Humans are known as homeotherms--animals that maintain constant body temperatures regardless of environmental temperatures.  The ability to maintain this constant temperature is critical to the functioning of the body's enzymatic systems regulating cellular functions.  Normal body temperature represents the optimal thermal condition required to support these internal functions.  To maintain this constant temperature, which is often referred to as "normal" temperature, the body must balance the amount of heat produced from within the body with the amount of heat lost from the body.  Body heat is produced through metabolism and metabolic activity and is balanced against heat lost through conduction, convection, evaporation, and radiation.  These mechanisms will be discussed later.

            Since the late 19th century, health professionals caring for infants have known the importance of adequate environmental temperature and its relationship to the survival of infants.  A French obstetrician and his student, Pierre Budin, reported improving the survival of premature infants by placing them in crude incubators and manipulating the incubator's inside environmental temperature through manual adjustments of gas flames.  While crude in its development, these incubators did dramatically improve the survival rate of these infants and prove that maintenance of normal body temperature in response to changing environmental temperatures is essential for survival.  Our ability to maintain a normal body temperature in response to changing thermal environment improves physically and physiologically with age.  Infants, especially the premature, the very-low-birth-weight infant, and/or the compromised neonate, are not capable of responding physically nor physiologically in the same manner as the older child or the adult; when they do respond, their responses are different and limited in nature.  Responses to changing environmental temperatures, in particular cold stress, to maintain normal body temperature involve activities that prove costly in both oxygen and substrate consumption.  Two items at a premium in this population. 

            Thermoregulation is controlled by the hypothalamus.  The hypothalamus receives thermal stimuli from the body's skin and deep thermal receptors as well as thermal receptors within the hypothalamus itself.  The hypothalamus' job, is to process the information received, compare it to the body's heat (body's temperature), and modify the temperature by altering body functions to either increase heat production or heat loss.  The body accomplishes this by altering metabolism, motor tone and activity, vasomotor activity, or sweating.  Metabolism reflects the body's overall energy needed to maintain and repair itself and to grow.  Infants, in addition to physiologic differences, have a high metabolic rate.  This higher metabolic rate is due to energy demands needed for growth and to the increased maintenance requirements related to the infants large body surface and large surface-to-mass ratio making it more difficult for them to maintain normal temperature. 

            A neutral thermal environment (NTE) is the range of environmental temperature at which calorie and oxygen consumption is minimal, and the infant's temperature remains within normal range.  The regulation of a normal temperature is a balance between heat losses and heat production.  Remember the transfer of heat is always down a heat gradient (from hot to cold temperatures).