Heat-regulating mechanisms of the human body

As discussed earlier, the human body must maintain a fairly constant temperature over a considerable range of external air temperatures. The human body is subject to the same laws of physics as other objects, gaining and losing heat by the processes described above, namely: radiation through space; conduction between bodies and/or substances in contact; convection involving the transfer of heat from a warm body to a body of air above it, which then rises to be replaced by cooler air; and evaporation, which requires that the evaporating surface give up some heat. However, the human body is not simply a passive object warmed or cooled like metal or water. Its metabolic processes generate its own heat as well, similar to a heat-producing engine. Like any other engine, it burns fuel, in the form of food, and converts this into heat and work. As with an engine, work cannot be generated without producing some heateven if unwanted-which must be dissipated just as for an automobile.

In a hot environment, the heat generated by the human body must be dissipated. Body heat regulation is essentially the maintenance of a balance between heat gains and losses. The body has an excellent heat-regulating mechanism, which under normal conditions can adjust its temperature to maintain the appropriate heat balance. Only when it is exposed to prolonged severe conditions do serious difficulties arise.

The metabolic processes of the living human body continuously generate heat. Even at complete rest, an important quantity of heat is produced. This basal heat production amounts to 73 kcal/h (290 Btu/h) for an average adult male. For a short time he can increase this rate eightfold through violent exercise, although over 24 hours the average heat production would not amount to more than 130 % of the basal rate for sedentary work and 300% for heavy manual labor.

Table 3. Heat gain and loss processes for the human body

Table 3 shows the modes of heat gain and loss between the human body and its surroundings for the metabolic activities and three mechanisms of physical heat exchange, namely, radiation, conduction, and evaporation.

Air movement has a significant influence on the heat transfer between the skin and air and will increase the transfer rate in whichever direction it is proceeding, i.e., either to or from the body. Air movement increases the rate of heat loss by evaporation. For continued heat loss, the evaporated water vapor must be free to move away from the site of evaporation. Thus the difference between the vapor pressure at the skin surface and that of the surrounding air controls the ease with which evaporation cools the skin. The vapor pressure at the skin surface results largely from the extent to which a water film covers the skin, which may vary from less than 10% of the skin area on a cool, dry day, to 100% when the skin is bathed in perspiration.

The consequences of heat stress can be important. When the human body has difficulty losing heat, the blood vessels of the skin dilate, allowing much more blood to circulate and cooling by heat loss through any of the processes discussed above. But this increase in blood-vessel volume may exceed the body's ability to provide a corresponding amount of blood. To compensate, other blood vessels in the internal organs may receive less blood, although this still may not yield sufficient blood. During such a relative blood shortage, the brain, located at the highest part of the body, may be deprived of an adequate supply. Brain tissue is most sensitive to the shortage of oxygen and quickly produces the characteristic symptoms of "heat exhaustion": lassitude, headache, nausea, dizziness, uneasiness, and ultimately fainting. However, a wide range of lesser disturbances probably interfere with efficiency without resulting in total exhaustion. In addition, the human body has a remarkable sweating capability. With moderately hard work under hot dry conditions, a man can produce about 1.5 liters (3 pt) of perspiration per hour. Although he probably would not keep this up for more than two or three hours, he could lose as much as 8 liters (4 gal) in one day, which must be compensated for by drinking water. Eight liters is a large quantity of water for the body to handle, and even at lower sweating rates there probably will be periods when water loss exceeds supply. Then the already precarious blood supply is depleted still further and the risk of heat exhaustion is increased. Further indirect consequences of heat stress are lowered alimentary activity due to the insufficient blood supply, discomfort from hot and moist skin, the risk of skin disturbances when moist skin is chafed, possible salt deficiencies due to sweat loss, and perhaps urinary stones from reduced urine flow.

Thus it is important to avoid conditions that stress human heat-regulatory processes until they interfere with normal body functions or health. A permanent state of human comfort need not be guaranteed, but there is a range of microclimatic conditions that can be maintained with an effort that is more than recovered by the saving in human efficiency. Securing this degree of climatic improvement should be the aim of tropical architecture.