For the human being it is crucial to keep the body's core temperature at a constant level (37°C) in order to ensure functioning of the inner organs and of the brain. In contrast the temperature of the shell, i.e. skin and extremities, can vary strongly depending on the volume of blood it contains, which in turn, depends on metabolic and environmental heat loads. Heat is produced by metabolism as a result of activity, sometimes increased by shivering or slightly reduced by mechanical work where applicable, e.g. when climbing. The surplus heat must be released to the environment. The body can exchange heat by convection (sensible heat flux), conduction (contact with solids), evaporation (latent heat flux), radiation (long- and short-wave), and respiration (latent and sensible).

From the analytical point of view, the human thermoregulatory system can be separated into two interacting sub-systems: (1) the controlling active system which includes the thermoregulatory responses of shivering thermo genesis, sweat moisture excretion, and peripheral blood flow (cutaneous vasomotion) of unacclimatized subjects, and (2) the controlled passive system dealing with the physical human body and the heat transfer phenomena occurring in it and at its surface (Fig. 2.1). That accounts for local heat losses from body parts by free and forced convection, long-wave radiation exchange with surrounding surfaces, solar irradiation, and evaporation of moisture from the skin and heat and mass transfer through non-uniform clothing. Under comfort conditions the active system shows the lowest activity level indicating no strain. Increasing discomfort is associated with increasing strain and according impacts on the cardiovascular and respiratory system. The tolerance to thermal extremes depends on personal characteristics (Havenith 2001, 2005): age, fitness, gender, acclimatization, morphology, and fat thickness being among the most significant. Of these, age and fitness are the most important predictors and both are closely correlated. High age and/or low fitness level means low cardiovascular reserve which causes low thermal tolerance. The strain for the organism due to thermal stress can be quantified e.g. by an Physiological Strain Index PSI that is based on heart rate and Tcore (Hyperthermia) (Moran et al. 1998) and on Tskin and Tcore (Hypothermia) (Moran et al. 1999).

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