Physiological Reactions to Cold and Their Effects on the Retention of Acclimatization to Heat

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Abstract
Three healthy, unacclimatized men were intermittently exposed successively to a 2-week period of physical conditioning, to 19 5.25-hr. periods of heat (107[degree]F, D.B.; 89[degree]F, W.B.; wind 3 m.p.h.); to 14 5-hr. periods of cold (-20[degree]F; wind 3-4 m.p.h.); to 5 re-exposures to heat; to a 5-week interval of no exposures to environmental stress; and finally to 3 re-exposures to heat. Through the exptl. periods, measurements were made of cardiovascular and metabolic functions, water and chloride balance and body water partition. The results reveal that: (a) in heat-acclimatized men, no acceleration of de-acclimatization is caused by repeated intermittent exposures to cold; (b) acclimatization to heat may be maintained for periods of several months by occasional re-exposure to the original environmental stress; (c) toe temps. during repeated cold exposures decreased more rapidly with successive exposures, suggesting more rapid and complete vasoconstriction, which does not necessarily represent acclimatization to cold since no increased tolerance to cold was demonstrated; (d) marked diuresis and negative Cl balance were observed in the period of cold exposures, and these phenomena tended to persist during subsequent re-exposures to heat; and (e) no significant differences in total blood, plasma or "available fluid" (thiocyanate space) volumes were found in any of the exptl. periods, although the plasma proteins and hematocrit values suggested hemodilution in heat and hemoconcentration in cold.
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