The core temperature (the temperature of organs within the cranial, thoracic and abdominal cavities) remains relatively constant - within the range 36.1 - 37.8 ° C (Heath, 1995). The peripheral temperature fluctuates according to both metabolic activity and the surrounding environmental temperature. This fluctuation is essential in order to maintain core temperature within the normal range. It is important to appreciate that heat gains and heat losses must be kept in balance. If the heat gained exceeds the amount of heat lost, the body temperature will gradually rise, and if this is not corrected a fatal heat stoke will develop. Conversely, if heat loss exceeds the amount of heat gained then eventually hypothermia will develop.
The skin plays an important part in the regulation of body temperature. The sympathetic division of the autonomic nervous system regulates skin blood flow. When sympathetic stimulation increases, the smooth muscle fibres in the walls of the vessels contract and the vessels become narrowed, reducing the flow of blood close to the surface of the body. As a result, the amount of heat being lost from the skin surface falls and body heat is conserved. A decrease in sympathetic tone produces the opposite effect: the blood vessels dilate and skin blood flow increases, increasing heat dissipation. The degree of sympathetic tone that exists is governed by information sent from the hypothalamus to the vasomotor centre, a specialised area within the brainstem.
Fever (pyrexia) occurs when the body temperature is maintained at an above-normal level. There can be various causes: infection, inflammation, dehydration, and abnormalities in the brain.
Chemical signals that induce fever are called pyrogens. They can originate from infectious pathogens, in which case they are referred to as exogenous pyrogens, and from activated cells of the immune system - endogenous pyrogens. These signals change the hypothalamic set point for body temperature and raise it by several degrees, apparently by increasing the production of prostaglandins in the hypothalamus. With the set point raised, all the mechanisms for raising body temperature are mobilised, including heat conservation and increased heat production. Within a few hours the body temperature has been brought to the new level.
As the body temperature rises to fever level, the affected person usually feels very cold. There is vasoconstriction of the blood vessels in the skin to conserve heat, and the skeletal muscles contract spontaneously to produce shivering, which generates more metabolic heat. The person puts on more clothing or bed covers in an attempt to feel warm. The chills continue until the new body temperature is reached, after which the person feels neither hot nor cold, even though their temperature is raised.
The febrile (feverish) person experiences feelings of malaise and lethargy. The degree of fever and progress, both from the point of view of the disease and any treatment measures instigated, can be objectively assessed by regularly taking the person's peripheral temperature.
When the level of circulating pyrogens falls, either because the infection has been overcome or because the inflammatory process has been resolved, the hypothalamic set point returns to its normal level. The person feels hot, becomes flushed and warm to the touch, and sweats profusely. Excess body heat is dispersed until the body temperature returns to its normal level.
It is thought that fever helps the body’s defence mechanisms to deal with problems such as infection and tissue damage. However, if the body temperature rises above 41 - 42 ° C for any length of time, cells within the body are damaged and the functioning of vital organs such as the heart, brain, liver and kidneys can be affected. Several antipyretic drugs, for example aspirin, are available and can be given to control a potentially damaging fever. These drugs appear to act by reducing prostaglandin production and allowing the hypothalamic set point to return to normal.