The normal range of human body temperature is 96.5° to 99.5°F (36° to 38°C), with an average oral temperature of 98.6°F (37°C). (A 1992 study suggested a slightly lower average oral temperature: 98.2° or 36.8°. But everyone seems to prefer the “traditional” average temperature.) Within a 24-hour period, an individual’s temperature fluctuates 1° to 2°, with the lowest temperatures occurring during sleep.
At either end of the age spectrum, however, temperature regulation may not be as precise as it is in older children or younger adults. Infants have more surface area (skin) relative to volume and are likely to lose heat more rapidly. In the elderly, the mechanisms that maintain body temperature may not function as efficiently as they once did, and changes in environmental temperature may not be compensated for as quickly or effectively.
Cell respiration, the process that releases energy from food to produce ATP, also produces heat as one of its energy products. Although cell respiration takes place constantly, many factors influence the rate of this process.
Heat Loss through the Skin
Because the skin covers the body, most body heat is lost from the skin to the environment. When the environment is cooler than body temperature (as it usually is), heat loss is unavoidable. The amount of heat that is lost is determined by blood flow through the skin and by the activity of sweat glands.
Blood flow through the skin influences the amount of heat lost by the processes of radiation, conduction, and convection. Radiation means that heat from the body is transferred to cooler objects not touching the skin, much as a radiator warms the contents of a room (radiation starts to become less effective when the environmental temperature rises above 88°F).
Heat Loss through the Respiratory Tract
Heat is lost from the respiratory tract as the warmth of the respiratory mucosa evaporates some water from the living epithelial surface. The water vapor formed is exhaled, and a small amount of heat is lost. Animals such as dogs that do not have numerous sweat glands often pant in warm weather. Panting is the rapid movement of air into and out of the upper respiratory passages, where the warm surfaces evaporate large amounts of water. In this way the animal may lose large amounts of heat.
REGULATION OF BODY TEMPERATURE
The hypothalamus is responsible for the regulation of body temperature and is considered the “thermostat” of the body. As the thermostat, the hypothalamus maintains the “setting” of body temperature by balancing heat production and heat loss to keep the body at the set temperature.
To do this, the hypothalamus must receive information about the temperature within the body and about the environmental temperature. Specialized neurons of the hypothalamus detect changes in the temperature of the blood that flows through the brain. The temperature receptors in the skin provide information about the external temperature changes to which the body is exposed.
Mechanisms to Increase Heat Loss
This is accomplished by vasodilation in the dermis and an increase in sweating. Vasodilation brings more warm blood close to the body surface, and heat is lost to the environment. However, if the environmental temperature is close to or higher than body temperature, this mechanism becomes ineffective. The second mechanism is increased sweating, in which excess body heat evaporates the sweat on the skin surface. As mentioned previously, sweating becomes inefficient when the atmospheric humidity is high.
Mechanisms to Conserve Heat
In a cold environment, heat loss from the body is unavoidable but may be reduced to some extent. Vasoconstriction in the dermis shunts blood away from the body surface, so that more heat is kept in the core of the body. Sweating decreases, and will stop completely if the temperature of the hypothalamus falls below about 98.6°F. (Remember that the internal temperature of the brain is higher than an oral temperature, and is less subject to any changes in environmental temperature.)
Food is needed for the synthesis of new cells and tissues, or is utilized to produce the energy required for such synthesis reactions. As a consequence of metabolism, heat energy is released to provide a constant body temperature and permit the continuation of metabolic activity. The metabolic pathways described in this chapter are only a small portion of the body’s total metabolism. Even this simple presentation, however, suggests the great chemical complexity of the functioning human being.
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