Fluid Replacement and Heat Stress, 1993

Pp. 127-142. Washington, D.C.

National Academy Press

11

Shift in Body Fluid Compartments After Dehydration in Humans

Hiroshi Nose1, Gary W. Mack, Xiangrong Shi, and Ethan R. Nadel

INTRODUCTION

Maintenance of blood volume is important for optimal regulation of both arterial blood pressure and body temperature during exercise and thermal stress (Fortney et al., 1981a,b; Fortney et al., 1983; Nadel, 1984). A reduction of the central circulating blood volume, due either to hypovolemia accompanying dehydration or dilation of the peripheral vasculature, results in a fall in cardiac filling pressure and stroke volume and, if uncompensated, also in cardiac output (Fortney et al., 1983; Miki et al., 1983a). Among the possible compensations is the body's ability to mobilize water from the extravascular to the intravascular space (Miki et al., 1983b; Mohsenin and Gonzalez, 1984; Morimoto et al., 1981; Nose et al., 1983).

Senay (1979) recently reviewed the dehydration literature and reported that water appeared to be lost from the plasma at a rate one to five times that of other fluid compartments during dehydration. Costill (1977) ascribed the relatively greater plasma water loss to movement accompanying the major ions lost in sweat and urine, which are those of the extracellular

1  

Hiroshi Nose, Foundation Laboratory and Departments of Epidemiology and Public Health and Physiology, Yale University School of Medicine, New Haven, CT 06519



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FLUID REPLACEMENT AND HEAT STRESS Fluid Replacement and Heat Stress, 1993 Pp. 127-142. Washington, D.C. National Academy Press 11 Shift in Body Fluid Compartments After Dehydration in Humans Hiroshi Nose1 , Gary W. Mack, Xiangrong Shi, and Ethan R. Nadel INTRODUCTION Maintenance of blood volume is important for optimal regulation of both arterial blood pressure and body temperature during exercise and thermal stress (Fortney et al., 1981a,b; Fortney et al., 1983; Nadel, 1984). A reduction of the central circulating blood volume, due either to hypovolemia accompanying dehydration or dilation of the peripheral vasculature, results in a fall in cardiac filling pressure and stroke volume and, if uncompensated, also in cardiac output (Fortney et al., 1983; Miki et al., 1983a). Among the possible compensations is the body's ability to mobilize water from the extravascular to the intravascular space (Miki et al., 1983b; Mohsenin and Gonzalez, 1984; Morimoto et al., 1981; Nose et al., 1983). Senay (1979) recently reviewed the dehydration literature and reported that water appeared to be lost from the plasma at a rate one to five times that of other fluid compartments during dehydration. Costill (1977) ascribed the relatively greater plasma water loss to movement accompanying the major ions lost in sweat and urine, which are those of the extracellular 1   Hiroshi Nose, Foundation Laboratory and Departments of Epidemiology and Public Health and Physiology, Yale University School of Medicine, New Haven, CT 06519

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FLUID REPLACEMENT AND HEAT STRESS compartment. There would then be less mobilization of water from the intracellular fluid (ICF) space due to the smaller increase of extracellular osmolality. Durkot et al. (1986) reported that in rats a higher water loss occurred from the extracellular fluid (ECF) space than from the ICF space during dehydration of 10% body wt. Water movement might be linked to the electrolyte losses from each compartment, as suggested by Nose et al. (1985). These results suggest the necessity of measuring electrolyte balance to analyze water balance between fluid compartments. It is well known that sweat [Na+] decreases during the process of heat acclimation (Kirby and Convertino, 1986; Locke et al., 1951). However, there is no experimental evidence that shows a relation between [Na+] and water mobilization from the ICF space. The purpose of this study was to clarify the effect of sweat [Na+] on water mobilization from the ICF compartment in conditions of thermal stress. We hypothesized that a lower sweat [Na+] would be accompanied by a smaller reduction in plasma water loss during dehydration. METHODS Ten volunteers (nine male and one female) participated in this study. Certain of their physical characteristics are shown in Table 11-1. Each subject gave informed consent and passed a physical examination to screen for medical reasons that would prevent participation. The experimental protocol was approved by the Human Investigation Committee of the Yale University School of Medicine. Experiments were performed in the spring months. Table 11-1 Characteristics of subjects (n = 10)   Age (years) Wt (kg) a (ml.kg−1.min−1) Blood Volume (ml/kg) Plasma Volume (ml/kg) Mean 27.8 68.9 50.2 81.2 46.5 Range 23-33 48.6-83.7 36.4-62.9 61.0-102.3 33.5-61.4 a Maximal aerobic power.

