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Warm-blooded Animals (Homeotherms)
In humans and other mammals, temperature regulation represents the balance between heat production from metabolic sources and heat loss from evaporation (perspiration) and the processes of radiation, convection, and conduction. In a cold environment, body heat is conserved first by constriction of blood vessels near the body surface and later by waves of muscle contractions, or shivering, which serve to increase metabolism. Shivering can result in a maximum fivefold increase in metabolism. Below about 40℉ (4℃) a naked person cannot sufficiently increase the metabolic rate to replace heat lost to the environment. Another heat-conserving mechanism, goose bumps, or piloerection, raises the body hairs; although not especially effective in humans, in animals it increases the thickness of the insulating fur or feather layer.
In a warm environment, heat must be dissipated to maintain body temperature. In humans, increased surface blood flow, especially to the limbs, acts to dissipate heat at the surface. At environmental temperatures above 93℉ (34℃), or at lower temperatures when metabolism has been increased by work, heat must be lost through the evaporation of the water in sweat. People in active work may lose as much as 4 quarts per hour for short periods. However, when the temperature and humidity are both high, evaporation is slowed, and sweating is not effective. Most mammals do not have sweat glands but keep cool by panting (evaporation through the respiratory tract) and by increased salivation and skin and fur licking.
Temperature regulatory mechanisms act through the autonomic nervous system and are largely controlled by the hypothalamus of the brain, which responds to stimuli from nerve receptors in the skin. Continued exposure to heat or cold results in some slow acclimatization, e.g., more active sweating in response to continued heat and an increase in subcutaneous fat deposits in response to continued cold.
Environmental extremes may result in failure to maintain normal body temperature. In both increased body temperature, or hyperthermia, and decreased body temperature, or hypothermia, death may result (see heat exhaustion). Controlled hypothermia is used in some types of surgery to temporarily decrease the metabolic rate. Fever, caused by a resetting of the temperature regulatory mechanism, is a response to fever-causing, or pyrogenic, substances, such as bacterial endotoxins or leucocyte extracts. The upper limit of body temperature compatible with survival is about 107℉ (42℃), while the lower limit varies.
In humans the inner body temperature alternates in daily activity cycles; it is usually lowest in early morning and is slightly higher at the late afternoon peak. In human females there is also a monthly temperature variation related to the ovulatory cycle. In many mammals and birds the body temperature shows more pronounced cyclic variations than in humans. For example, in hibernators the body temperature may lower to only a few degrees above the environmental temperature during the dormant periods; mammalian hibernators reawake spontaneously and in their active period are homeothermic.
Cold-blooded Animals (Poikilotherms)
a complex indicator of the thermal state of animals and man. It results from the complex relations between the heat produced by the organs and tissues and the transfer of heat between the organs and tissues and the environment. In man and homeothermic animals, the body temperature is maintained by specialized thermoregulatory mechanisms at temperatures ranging from 36° to 39°C; the body temperature in birds ranges from 40° to 42°C.
Physiological fluctuations in the body temperature occurring during a 24-hour period follow circadian rhythms; in man, the difference between the body temperature in the early morning and in the evening varies from 0.5°–1.0°C. The differences in temperature among the internal organs amount to several tenths of a degree. The difference between the temperatures of the internal organs, muscles, and skin may vary as much as 5°–10°C. This makes it difficult to determine the average body temperature, which must be known in order to ascertain the thermal state of the body as a whole.
Body temperature is measured with a thermometer, which is usually placed in the armpit, rectum, mouth, or ear. In poikilotherms (cold-blooded animals), the body temperature differs little from the ambient temperature, and in some species it exceeds the ambient temperature only during intensive muscular activity.
The lowering or raising of body temperature (hypothermia and hyperthermia) by several degrees disturbs the body’s vital processes and may result in cooling, hyperpyrexia, or even death. In many diseases, such as fever, the body temperature rises to a certain level and is regulated at the new level.
REFERENCESBurton, A., and O. Edholm. Chelovek v usloviiakh kholoda. Moscow, 1957. (Translated from English.)
Prosser, C. L., and F. Brown, Jr. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Hensel, H. “Neural Processes in Thermoregulation.” Physiological Reviews, 1973, vol. 53, no. 4.
K. P. IVANOV