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 disappears below 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 FL.1) 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 FL.2. 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

Figure FL.1 Body fluid compartments as percentage of body weight

VARIATION OF TBW (% BODY WEIGHT) AND ECF (% BODY WEIGHT), WITH AGE

Age

TBW

ECF

Neonate

80

45

6 months

70

35

1 year

60

28

5 years

65

25

Young adult

60

22

Elderly

50

Figure FL.2

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/100 g water.

Extracellular Fluid (ECF)

About one-third of total body water is ECF. Although ECF as a percentage of body weight, varies with age (Figure FL.2), when expressed as an index of body surface area it remains relatively constant throughout life. The ECF volume to body surface area index is about 7.5 litre/m2.

ECF is composed of several components (Figure FL.3).

COMPONENTS OF ECF

IN A 7G KG ADULT

Fluid

% Body weight

Volume (litres)

Interstitial fluid

15

10.5

Plasma

5

3.5

Transcellular fluid

1

0.7

Total ECF

21

Plasma

Plasma volume equates to the intravascular component of ECF and amounts to about 5% of body weight. The total blood volume is composed of plasma and the red blood cell volume. Plasma volume (VPL) can be measured using a dilution technique. The fractional red blood cell volume is readily available as the haematocrit (%). Given these values, the total blood volume (VBL) can be calculated:

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