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FLUID REPLACEMENT AND HEAT STRESS Design About 1-2 weeks before the experiment, maximal aerobic power () (cycle ergometer) and plasma volume [Evans blue dye dilution (Greenleaf et al., 1979)] were measured on each subject. On the day of the experiment, subjects reported to the laboratory normally hydrated but without breakfast, then entered an environmental chamber and rested in the sitting position for 30 min at a thermoneutral temperature [28°C, <30% relative humidity (rh)]. After a control blood sample (5 ml) was taken, subjects emptied their bladders and entered another environmental chamber (36°C, <30% rh). Body weight was measured to the nearest 1 g. Subjects then sat in the contour chair of the cycle ergometer in a semirecumbent position and had electrocardiogram electrodes put in place and a vinyl bag for collecting sweat placed on one forearm. Exercise (40% )began for an initial 30-min period, followed by alternating 5-min rest and 15 min exercise periods. Exercise continued until body weight decreased between 2 and 3%. Total exercise time was 90-110 min. Sweat from the forearm bag was collected every 30 min of exercise; the bag for collecting sweat was changed from one forearm to the other in consecutive collection periods to minimize the pore occlusion, which influences excretion rate and electrolyte concentrations in sweat (Sohar et al., 1965). Heart rate was monitored each 5 min and oral temperature was measured during each of the nonexercise intervals to ensure that subjects were not overly strained. After the cessation of exercise, subjects voided for collection of urine samples, were weighed, and then entered an adjacent chamber (28°C, <30% rh) for a seated recovery period without any fluid supply. A butterfly catheter was inserted into a superficial forearm vein and a 5-ml blood sample was taken within 10 min of the termination of exercise. Blood samples were also taken at the 30th and 60th min thereafter. Body weight was measured at 30 min. Blood samples were separated into a 4-ml aliquot, which was transferred to a heparinized tube and placed on ice to be centrifuged later, and a 1-ml aliquot, which was immediately treated for hematocrit (Hct) and hemoglobin (Hb) analysis. Measurements From each blood sample we determined plasma osmolality by freezing point depression (model 3DII, Advanced Instruments) and plasma electrolytes ([Na+] and [K+], flame photometry, Instrumentation Laboratory model 443; [Cl−], Cotlove chloride titrator). These were expressed in meq/kg plasma

