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Fluid Replacement and Heat Stress, 1993

Pp. 195-214. Washington, D.C.

National Academy Press

15

Environmental Issues That Influence Intake of Replacement Beverages

John E. Greenleaf1

INTRODUCTION

Water is the major constituent by weight and volume in the human body. The volume of water in normal healthy people is regulated to within ±0.22% (±165 g) of the body weight each day (Adolph, 1943), and plasma volume varies by less than ±0.6% (±27 ml) of the blood volume (Greenleaf et al., 1979). Such precise regulation underscores the degree of integrated coordination for maintenance of the volumes of cellular water (33 liters, 41% of body weight) and extracellular water (20 liters, 25% of body weight) in an 80-kg man as well as the importance of water for life. Muscle cells contain more water than fat cells, and men have a greater percentage of their weight as muscle than women; thus, men have a greater percentage of intracellular water than women. The rate of increase of total body water volume in infants and children is essentially the same until puberty, when female total body water levels off at about 28 liters and male total body water volume increases to about 44 liters (Figure 15-1, upper half). There is a gradual decline in the percentage of body water content to body weight

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John E. Greenleaf, Laboratory for Human Environmental Physiology, Life Science Division (239-7), NASA, Ames Research Center, Moffett Field, CA 94035-4000



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