a constancy of the body temperature. It is essential to the normal realization of the metabolic processes in the body.
The temperature of the body is not one and the same in all its parts.
The temperature inside the organism is higher than the one on its surface and more stable. It is called inner or core temperature. With healthy people the core temperature measured in the rectum is about 37,2 °С, and in the mouth – approximately 37,0 °С. The surface temperature is not one at the same in the different body parts either. Body temperature also varies in the different parts of the day. It is lowest at about 4a.m. and highest – at about 4 p.m. The body temperature rises during physical labor, emotional stress, prolonged stay in a very warm environment and illness. Body temperature decreases during sleep, prolonged exposing to cold and decreased production of hormones by the thyroid gland.
Transfer of heat from the inside of body to its surface is facilitated through better blood supply of the skin. Lessening the blood influx to the skin decreases temperature release to the environment.

Transfer of heat to the surface of the body is carried out in several ways:
1. By radiation. Exchange of heat between the environment and the organism is realized through heat electromagnetic waves. When the temperature of the environment is high, the organism takes heat from the sun for example. When the temperature of the environment is low, the organism releases heat.
2. By convection. This is due to the movement of gas or liquid molecules that are close to the body surface. When the atmospheric or water medium is mobile the loss of heat through convection is greater.
3. By conduction. Release of heat from the body surface to the environment is observed when in touch with objects or liquids that have lower temperature. When the temperature of the skin is higher than the one of the environment, release of temperature occurs. It is proportional to the temperature difference.
4. By evaporation. A great quantity of heat is released when water evaporates from the surface of the body.   There always exists evaporation from the body surface of water passing through the skin without the participation of the sweat glands. Thus the organism constantly loses heat. This loss cannot be regulated and new heat has to be produced constantly. At a temperature of the environment of above 20 °С sweat glands start producing sweat. When the temperature is higher than 34 °С evaporation of the sweat is the only way of releasing heat from the body. High humidity of the surrounding air hinders evaporation and can lead to overheating of the organism.  When the air is warm and dry evaporation and release of heat is easily done.