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FLUID REPLACEMENT AND HEAT STRESS H2O by correcting for plasma solids. To calculate interstitial [Cl−], we multiplied plasma [Cl−] by the Donnan factor (1.05). We also measured microhematocrit, hemoglobin concentration (refractometry). Plasma solid concentration was determined by drying plasma in a heating chamber at 90°C for 24 h. We determined volume and electrolyte concentrations from each sweat and urine sample. Average electrolyte concentration in sweat was also determined from all samples. Calculations Total body water loss (ΔTW) was estimated from body weight loss. Total sweat loss was calculated by subtracting total urine loss from total body water loss, assuming that water loss due to respiration was negligible (Morimoto et al., 1981). Electrolyte losses in sweat and urine were calculated by multiplying the volume of water loss by the electrolyte concentration of each fluid, respectively. The change in plasma volume (ΔPV) during an experiment was calculated from changes in the Hct and Hb concentrations (Elkinton et al., 1946). We made determinations of ECF and ICF water by the Cl− method (Costill et al., 1976). This method requires the following assumptions: (1) Cl− loss in sweat and urine comes only from the ECF space; (2) the Donnan factor for Cl− between plasma and the interstitial fluid (ISF) space is 1.05; and (3) Cl− loss from the plasma and ISF spaces is proportional to the water loss from each space. The calculation was as follows: ΔCl−ECF = Cl−U + Cl−S ΔCl−ISF = ΔCl−ECF − ΔCl−Pl ΔISF = 1/1.05 × ΔCl−ISF/ΔCl−pl × ΔPV ΔECF = ΔPV + ΔISF ΔICF = ΔTW − ΔECF where subscripts Pl, ISF, ECF, U, and S indicate plasma, interstitial and extracelluar fluid spaces, urine, and sweat, respectively. The use of the Cl− method in this experiment was based on the results of a previous experiment on rats (Nose et al., 1985): the decreases in distribution of 51Cr-EDTA in various tissues after thermal dehydration were strongly correlated with their losses of sodium (r = 0.97, P < 0.0001), which itself is highly correlated with

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FLUID REPLACEMENT AND HEAT STRESS Cl− losses. Changes in the extracellular fluid space determined by the Cl− method were almost identical to those determined by the 51Cr-EDTA dilution method. Free water loss (ΔFW) during dehydration was calculated as follows: ΔFW = ΔTW − {([mosmol]s × vols) + ([moamol]U × volU)}/P0osmol = ΔTW − (cation loss × 2)/P0osmol cation loss = [cation]S + volS + [cation]U × volU ΔTW = volS + volU where P0osmol is the control plasma osmolality, subscripts s and u indicate sweat and urine, respectively, and [cation] denotes the sum of [NA+] and [K+]. The amount of osmotically active substances lost from the body was estimated by doubling the cation loss because the main osmotically active substances in body fluids are Na+ and K+ and their combined anions. The concept of free water loss is analogous to “free water clearance” in renal function. Statistics One-way analysis of variance for repeated measures was used to determine differences between predehydration and postdehydration conditions, with significant differences between pre- and postdehydration at various times determined with Tukey's minimum significant difference (MSD) test (Sokal and Rohlf, 1981). The null hypothesis was rejected when P was <0.05. Regression formulas were calculated using Brace's method (Brace 1977). Values are represented as means ± SE of 10 subjects. RESULTS The amounts of water and electrolytes excreted in sweat and urine during the dehydration period are shown in Table 11-2. Eighty-seven percent of the total Na+ loss was excreted in sweat and only 13% in urine. Only 21% of the total cation loss was K+. [Na+] and [K+] in sweat averaged 56.4 ± 7.3 meq/liter (ranging from 6.9 to 11.5 meq/liter), respectively.

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FLUID REPLACEMENT AND HEAT STRESS Table 11-2 Water and electrolyte losses 60 min after dehydration (means ± standard errors)     Electrolyte loss (meq/kg)   Volume (ml/kg) Na+ K+ Cl- Sweat 21.6±1.5 1.04±0.12 0.20±0.04 0.97±0.12 Urine 0.9±0.9 0.15±0.18 0.11±0.03 0.17±0.03 Total 22.5±1.5 1.19±0.14 0.31±0.06 1.14±0.13 Figure 11-1 shows the Hct, Hb, and plasma solids before and after dehydration. Immediately after exercise, these increased from 42.7 ± 0.5% to 44.7 ± 0.5%, 14.8 ± 0.2 g/dl to 15.8 ± 0.2 g/dl, and 8.4 ± 0.1 g/dl to 9.1 ± 0.1 g/dl, respectively. These levels were maintained for the next 30 min (43.6 ± 0.5%, 15.5 ± 0.1 g/dl, and 8.8 ± 0.1 g/dl at 60 min). In all variables, FIGURE 11-1 Hematocrit (Hct), hemoglobin (Hb) concentration, and plasma (Pl) solids are shown as means ± SE of 10 subjects before (C) and 0, 30, and 60 min after dehydration. Significant differences were observed for all variables between control and dehydrated conditions (0, 30, and 60 rain). Significant differences between 0 min and the other 2 dehydrated conditions (30 and 60 min).

