ELEVATION OF BODY TEMPERATURE IN HEALTH

Under steady state conditions, heat fows at uniform rates from the sites of its production in the body to cooler tissues or blood which carries the heat to the body surface from which it is dissipated to the external environment by radiation, convection, and evaporation. Temperature gradients between regions of the body core and skin surface thus remain fixed. Increased heat production, for example, during exercise, or decreased heat loss in hot environments results in transient changes in these gradients, thus leading to thermoregulatory responses which establish a new body temperature equilibrium at a higher level if heat loads are within tolerable limits. Internal temperature gradients at rest. In a recent review by the two senior authors o n the subject of internal temperature gradients in man at thermal equilibrium and during transient heat storage,' we noted that the existence of temperature gradients between internal regions of the homeotherm a t rest has long been known. Claude Bernard' in presenting his own findings cites numerous references to the literature on this subject dating from the work of Haller in 1760. The results of these earlier workers have recently been confirmed and extended in the dog by Horvath et ~ 1 . ~ and in the human by Eichna e f aL4 Internal temperatures in resting man range from rectal temperature near the upper limit with a mean value of 37.1"C. (98°F.) and a standard deviation of 0.23"C. (0.42°F.)s to mouth temperature a t the lower limit with mean values of 36.7% (98.1 OF.) and a standard deviation of 0.22"C. (0.40°F.)5.6 Temperatures measured in the esophagus, at the tympanic membrane, in the stomach, in the liver, in the chambers of the heart, in central vessels, and in the jugular vein are intermediate.',4 In the range from 22OC. (71.6'F.) to 35°C. (95'F.) in ambient temperature, mean skin temperature in the resting nude man derived from weighted readings obtained with the Hardy radiometer varies from 3 0 5 ° C . (869°F . ) under the coolest condition to 35.3"C. (95.5OF.) under the warmest condition. Differences between rectal and skin temperatures are greatest for the extremities, less for the trunk, and least for the head.' Temperature gradients during transient heot storage. In nonsteady states many investigators have observed that. rectal temperature lags behind temperatures measured in the mouth,& lower esophagus,9 liver,'" stomach,ll central ve~se ls ,~J2 paranasal sinuses and tympanic Thus during transient heat storage, in contrast to equilibrium conditions at rest, internal temperatures measured in these regions may temporarily exceed rectal temperature. Equilibrium temperatures during work. According to the classical studies of Nielsen15 and his followers cited in our review,' the elevation of rectal temperature to a new plateau during work at a constant rate is a regulated rise in which peripheral as well as central thermoreceptors play a role and thus is not to be regarded as merely a manifestation of strain on, or insufficiency of, thermo-