The human body is in many ways like a machine, and like any machine it is not 100% efficient. Every time the human body does work of any sort, heat is generated, in much the same way as heat is generated in machines by friction. It is often said that our bodies have a temperature of 37 Degrees Celsius. This is in fact only an average temperature and different body parts have different temperatures. The mouth for example has a temperature of about 36.8 degrees Celsius
Core temperature, also called core body temperature in deep structures of the body such as the liver. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Significant core temperature elevation (hyperthermia) or depression (hypothermia) that is prolonged for more than a brief period of time is incompatible with human life.
The core temperature is kept at about 37 Degrees Celsius, even though conditions outside the body can vary dramatically. Our metabolic rates can increase by a factor of up to twenty during metabolic exercise. The heat energy generated by our body not being a completely efficient machine – some estimates say it is maybe only 30% efficient – can be useful – as on polar expeditions, or during storms at sea, but it must often be removed somehow. If not removed the body's core temperature would rise by about 6 Degrees Celsius in 15 minutes, causing severe damage.
If our bodies become too cold, than exercise becomes an involuntary act – we shiver, and shivering is a form of exercise which generates heat. Even in the absence of any exercise the human body generates heat as a by product of respiration, and the problem is mainly to dissipate heat at such a rate as to maintain the right core body temperature.
The human body loses heat by several routes:
Sweating – salty fluid is excreted from pores in the skin, and as it evaporates it carries away body heat.
Exhalation of air from the lungs. The air we breath out is warmer than the air we breath in and the net effect is a loss of heat energy.
All excreted material takes heat away from the body.
We can take off some clothes which serve as insulation. Taking off clothes means that heat can more rapidly escape from the surface of the skin.
Going swimming is a very efficient method of losing heat energy. Water has a much higher capacity to absorb heat than air, so will enable much quicker cooling. Eving more effective is drinking cold water or eating ice, since it gets most closely to the core and can absorb heat faster, which is then excreted.