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FLUID REPLACEMENT AND HEAT STRESS significant differences occurred between control and dehydrated conditions (0, 30, and 60 min) and between 0 min and the other two dehydrated conditions (30 and 60 min), but no significant differences occurred between 30 and 60 min. Plasma protein concentration paralleled changes in plasma solids. Figure 11-2 shows the plasma electrolyte concentration before and after dehydration. Plasma osmolality increased significantly from 284 ± 1 to 290 ± 1 mosmol/kg H2O. Changes in [Na+], [Ka+], and [Cl-] from 155 ± 1 to 160 ± 1, from 4.28 ± 0.08 to 4.47 ± 0.07, and from 116 ± 1 to 119 ± 1 meq/kg H20 just after the dehydration (0 min) were significant. These all showed a tendency to return toward control levels between 0 and 60 min, but no significant differences occurred. FIGURE 11-2 Changes in electrolyte concentrations and osmolality (Posmol) in plasma after dehydration. Significant differences were observed for all variables between control and dehydrated conditions (0, 30, and 60 min).

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FLUID REPLACEMENT AND HEAT STRESS Figure 11-3 shows the changes in the body fluid compartments after dehydration. The ΔTW was 20.3 ± 1.3 ml/kg body wt at time 0 and 22.5 ± 1.5 ml/kg body wt at 30 min, because the subjects continued to sweat after the dehydration until their body temperatures returned to the control level. We did not measure body weight at 60 min, because sweating was negligible after 30 min. Changes in the ICF space after dehydration were −8.7 ± 1.3 (0 min), −10.3 ± 1.2 (30 min), and −10.0 ± 1.2 (60 rain) ml/kg body wt. There were no significant differences between these values. Changes in ECF space were −11.6 ± 1.2 (0 min), −12.3 ± 0.7 (30 min), and −12.5 ± 0.9 (60 min) ml/kg body wt. There were no significant differences between these values. The ΔPV was −4.3 ± 0.6 ml/kg body wt (−9.4%) at time 0. This loss was partially recovered at 30 and 60 min, with ΔPV of −2.3 ± 0.4 (30 min) and −2.6 ± 0.5 ml/kg body wt (60 min). The differences were significant between the control and dehydrated conditions (0, 30, and 60 min) and between 0 min and the other dehydrated conditions (30 and 60 min). The change in the ISF volume, determined by subtracting the ΔPV from ΔECF, was −7.3 ± 1.0 ml/kg body wt at time 0 and decreased further to −10.0 ± 0.5 ml/kg body wt at 30 min and −9.9 ± 0.8 ml/kg body wt at 60 min. The difference was significant between 0 and 30 min but not between 30 and 60 min. FIGURE 11-3 Changes in body fluid compartments after dehydration. Values are changes with respect to control values. ΔPV, ΔISF, ΔECF, and ΔICF denote changes in plasma, interstitial, extracellular, and intracellular fluid volumes. Significant differences were observed for all variables between control and dehydrated conditions (0, 30, and 60 rain).Significant differences between 0 min and the other 2 conditions (30 and 60 min).

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FLUID REPLACEMENT AND HEAT STRESS Figure 11-4 shows the linear relationship between the change in Posmol and in ΔFW during the steady state after dehydration (r = −0.79, P < 0.01). Individual points are the mean steady-state values between 30 and 60 min. Figure 11-5 shows the change in ICF volume as a function of the change in Posmol (r = −0.74, P< 0.02). Figure 11-6 depicts a linear relationship between [Na+] in sweat and free water loss, normalized for total body water loss in each subject (r = −0.97, P < 0.001). It also shows that the loss in ECF was correlated with the [Na+] in sweat (r = −0.80, P<0.01). Changes in ICF would then be inversely correlated with the changes in [Na+] in sweat (r = −0.80, P < 0.01). FIGURE 11-4 Relationship between loss of free water (ΔFW) and change in plasma osmolality (ΔPosm). Solid line, regression line; dashed line, theoretical line. FIGURE 11-5 Relationship between changes in plasma osmolality (ΔPosm) and intracellular fluid volume (ΔICF). Solid line, regression line; dashed line, theoretical line.

