Four trained men worked 4 h/day at 40-50% of their maximum aerobic capacity first for 3 days at 25 degrees C db, 18 degrees C wb and then for 10 consecutive days at 45 degrees C db, 32 degrees C wb. Between days 1 and 2 of heat exposure mean total circulating protein (TCP) and plasma volume (PV) increased 11.6% and 9%, respectively. Preexposure TCP and PV increased until day 6 of heat exposure. Of the protein fractions beta-globulins underwent the largest relative increase. During work movement of protein into and out of the vascular compartment was similar in control and acclimatizing subjects but the latter generally maintained a greater amount of protein and fluid within the vascular volume. There was no evidence of salt and water retention. The increase in vascualr volume was ascribed to transfer of interstitial protein and water to the vascular volume. Regression coefficients indicated significant correlations for changes in plasma volume versus heart rate, stroke volume, and cardiac output during acclimatization. It was concluded that the most critical event in heat acclimatization is the expansion of the plasma volume.

Four trained young men worked for 4 h/day at 40-50% of their maximum aerobic capacity first for 3 days at 25 degrees C db, 18 degrees C wb, and then for 10 consecutive days at 45 degrees C db, 32 degrees C wb. This portion of the study was mainly concerned with central circulatory changes during acclimatization. The central circulatory adaptation to work in heat could be divided into four distinct phases: phase I (day 1) was characterized by a progressive fall in stroke volume (SV) during heat exposure but cardiac output (CO) was maintained above control values by high heart rates. Phase II (days 2 and 3) was marked by increases in SV ande decreases in heart rate but with little change in CO from phase I. During phase III (days 4-8 of acclimatization), CO increased due to increases in SV. Phase IV (days 6-8) was associated with decreases in rectal and skin temperature towards control levels. SV and HR both decline in this phase so that CO was not elevated greatly above control levels. The results indicated that central circulatory and temperature regulating events are not casually associated in acclimatization.

Four trained young men, worked for 4 h/day at 43-50% of their maximum aerobic capacity for 3 days at 25 degrees C db, 18 degrees C wb and then for 10 consecutive days at 45 degrees C db, 32 degrees C wb. Their thermal status was assessed using direct calorimetry. As a group, the men showed classical acclimization responses, but there were marked individual differences. The calorimetric analysis revealed that reductions in strain were associated with minor changes in heat balance confined to the first and last hours of exposure. Events occurring within the first 4 days appeared to have little effect on body temperatures. Significant decreases in body temperature took place only when sweat and evaporation rate increased. A 10% increase in evaporation rate was accompanied by a 30% increase in sweat rate and a 200% increase in unevaporated sweat; thus, there is a wasteful overproduction of sweat. By the 10th day skin temperature was confined to the level necessary to evaporate sufficient sweat to achieve thermal balance with a fully wet body surface. The efficiency of heat transport within the body did not change with acclimatization.

SUMMARY1. Ten male subjects were trained in stair stepping for 2 weeks. Group A (six subjects) was thereupon sequentially exposed for 45 min to dry bulb temperatures of 20, 40 and 300 C, a vapour pressure of 10-11 mm Hg and a wind speed of 1 m/sec in a climate tunnel. While temperature changes were being effected the subjects rested in an antechamber. Group B (four subjects) was exposed to a sequence of 40, 20 and 300 C. Work rate was the same for all subjects, i.e. 216 kg m/min (-to an oxygen consumption of 0 9 1./min). Duplicate experiments were run on both groups of subjects before and after acclimatization to heat.2. Throughout, periodic samples of venous blood, water and protein movement into or out of the extravascular compartment was assessed during exercise periods wherein blood flow was increased to exercising muscles (Group A, 20°C) or to both exercising muscles and skin (Group B, 400 C; Groups A and B, 300 C.) 3. Mild exercise in a cool environment before and after acclimatization to heat was accompanied by expansion of the vascular volume and an increase in the amount of circulating protein.4. Mild exercise in a warm environment for 45 min was accompanied by haemoconcentration and loss of protein from the vascular volume before subjects were heat acclimatized. The results were reversed following heat acclimatization; i.e. exposure of Group B to 400 C and of Groups A and B to 300 C was accompanied by haemodilution and addition to (or maintenance of) plasma protein concentration.5. Effects of heat acclimatization on exposure of Group A to 400 C were also noted.6. The effects of heat acclimatization were ascribed to: (a) a change in permeability of cutaneous capillaries to large molecules, (b) an increased availability of translocatable protein within cutaneous interstitial spaces, and (c) a combination of both a and b. 7. Further, the results supported a previous suggestion that addition or loss of water and protein from the vascular volume is dependent on the ratio of cutaneous to muscle blood flow.

Progressive dehydration of resting male subjects was accomplished by exposure to 43.3 C dry bulb, 29 C wet bulb for 12 hr. For control experiments, evaporative weight loss was replaced with 0.1% saline. For dehydrating subjects, the following relationships with evaporative weight losses were obtained: %Delta osmotic pressure = 1.14 (% evap wt loss) — 0.43; %Delta [Na+] = 1.37 (% evap wt loss) — 0.45; %Delta [K+] — 2.19 (% evap wt loss) — 1.29; %Delta hematocrit = 1.4 (%Delta evap wt loss) — 2.65. Comparison of dehydration with rehydration results indicated augmentation of plasma volume with fluids containing high concentrations of potassium. An average decrease in plasma volume of 13.6% (T-1824) from 2.5 to 11 hr after initial heat exposure in dehydrating subjects was accompanied by a 15.7% increase in total protein. Albumin increased 11.6% while globulin increase averaged 22.5%. Plasma protein fractions are not static during dehydration, probably due to augmentation of each protein fraction, particularly globulin. Measurement of plasma volume by T-1824 more likely indicates changes in protein concentration rather than plasma volume changes. A relationship between hematocrit changes and plasma albumin concentrations is suggested. plasma sodium; plasma potassium; plasma osmolarity; hematocrits; plasma albumin; plasma globulins Submitted on February 23, 1965