The Mechanisms of Organism Thermoregulation
It is known that body core temperature is kept on basic level 37,0 ± 2,0 oC due to the thermoregulation mechanisms in human body. All living processes are dependent upon the temperature. The optimal core temperature is maintaining the normal heart beating, muscles working and nervous system functioning. The correct living organism functioning due to normal metabolism and organs functioning is possible thanks to optimal temperature conditions called thermal homeostasis (Górski, 2010). However, the temperature of human body (core as well as the surface) can change due to diseases, fever, stress and physical effort (Wendt, 2007; Pilawski, 1983).
The human body raises the energy needed for living from nutrition (oxygen and food metabolism). Part of this energy is used for work and the rest is radiated as a thermal energy (heat) (Herman, 2007). To keep the thermal homeostasis the overcapacity of energy has to be removed from the organism. Heat transfer between human body and environment can be done by mechanisms like: conducting, convection, radiation and evaporation. It can be noticed that the fragmented streams from convection ΦK, radiation ΦR and evaporation ΦE have contribution to the total heat flow Φ (Equation 1):
(1)However the heat transfer due to the conducting is observed only in the thermal energy transport between the tissues and skin (Pilawski, 1983; Herman, 2007).
Human body as a subject with temperature higher than absolute zero (0oK) transfers heat also by radiation (Usamentiaga, 2014). The power of radiation loss can be estimated from the surface of the body using the Stefan-Boltzmann law for biological object (Pilawski, 1983; Cholewka, 2004; Cholewka, 2005; Tuszynski, 2002; Wiśniewski, 2009):
(2) where:
σ – black body radiation constant,
,a – radiation surface emissivity,
A – body surface.
Another way of heat transfer occurring in the human organism is sweating and evaporation, which should be taken into consideration from the thermal imaging point of view. It is known that the human body excretes the water due to sweating. In normal conditions the 50 ml of water is lost per hour in evaporation way. Obviously the evaporation speed depends on the level of environment humidity and the heat exerted by the body i.e. during training (Pilawski, 1983; Cholewka, 2013; Cholewka, 2015).
The first thermodynamics rule is interrelated with the biological processes. For the human body it can be written as:
(3) where:
When the work is done (ΔW > 0) the heat is transferred out of the body (ΔQ < 0) and the stored energy is rising down (ΔU < 0). The heat from metabolic processes Qmet (called also the metabolic rate, MR) and the heat loss due to the heat transfer Qloss contributes to the total heat Q inside the body. The body temperature is regulated by increase of heat production (thermogenesis) when the heat is losing passive and active heat loss (thermolysis) when the organism is overheated (Pilawski, 1983; Herman, 2007; Kasprzyk, 2014).
However the thermoregulation system in human body is working to keep the constant temperature of body core and not to keep the constant level of heat in organism (Górski, 2010).