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FLUID REPLACEMENT AND HEAT STRESS FIGURE 11-6 (Top): relationship between [Na+] in sweat and loss of free water normalized by total body water loss (ΔFW/ΔTW). (Bottom): Relationship between [Na+] in sweat and loss of extracellular or intracellular fluid normalized by total body water loss (ΔECF/ΔTW). FIGURE 11-7 Relationship between loss of plasma volume (ΔPV) and loss of extracellular fluid (ΔECF). Solid line, regression line; dashed line, theoretical line.

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FLUID REPLACEMENT AND HEAT STRESS Figure 11-7 shows that the decrease in PV was correlated with that in ECF volume (r = 0.77, P < 0.01). DISCUSSION The loss of plasma water during exercise in a hot environment is governed by several factors (Senay and Pivarnik, 1985). One of these is the shift of plasma water from the intra- to the extravascular fluid space caused by a change in the Starling forces accompanying increased perfusion pressure and capillary area. Another is plasma water movement accompanying whole-body dehydration due to the excretion of sweat. The decrease in plasma water must be compensated for, partly by shifting water from other fluid compartments, to maintain cardiac filling pressure and to provide for an adequate distribution of cardiac output (Nadel, 1984; Rowell, 1983). The purpose of this study was to investigate the water movement between fluid compartments during and after dehydration and to quantify the relationship between [Na +] in sweat and the maintenance of circulating blood volume. Immediately after the dehydration exposure, we found plasma volume to be decreased by 9.4% with respect to the preexposure value, and this deficit recovered to −5.0% at 30 min and −5.6% at 60 min of recovery when the subjects had no access to fluids. The water losses from the ICF and ECF spaces were −8.7 and −11.6 ml/kg body wt, respectively, immediately after the exposure. There was a small additional water loss between 0 and 30 min of recovery due to the continued secretion of sweat. However, electrolyte concentrations in plasma were unchanged during the hour after the dehydration exposure. These results suggest that fluid movement between the ICF and ECF spaces was at a steady state at the end of the dehydration exposure and the partial recovery of the lost plasma volume between 0 and 30 min of recovery was from the interstitial fluid space rather than from the intracellular space. Many investigators have documented the existence of fluid movement from the intra- to extravascular space during exercise; these data were recently summarized by Senay and Pivarnik (1985). Mohsenin and Gonzalez (1984) reported that interstitial fluid pressure in muscle immediately after maximal exercise was increased by 2.5 cm H2O above base line, and this increase was sustained up to 14 min after the termination of exercise. Convertino et al. (1981, 1983) demonstrated that there was a linear relationship between the amount of plasma water loss and the relative exercise intensity (% ). Sjogaard and Saltin (1982) showed, from estimates of ECF space in humans using 3H-inulin distribution, that this fluid movement was due primarily to increased filtration caused by the rise

