Core body temperature is the physical state at which the internal organs and bodily systems function at an optimal level. Core body temperature is an aspect of thermoregulation, the body's ability to control its operating temperature within a constant range.
The ideal core body temperature has traditionally been stated as 98.6°F (37.7°C). This stated ideal temperature is in fact the approximate midpoint of the range of optimal temperatures that are sought to be maintained by the body, from a low of 96°F to a maximum of 100°F (36°C to 39°C). Every human possesses individual physiological factors that contribute to the variability of a healthy core body temperature, including the base metabolic rate (BMR, the rate at which the body consumes energy while at rest), physical conditions such as pregnancy, and the ingestion of various medications.
When the core body temperature approaches the lower part of the desired range, the body will take corrective measures through its temperature control regulation system centered in the hypothalamus region of the brain. The body approaches a hypothermic state at these lowered temperatures. To preserve the ability of the internal organs to function, the hypothalamus initiates a reduction in the volume of blood circulating near the surface of the body in order to retain a greater volume of warm blood near the internal organs. These conditions often occur during participation in cold weather sports, such as cross-country skiing, or other circumstances in which athletes have significant exposure to extreme weather.
If the core body temperature increases beyond the upper safe limit of approximately 100°F (39° C), the hypothalamus takes an opposite action to that employed during hypothermic conditions. To counter a hyperthermic state, the hypothalamus initiates an increase in blood volume, directing the warmed blood toward the surface of the skin to promote its cooling. The body also seeks to dissipate the increased internal heat by promoting the production of greater amounts of perspiration, which is released from the eccrine sweat glands for eventual evaporation at the skin surface. When the body has sustained an illness such as a fever or an infection, or when the internal temperature has increased due to warm weather exercise, the release of perspiration will also occur.
A number of instruments and techniques exist to obtain a core temperature reading. The traditional method involves the insertion of a thermometer under the tongue of the individual; this technique is subject to variables, such as the manner in which the tongue is actually positioned in the mouth or the presence of fluids in the mouth that may affect the temperature reading. An accepted alternative means for determining body temperature is the positioning of the thermometer in the armpit of the person. Again, an incorrect position that created exposure of the thermometer to either cool skin or the air itself would also produce an imprecise reading.
The two most accurate methods of taking core temperature are through the use of a rectal thermometer or a tympanic thermometer. Rectal thermometers are designed to be inserted a sufficient distance into the rectum of the individual to obtain a measure at a place close to the internal organs that are maintained at the optimum body temperature. The tympanic thermometer is a modern device that is attached to a handheld instrument and inserted into the inner ear; the blood flow in the vicinity of the tympanum, the middle ear, provides an accurate indication of the temperature at the body core.