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1.2. THERMAL BALANCE
In order for the body temperature to remain constant, it is necessary that the inflow and outflow of heat be constant. The heat exchange of the body with the environment occurs due to 4 main mechanisms:
I) direct heat transfer;
2) convection;
3) radiation;
4) evaporation.
This can be written as a formula:
M + P + K + P-I \u003d 0, where M is metabolic heat production;
P - direct heat transfer;
K - heat transferred due to convection (this is a positive value if the air temperature is higher than the skin temperature and negative otherwise);
P - heat exchange due to radiation (also a positive value if the temperature of the external environment is higher than the temperature of the skin);
I - heat loss due to evaporation.
The above equation is valid only in tea if the body temperature remains constant. If the body temperature changes, then an additional term must be introduced into this equation, and then it will look like this: M + P + K + P- I ± T \u003d 0, where
T - changes in body temperature. It is customary to consider T - positive if the body temperature drops, and negative if the body temperature increases.
The specific heat capacity of various body tissues is not quite
the same, however, in principle, varies not very significantly.
Therefore, when determining the relationship between body temperature and the heat transferred to it, one proceeds from the average value of the heat capacity of 0.83.
Consider each of the above ways of heat energy exchange:
1. The direct transfer of body heat occurs when individual parts of the body come into contact with others (without taking into account the transfer of heat through the air). Under normal conditions, a relatively small amount of heat is transferred in this way. Typical examples of direct heat transfer would be standing barefoot on hot sand, heat transfer while sitting in a hot seat (e.g. in a car)
in light clothes, etc. Direct heat transfer is the main mechanism of heat exchange in bathing, swimming and generally when immersed in water.
2. Convection. Convection, as you know, is the transfer of heat by masses of moving air or water. Even in calm weather, the air around the body is in constant motion, and therefore a significant part of the heat is transferred in this way. There are three barriers to heat transfer through convection. This is, firstly, subcutaneous fat; secondly, clothes; thirdly, a thin layer of still air or water, which is in direct contact with the body or clothing.
The rate of energy exchange through each of these barriers depends on such factors as the temperature gradient, the thermal conductivity of the tissue or material, and the intensity of the air or water current. Heat loss from the skin surface increases dramatically if there is significant air movement. In the highlands, where atmospheric pressure is reduced, the number of air gas molecules per unit volume is also reduced, and heat transfer from the skin surface due to convection worsens. Conversely, as the pressure increases, the possibility of heat transfer increases. So, for example, in scuba diving, the rate of heat transfers due to the two mechanisms under discussion (direct heat transfer and convection) can lose similar performance on the water surface by about 20 times.
3. Radiation. Radiation, as you know, is the transfer of "heat due to thermal radiation. All objects radiate energy at a rate that depends on their temperature and surface properties, air temperature in the range of 20-50 ° C, radiation occurs mainly due to infrared radiation. The vast majority of materials, including clothing and skin, emit absorbing-like radiation in much the same way as dark surfaces.
Solar radiation occurs at a much higher temperature and, accordingly, in a different particular range with thinner waves). Radiation of this kind can be significantly reflected from the skin surface, as well as when wearing clothes of appropriate reflective materials in light colors. Radiative heat load is usually measured with a thermometer placed in a black ball that does not reflect the sun's rays.
4. Evaporation. Evaporation of 1 g of water corresponds to the dissipation of 2.43 kl of energy. In humans, evaporation from the surface of the body occurs mainly due to three main sources:
I - due to evaporation of the body;
2 - due to the liquids contained in the exhaled gases;
3 - evaporation due to a liquid that can be poured onto the human body. In this case, it is advisable to wash also the parts of the body where evaporation and convection are maximum. For example, cyclists and a runner are advised not to water on their shoulders, but on their hips. Light clothing facilitates evaporation.
The intensity of evaporation depends on several factors, such as skin temperature, air flow rate, atmospheric air humidity, barometric pressure, etc. Accounting for these factors is important when assessing the conditions in which the body's heat balance is carried out. Under different conditions, the ratio between the contributions of different thermoregulation mechanisms is different.
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