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FLUID REPLACEMENT AND HEAT STRESS in precapillary hydrostatic pressure during exercise rather than from an induced hyperosmolality of the ICF space. The dashed line in Figure 11-4 is the theoretical line describing the relationship between free water loss and P0osmol, assuming that total body water is 65% of body weight (Wyndham et al., 1968) and plasma osmolality represents that of total body water in the equilibrium state. The equation is as follows: ΔPosmol = (P0osmol × TW0 − cation loss × 2)/ (TW0 − ΔTW) − P0osmol = P0osmol/TW0 × ΔFW where TW0 indicates control values of total water volume (see Appendix this chapter). The amount of osmotically active substances lost from the body was estimated by doubling the cation loss (Na+ and K+). The slope of this relation was 0.41 (mosmol/kg H2O)/(ml/kg body wt), which was almost identical to that of the best-fit regression line (solid line) from the experimental data. In other words, the increase of plasma osmolality during dehydration was a function of the loss of free water from the body. The relationship between the change in plasma osmolality and the change in ICF volume is shown in Figure 11-5. The dashed line, again, is the theoretical line determined by assuming that the initial volume of the ICF space is 38% of body weight (Bass et al., 1955), that the potassium in urine and sweat comes from only the ICF space (Nose et al., 1985; Wallace et al., 1970), and that plasma osmolality represents that of the ICF space in the steady state. The equation is as follows: ΔPosmol = (P0osmol × ICF0 − K0 loss × 2)/ (ICF0 − ΔICF) − P0osmol where ICF0 indicates the control values of ICF volume and K+ loss is the K+ loss in sweat and urine. The amount of osmotically active substance lost from the ICF space was estimated by doubling the K+ loss. The slope of the theoretical line is −1.67 (ml/kg body wt)/(mosmol/kg H2O). The slope of the regression line determined from the experimental data was −2.00 (ml/kg body wt)/(mosmol/kg H2O). The difference between the two may be partially explained by the fact that ions other than K+--i.e., Mg2+ and Ca2+--were lost from the ICF space into sweat and urine during dehydration. [Mg2+] and [Ca2+] in sweat have been reported to be 1.5-5.0 and 5.0-10 meq/liter, respectively (Durkot et al., 1986). On this basis, we estimated the losses in sweat to be 0.03 - 0.11 meq/kg for Mg2+ and 0.11 - 0.22 meq/kg body wt for

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FLUID REPLACEMENT AND HEAT STRESS Ca2+. Costill et al. (1976) reported that the Mg2+ loss during thermal dehydration of 2.2% of body weight was 0.07 meq/kg body wt. A second possibility for the difference between the theoretical and observed regression relations is that the K+ lost from the ICF space partially accumulated in the interstitial fluid space without being excreted into the sweat and urine, but this seems rather unlikely. The plasma [K+] increased by 9.8% above the control value, which was entirely within the range of the expected increase in [K+] by extrapolation from the decrease of the ECF space after dehydration. It is obvious that the increase in the ICF space was highly correlated with the change in plasma osmolality and that water movement from the ICF space followed the osmotic gradient. We also found a strong correlation between free water loss and the [Na+] in sweat over a wide range (Figure 11-6, top). The free water loss caused the increase in Posmol (Figure 11-4), resulting in fluid mobilization from the ICF space (Figure 11-5) to maintain ECF volume (Figure 11-6, bottom). The dashed line in Figure 11-7 is a theoretical line drawn with the assumption that water losses from plasma and the ECF space were proportional to the initial volume of each compartment (Spector, 1956). The regression line determined from the experimental data (Figure 11-7, solid line) was practically identical to the theoretical line, and the slope relating ΔPV to ΔECF was 0.22. The ratio of plasma water loss to total body water loss was 0.11 ± 0.02, which was approximately 60% higher than the theoretically expected value, assuming that body water was lost proportionally from each compartment (Spector, 1956). This is likely due to the fact that the major electrolytes excreted during thermal dehydration are Na+ and Cl−, which are the main electrolytes in the ECF. At a given level of dehydration, the [Na+] in sweat determines the volume of fluid mobilized from the intracellular fluid compartment, thereby determining the effective maintenance of circulating blood volume. This conclusion emphasizes the importance of producing a more dilute sweat in the heat adaptation process. APPENDIX The content of osmotically active substances in the body and their loss during dehydration are P0osmol × TW0 and [mosmol]S+U × ΔTW, respectively, where P0osmol and TW0 are control plasma osmolality and total water, and [mosmol]S+U and ΔTW are mean values of osmolality of sweat and urine and their total volume, respectively. Therefore plasma osmolality after dehydration (P'osmol) is represented as follows:

