THERMAL REGULATION DURING ACCLIMATIZATION IN A HOT, DRY (DESERT TYPE) ENVIRONMENT

Abstract

Thermal balance during acclimatization in a hot environment was studied to determine the adjustments by which the initially elevated body temp. is lowered. Three healthy young men performed 1 hr. of standard work on a treadmill daily; first for 9 days in a cool environment (D.B. 78[degree]F, W. B. 62[degree]F) then for 10 days in a hot, dry environment (D. B. 123[degree]F, W. B. 80[degree]F) and finally for 2 days in the initial cool environment. Observations were made of (a) heat content of the body and heat distribution in the deep and peripheral tissues, (b) heat interchanges between man and his environment, partitioned into metabolism, convection, radiation and evaporation and (c) flow of heat within the body as detd. by the "peripheral blood flow." The acclimatization process returned the initially (1st day) elevated deep tissue (rectal) temp. to the level originally shown in the cool environment, and lowered somewhat the initially high peripheral tissue (skin) temp. but did not return it nearly to the level in the cool. Skin temp. appeared to be adjusted to a new level, which permitted thermal equilibrium between the body and the environment on one hand, and on the other, permitted a more efficient transport of deep heat to the periphery, with the result that the circulation was not overtaxed as it had been on day 1 in the heat. The principal adaptive mechanism was an increase of 10% in sweat secretion above that on the first day in the heat. The resultant increase in evaporative cooling accounted for 75-90% of the reduction in body temp. and heat content. Metabolic heat production decreased by a mere 4% and accounted for only 5-20% of the reduction in body heat content. Due to the lowering of skin temp. by the increased evaporative cooling, heat gains by convection and radiation actually increased (+8%) as acclimatization developed and this necessitated twice as large an increase in sweat secretion as would have been required had the skin temp. not fallen. However, the lower skin temp. widened the internal thermal gradient so that 70% of the initial "peripheral blood flow" sufficed to transfer the metabolic heat from the deep tissues to the periphery and to that extent spared the circulation.