The incidence of heat-related illness is as high as 1 in every 100 men working for 1 year … 50 US football players are said to have died of heat illness in the 10 years to 1975 … Heat strain occurs whenever the body generates more heat than it can lose … even in cold conditions … A British soldier marching in outdoor temperatures close to 12 degrees Celsius died of heat-related illness.
(G. W. Crockford, 1999)
Because the rates of most biochemical reactions are temperature dependent, it is advantageous to control the rates of these reactions by means of a thermoregulatory system.
Humans have a remarkably well-adapted ability to tolerate heat compared with other primates. This applies equally to 'Eskimos' as to tropical rainforest dwellers, despite small differences. This is because humans are hairless and have a large proportion of high-capacity sweat glands - known as eccrine glands - in their skin.
Thermoregulation is achieved by balancing the two main factors that determine body temperature - the metabolic heat produced and the rate of heat loss. The thermoregulatory goal is to maintain the core temperature at approximately 36-37°C. Core temperatures over 39.5°C are disabling and over 42°C they are usually fatal. The lower acceptable limit is 35.5°C and 33°C marks the onset of cardiac disturbances. Further drops in core temperature are extremely dangerous and temperatures as low as 25°C are fatal. The temperature of the peripheral body tissues, particularly the skin, can safely vary over a much wider range. From a thermal point of view, the body can be considered to have a warm core where much of its heat is produced. This is surrounded by a shell of cooler, insulating tissues, particularly subcutaneous fat.
The principal sources of heat are the liver, the brain, the heart and the working muscles. Muscular work is a source of heat because the mechanical efficiency of muscles is only about 20%. Little heat transfer from the interior to the surface of the body takes place via conduction: the body tissues are poor heat conductors. Heat is transferred to the skin from the deep body tissues by convection. Blood is an ideal