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FLUID REPLACEMENT AND HEAT STRESS ΔP'osmol = P0osmol × TW0 − [mosmol]S+U × ΔTW)/(TW0 − ΔTW) therefore the change in plasma osmolality (ΔPosmol) is as follows: Posmol = P'osmol − P0osmol = P0osmol × TW0 − [mosmol]S+U × ΔTW)/ [TW0 × (1 − ΔTW/TW0)] − P0osmol = (Posmol × TW0 − [mosmol]S+U × ΔTW)/ TW0 ×(1 + ΔTW/TW0)] − P0osmol because ΔTW/TW0 <<< 1 = (P0osmol − ΔTW0/TW0 × [mosmol]S+U) × (1+ ΔTW/TW0) − P0osmol = (P0osmol − [mosmol]S+U × ΔTW/TW0 because [mosmol]S+U × (ΔTW/TW)2 = 0 = P0osmol × (1 − [mosmol]S+U/P0osmol) × ΔTW/TW0 whereas ΔFW = ΔTW × (1 − [mosmol]S+U/P0osmol) therefore ΔPosmol =P0osmol/TW0 × ΔFW We gratefully acknowledge the technical assistance of Sandra DiStefano, the statistical advice of Loretta DiPietro, and the cooperation of all our subjects. We also thank Barbara Cangiano and Elise Low for preparing the manuscript. This study was partially supported by National Heart, Lung, and Blood Institute Grant HL-20634.

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FLUID REPLACEMENT AND HEAT STRESS REFERENCES Bass, D.E., C.R. Kleeman, M. Quinn, A. Henschel, and A.H. Hegnauer. 1955 Mechanisms of acclimatization to heat in man. Medicine 34:323-380. Brace, R.A. 1977 Fitting straight lines to experimental data. Am. J. Physiol. 233(Regulatory Integrative Comp. Physiol. 2):R94-R99. Convertino, V.A., L.C. Keil, E.M. Bernauer, and J.E.Greenleaf. 1981 Plasma volume, osmolality, vasopressin, and renin activity during graded exercise in man. J. Appl. Physiol. 50:123-128. Convertino, V.A., L.C. Keil, and J.E. Greenleaf. 1983 Plasma volume, renin, and vasopressin responses to graded exercise after training. J. Appl. Physiol. 54 :508-514. Costill, D.L. 1977 Sweating: its composition and effects on body fluids. Pp. 160-174 in The Marathon: Physiological, Medical, Epidemiological and Psychological Studies, Paul Milvy, ed. New York: N.Y. Acad. Sci. Costill, D.L., R. Cote, and W.J. Fink. 1976 Muscle water electrolytes following varied levels of dehydration in man. J. Appl. Physiol. 40:6-11. Durkot, M.J., O.Martinez, D. Brooks-McQuade, and R. Francesconi. 1986 Simultaneous determination of fluids shifts during thermal stress in small-animal model. J. Appl. Physiol. 61:1031-1034. Elkinton, J.E., T.S. Danowski, and A.W. Winkler. 1946 Hemodynamic changes in salt depletion and in dehydration. J. Clin. Invest. 25:120-129. Fortney, S.M., E.R. Nadel, C.B. Wenger, and J.R. Bove. 1981a Effect of acute alteration of blood volume on circulatory performance in humans. J. Appl. Physiol. 50:292-298. Fortney, S.M., E.R. Nadel, C.B. Wenger, and J.R. Bove. 1981b Effect of blood volume on sweating rate and body fluids in exercising humans. J. Appl. Physiol. 51:1594-1600. Fortney, S.M., C.B. Wenger, J.R. Bove, and E.R. Nadel. 1983 Effect of blood volume on forearm venous and cardiac stroke volume during exercise. J. Appl. Physiol. 55:884-890. Greenleaf, J.E., V.A. Convertino, and G.R. Mangseth. 1979 Plasma volume during stress: osmolality and red cell volume. J. Appl. Physiol. 47:1031-1038. Kirby, C.R., and V.A. Convertino. 1986 Plasma aldosterone and sweat sodium concentrations after exercise and heat acclimation. J. Appl. Physiol. 61:967-970. Locke, W., N.B. Talbot, H.S Jones, and J. Worcester. 1951 Studies on the combined use of measurements of sweat electrolyte composition and rate of sweating as an index of adrenocortical activity J. Clin. Invest. 30:325-337. Miki, K., T. Morimoto, H. Nose, T. Itoh, and S. Yamada. 1983a Circulatory failure during severe hyperthermia in dog. Jpn. J. Physiol. 33:269-278. Miki, K., T. Morimoto, H. Nose, T. Itoh, and S. Yamada. 1983b Canine blood volume and cardiovascular function during hyperthermia J. Appl. Physiol. 55:300-306.

