Body Temperature, Energy and Heat Loss

For the skin to lose (rather than gain) heat, the object in contact must be at a lower temperature than the skin, regardless of whether it is a good or poor conductor.

The amount of energy radiated as infrared depends on the body temperature (E ∝ T⁴).

Vasoconstricting blood vessel close to the body surface will withdraw blood from the surface and allow the skin temperature to cool.

Hyperthermia is a body temperature that is too high. Hence the body must lose heat by evaporating sweat. Vasodilation brings more blood closer to the surface allowing radiation loss to increase.

The water molecules in sweat that have the highest kinetic energy are the ones that evaporate first. This leaves the slower moving molecules behind, and they have a lower temperature. These slower molecules will gain energy from the body heat and eventually have enough kinetic energy to evaporate, again cooling the body as more kinetic energy is transferred away from the body.

Heat is the energy that is transferred between two objects due to their difference in temperature. Infrared radiation is one form of heat (but not the reverse).

Respiratory heat loss occurs because the air we inhale is usually at less than 37 °C, while it is warmed to 37 °C before it is exhaled. (Also exhaled air is saturated with water vapour that carries with it the heat of vaporisation.) These mechanisms are not affected by the amount of skin we expose.

Clothes trap a layer of air close to the skin. This air is warmed to skin temperature and is prevented from blowing away by being contained within the clothing.

Conduction is the transfer of heat between contacting objects at different temperatures.

Degrees Kelvin = degrees Celsius + 273