Body Heat Balance

Why does anyone feel limp in hot surroundings? Why is a breeze refreshing? These sensations relate very much to the efforts the body has to make to maintain its temperature. The less the effort the more comfortable we feel. Man is a warm blooded animal and must maintain the temperature of his vital organs within a few degrees of 37°C throughout life.

The human body, fuelled by the food we eat, continuously produces heat, associated with chemical change and muscular activity. This metabolic rate of heat generation can exceed 1kW with maximum exertion. The following table gives an idea of the range; individuals will vary with age, weight and other personal characteristics.

Activity	Metabolic Rate
Sleep and complete rest	50-100W
Seated, without manual work	100-140W
Light manual work	140-200W
Moderate work; walking	200-400W
Heavy labour	400-800W

The body has not much heat storage capacity because we cannot allow the greater part of our substance to get significantly hotter or cooler without distress. Apart from minor temporary deviations we must dissipate heat just as fast as it is generated - i.e. at the metabolic rate. There are three processes by which the body loses heat: convection; radiation; evaporation.

Convection and Radiation

A layer of cool air in contact with warm skin or clothing will pick up heat, and as its temperature rises its density will fall. The lighter air now rises up away from the body, taking heat with it, and is replaced by fresh, cool air which continues the process. This is natural convection.

Even the slightest air movement around the body will increase the rate at which warm air is replaced by cool, and thus increase the heat loss.

If the air temperature is on the high side the extra movement will be felt as a pleasant breeze, and can be increased with advantage by the use of ceiling fans or circulating fans. If, on the other hand, the temperature is normal or low, movement will be felt as a draught and should be kept below the limit of perception - about 0.25m/s - for maximum comfort.

Like all matter, the body transmits heat by radiation and receives heat by the same path. If all the surfaces surrounding us were at the same temperature as our skin there would be no net gain or loss of heat by radiation. In practice, of course, the walls and most of the surfaces are cooler and radiation carries part of the necessary heat loss. Radiant heaters have high temperatures over small areas, contributing a net heat gain to the body. As radiant heat, like light, travels in straight lines, only the ‘illuminated’ side of the body will be heated, but, in the absence of excessive draught, the blood stream will distribute the heat satisfactorily over the body.

Within the comfort zone of external air conditions, convection and radiation losses account for about 75% of the metabolic heat at rest, or with mild activity. When the temperature rises above or falls below the comfort zone an automatic reaction known as vasomotor regulation comes into play, with the object of preserving both internal temperature and heat loss unchanged. The tissue layers under the skin contain a network of veins which can be enlarged or contracted under the control of the nervous system. In a warm environment, a copious flow of blood is allowed through these vessels, bringing the skin temperature up towards a maximum of about 35°C, so as to maintain its temperature excess over the surroundings. In a cold environment the veins are constricted, reducing the thermal conductivity of the tissue layer so that the skin cools. The blood flow along legs and arms is also modified, allowing further cooling along their lengths to keep down the heat loss. Ultimately, hands and feet may be only a few degrees above the air temperature - even to the extent of allowing frost-bite of fingers and toes rather than loss of temperature at a vital centre.

Evaporation

At temperatures above about 30°C the system just described is unable to secure the necessary heat loss, even at rest. Indeed, above 35°C convection and radiation cease to be losses and become heat gains making the body’s task even harder. Reaction to this situation is known as evaporative regulation. It consists in the automatic activation of the sweat glands over an area of skin proportional to the corrective effort required.

The transformation of sweat into water vapour absorbs energy, just as the boiling of water does. This energy is taken from the wetted skin surface in a form known as the latent heat of evaporation.

The cooling effect is powerful: the worker in a hot industrial environment may easily produce and evaporate one kilogram of sweat each hour, which will remove heat at the rate of 680 watts. The water and salt lost by the body have to be replaced, and it is natural that such workers should be copious drinkers.

Apart from the sweat mechanism, evaporation of the water vapour in the exhaled breath carries away heat at a minimum rate of around 20 watts. Note that the metabolic rate cannot be reduced, though we may increase it if we are cold by voluntary activity - walking briskly, stamping the feet, swinging the arms, etc. Shivering is an automatic reaction with the same purpose.

In a still atmosphere the air next to the skin and trapped in the clothing becomes almost saturated, and its capacity to absorb and carry away moisture is severely limited. The sweat produced stays wet on the skin and the body’s effort to give off heat is retarded. The surplus sweat which drips off or is wiped away is virtually useless for heat removal.

Currents of air flowing round the body correct this situation. Saturated air is replaced by fresh and evaporation is helped in exactly the same manner as heat removal is helped by forced convection.