Heat and the body

Heat flows to and from the body are determined by the amount of heat produced by the body from metabolic processes, and heat gain or loss to and from the environment. The body loses and gains heat through conduction, convection, radiation and evaporation. Understanding these processes is necessary to understand heat or cold injury, and how they should be managed.

Conduction is the transfer of heat between objects by direct contact. Heat flows from higher temperature to lower temperature. In this way, heat may be lost from the body when a person is placed on the snow with little insulation, or heat may be provided to a hypothermic patient by contact with a warming device. Contact with clothing that has been cooled by evaporation of water or sweat is another example of heat loss by conduction. The well, alert person usually protects themselves from these heat losses by wearing insulating clothing and footwear and using appropriate equipment. The person who is injured, ill, intoxicated or has an altered conscious state is at risk of significant heat exchange when lying uninsulated on hot or cold ground for prolonged periods.

Convective heat exchange occurs when heat is transferred between the body and air. Increasing air movement will increase transfer by convection; for example, fanning a patient suffering a heat illness will facilitate heat loss from the body.

Radiation is the transfer of heat through space, by electromagnetic radiation, without any intervening medium. Radiation is a major source of heat gain to the body in hot climates. Protection from direct sunlight and the wearing of light coloured clothing reduces absorption of solar radiation.

Evaporative heat loss occurs when water vaporises from the body surface. This occurs with sweating and with breathing, because the air we breathe out contains water. The scalp, face and upper torso are the most important sites for cooling from sweating. The capacity for sweat to evaporate varies with humidity, ambient temperature and wind velocity. High ambient temperature, high humidity and low wind velocity minimises evaporative heat loss from the body and results in an increased risk of heat illness. When sweating is minimal, evaporative heat loss accounts for only 15% of total body heat loss. However, with maximal sweating, evaporative heat loss can increase to around 70% of body heat loss. Evaporative heat loss through breathing can be reduced by breathing warm, moist air. Loose fitting clothing increases both convective and evaporative heat losses.