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FLUID REPLACEMENT AND HEAT STRESS Mohsenin, V., and R.R. Gonzalez. 1984 Tissue pressure and plasma oncotic pressure during exercise. J. Appl. Physiol. 56:102-108. Morimoto, T., K. Miki, H. Rose, S. Yamada, K. Hirakawa, and C. Matsubara. 1981 Changes in body fluid volume and its composition during heavy sweating and the effect of fluid electrolyte replacement. Jpn. J. Biometeorol. 18:31-39. Nadel, E.R. 1984 Body fluid and electrolyte balance during exercise: competing demands with temperature regulation, Pp. 365-376 in Thermal Physiology, J.R.S. Hales ed. Raven, New York. Nose, H., T. Morimoto, and K. Ogura. 1983 Distribution of water losses among fluid compartments of tissues under thermal dehydration in the rat. Jpn. J. Physiol. 33:1019-1029. Nose, H., T. Yawata, and T. Morimoto. 1985 Osmotic factors in restitution from thermal dehydration in rats. Am. J. Physiol. 249 (Regulatory Integrative Comp. Physiol. 18): R166-R171. Rowell, L.B. 1983 Cardiovascular adjustments to thermal stress. Pp. 967-1023 in Handbook of Physiology. The Cardiovascular System. Peripheral Circulation and Organ, Blood Flow. Am. Physiol. Soc., Sect. 2, Vol. 3, Part 2, Chapt. 27. Bethesda, Md. Senay, L.C., Jr. 1979 Temperature regulation and hypohydration: a singular view. J. Appl. Physiol. 47:1-7. Senay, L.C., Jr. and J.M. Pivarnik. 1985 Fluid shifts during exercise. Pp 335-387 in Exercise and Sport Sciences Reviews, Vol. 13, R.L. Terjung, ed. Macmillan, New York. Sjogaard, G. and B. Saltin. 1982 Extra- and intracellular fluid spaces in muscles of man at rest and with dynamic exercise. Am. J. Physiol. 243 (Regulatory Integrative Comp. Physiol. 12):R273-R280. Sohar, E., Y. Shapira., M. Nir, and M. Hellman. 1965 Comparison of methods for determination of the sodium content of sweat. Nature 205:604-605. Sokal, R.R., and F.J. Rohlf 1981 Pp. 344-354 and 246-247 in Biometry. Freeman, New York. Spector, S.W 1956 P. 340 in Handbook of Biological Data. Saunders, Philadelphia, p. 340. Wallace, W.M., K. Goldstein, A. Taylor, and T.M. Teree. 1970 Thermal dehydration of the rat: distribution of losses among tissues Am. J. Physiol. 219:1544-1548. Wyndham, C.H., A.J.A. Benade, C.G. Williams, N.B. Strydom, A. Goldin, and A.J.A. Heyns. 1968 Changes in central circulation and body fluid spaces during acclimatization to heat. J. Appl. Physiol. 25:586-593.