Our body temperature is maintained within a narrow range despite extremes in environmental conditions and physical activity. In fact, fever is such a sensitive and reliable indicator of presence of disease that thermometry is probably the commonest clinical procedure in use. Heat is lost from the body in several modes. Some heat is lost during the intake of food and fluids, and during evaporation of moisture through respiration. Most heat is lost from the surface of the body, by convection that is transfer of heat. Heat loss by convection depends on the existence of a temperature gradient between the body surface and the surrounding air. Evaporation is the major mechanism for dissipating heat from the body and is particularly important when outside or environmental temperature exceeds body temperature.
The principal method of regulating heat loss is by varying the volume of blood flowing to the surface of the body. A rich circulation in the skin and the subcutaneous tissues carries heat to the surface where it can escape. In addition, sweating increases the heat loss by providing water to be vaporized. The sweat glands may produce as much as one liter per hour of sweat to loose the body heat. The amount of heat loss through sweating is also dependent upon the humidity in the air. The greater the humidity, the less the ability to lose heat through sweat. The transfer of body heat occurs between the adjacent organs by conduction and by circulation, which is governed by the bulk movement of body fluids which brings about transfer of heat between the cells and blood. Insulation of the body is relatively uniform throughout the body but some parts like digits are particularly susceptible to cold. The blood reaching the digits has been cooled on the way. Insulation can be increased by good clothing.
The high environmental temperature may give rise to ‘heat cramps’, ‘heat exhaustion’ and ‘heat pyrexia’. These complications occur during the first few days of heat wave before the effective acclimatization can occur. The basic mechanism by which man accommodates the excessive temperatures is unknown. Acclimatization does not increase the threshold for sweating. Sweating is the most effective natural means of combating heat stress and can occur with little or no change in the core temperature of the body. As long as sweating continues, man can withstand remarkably high temperatures, provided water and sodium chloride, the most important physiologic content of sweat are replaced.
If environmental temperatures remain persistently high than the body may retain heat giving rise to hyperpyrexia (high fever). Heat exhaustion or heat collapse is probably the most common. Weakness, vertigo, headache, nausea, fainting and collapse is common in such cases. Heat collapse occurs in both physically active and sedentary people. The onset is usually sudden and the duration of collapse is brief. The skin is cold and clammy, pupils are dilated and the pressure may be low. The body temperature may he subnormal or normal. The person should be removed to a cool place and replacement of fluids must be done after hospitalization.
Heat stroke or sun stroke is most common in people with pre-existing chronic diseases. Heat stroke can occur during any period of hot weather. High humidity is a prerequisite for heat stroke, and persons usually stop sweating before the onset of acute symptoms. The cessation of sweating is due to an intrinsic breakdown of the heat regulation mechanism for reasons not known. Some may complain of headache, vertigo, fainting, abdominal distress and confusion before getting a heat stroke.
Heat stroke requires heroic emergency measures. Time is most important. The person should be placed in a cool place with adequate circulation of fresh air, and most clothing removed, followed by early hospitalization and further treatment.