Living organisms require energy to sustain life and develop. Energy is derived from food which is metabolised, mostly into adenosine triphosphate (ATP) which can be readily extracted (anabolism). In these processes much of the available energy is released as heat, e.g., 60% during synthesis of ATP. Additional heat is produced when ATP-stored energy is transferred to the functional systems in the body. These energy losses leave, on the average, a mere 25% available for use by the functional systems (typical conversion efficiencies are: 65% for heart muscles, 30% for bicycling and 5% for respiration).
In all warm-blooded animals, life depends on homeostasis —the maintenance of fairly constant internal conditions. One of the chief manifestations of homeostasis is the maintenance of a constant body temperature. At rest these temperatures for mammals are within the narrow range of 35-39.5°C with species-specific levels. Most of the generated heat must be dissipated to the environment to maintain constant body temperature. Temperature control is achieved by the thermoregulatory system composed of 3 interconnected subsystems: sensory, control and regulatory. Thermal receptors, located throughout the body, form the major components of the sensory system. The control center is located in the hypothalamus, which lies at the base of the brain. Regulatory mechanisms mediate heat production and, for the most part, heat loss. Long-term adaptations to environmental changes are effected through the endocrine glands. Short-term responses of the thermoregulatory system to varying internal or external conditions, are produced by three basic mechanisms: metabolic, vasomotor and sudomotor which are depicted in Figure 1 . Other thermoregulatory activities are behavioral rather than physiological. These include seasonal migration to milder climates, seeking shelter, periodic basking in the sun, typical of poikilotherms (i.e., animals having a variable body temperature, "cold-blooded"), and, in the case of humans, design and utilization of garments, etc. A final reference is made to those animals, termed hibernators, who are usually homeotherms (i.e., animals having a relatively constant body temperature, "warmblooded"), e.g., ground squirrel. These animals possess the unique biological capability of lowering their body temperature to levels, which are lethal to nonhibernating species.
Shitzer, A. and Eberhart, R. C. Eds. (1985) Heat Transfer in Medicine and Biology—Analysis and Applications, Plenum Press, New York.
Whittow, G. C, Ed. (1971) Comparative Physiology of' Thermoregulation, Academic Press, New York.