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Water is the most plentiful constituent of the human body. Total body water (TBW) as a percentage of body weight varies with age. The fetus has a high TBW, which gradually decreases to about 75% of birth weight for a term infant. Premature infants have a higher TBW content than term infants. During the first year of life, TBW decreases to about 60% of body weight and basically remains at this level until puberty. At puberty, the fat content of females increases more than that of males, who acquire more muscle mass than females. Because fat has low water content, and muscle has high water content, by the end of puberty TBW in males remains at 60%, but it decreases to 50% of body weight in females. During dehydration, TBW decreases and is a smaller percentage of body weight.
TBW has two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF). In the fetus and newborn, the ECF volume is larger than the ICF volume. The normal postnatal diuresis causes an immediate decrease in the ECF volume. This decrease in ECF volume is followed by continued expansion of the ICF volume because of cellular growth. By 1 year of age, the ratio of the ICF volume to the ECF volume approaches adult levels. The ECF volume is 20% to 25% of body weight, and the ICF volume is 30% to 40% of body weight (Fig. 32-1). With puberty, the increased muscle mass of males results in a higher ICF volume than in females.
The ECF is divided further into plasma water and interstitial fluid (see Fig. 32-1). Plasma water is about 5% of body weight. The blood volume, given a hematocrit of 40%, is usually 8% of body weight, although it is higher in newborns and young infants. The interstitial fluid, normally 15% of body weight, can increase dramatically in diseases associated with edema, such as heart failure, protein-losing enteropathy, liver failure, and nephrotic syndrome.
The composition of solutes in the ICF and ECF is different.Sodium and chloride are the dominant cation and anion in the ECF. Potassium is the most abundant cation in the ICF, and proteins, organic anions, and phosphate are the most plentiful anions in the ICF. The dissimilarity between theanions in the ICF and the ECF is determined largely by thepresence of intracellular molecules that do not cross the cellmembrane, the barrier separating the ECF and the ICF. Incontrast, the difference in the distribution of cations—sodium and potassium—is due to the activity of the Na+,K+-ATPase pump, which extrudes sodium from cells in exchange forpotassium.
Proper cell functioning requires close regulation of plasma osmolality and intravascular volume; these are controlled by independent systems for water balance, which determines osmolality, and sodium balance, which determines volume status. Maintenance of a normal osmolality depends on control of water balance. Control of volume status depends on regulation of sodium balance.