Fluid compartment volumes

Total body water (TBW)

Water comprises the major part of body weight, forming about 60% of the body weight of a young adult male. This percentage varies with build, sex and age in the following manner:

• TBW in the adult can range from 45% to 75% of body weight. This large variation is the result of individual differences in adipose tissue, which contains relatively little water.

• TBW in the female adult is less than in the male of the same age. In the young adult female TBW only forms about 50% of body weight. This difference between the sexes develops during puberty and decreases in old age due to a reduction in adipose tissue.

• TBW as a proportion of body weight may be as high as 80% in the neonate.

TBW is divided into several fluid compartments (Figure FL1), the main division being between intracellular fluid (ICF) and extracellular fluid (ECF).

The variations of TBW (% body weight) and ECF (% body weight) with age are shown in Figure FL2.

Intracellular fluid (ICF)

The ICF compartment contains about 60% of TBW. The volume of the ICF compartment, including red blood cell contents (2 litres), amounts to about 27 litres. The water content and composition of ICF varies according to the function of the tissue. As noted above, adipose tissue will have much lower water content than lean body tissue, which contains about 70 ml per 100 g water.