Normal volumes, fluid compartments

Figure 1: The diagram shows the normal distribution of body water. Total body water (TBW) is about 60% of body weight, this is dependent on lean body mass so is higher in less fat muscular people. The young are also more watery than the elderly. The two main fluid compartments are divided by the cell membrane into the extracellular fluid (ECF) and the intracellular fluid (ICF). The ICF is further divided by the capillary endothelium into the intravascular space (plasma) and the interstitial space.

Body water is mainly related to non fat body weight. So in lean people about 65% of the body weight is water and for fat people this is about 45%. In an average adult male total body water is about 60% and the corresponding figure for an adult female is 50%. Infants are more watery than adults. About 70% of an infants body weight is water.

As 1 of water weighs 1 , in a 70 male the total body water would be

$$70 \times 0.6 = 42\ Litres\ldots\mathbf{Total\ body\ water}$$

The distribution of body water can be broken down into two main compartments which are divided by the cell membrane. These are the intracellular compartment (ICF ) and the extracellular compartment (ECF ). About ${\frac{2}{3}}$ of the total body water is intracellular and the remainder extracellular. This equates to 40% of the total body weight as ICF and the remaining 20% as ECF .

$$42 \times 0.66 = 28\ Litres\ldots\mathbf{ICF}$$

$$42 - 28 = 14\ Litres\ldots\mathbf{ECF}$$

The ECF may be subdivided by the capillary endothelium into the intravascular space and the interstitial space. 5% of the total body weight is plasma and 15% interstitial fluid. One quarter of the ECF therefore is plasma, because ECF is 20% of the total body weight and 5 goes into 20 four times.

$$14 \times 0.25 = 3.5\ Litres \ldots \mathbf{Plasma}$$

Table 1: Breakdown of body water

			70 Male		Infant
			% Body weight	Volume	% Body weight
Total body water			60 %	42 L	70 %
	Extracellular fluid		20 %	14 L	30 %
		Plasma	5 %	3.5 L	5 %
		Interstitial fluid	15 %	10.5 L	25 %
	Intracellular fluid		40 %	28 L	40 %

The normal values for body water in its various compartments are summarised in figure 1 and table 1.

The difference in body water between infants (70% body weight) and adults (60% body weight) is in the main due to a higher amount of interstitial fluid in infants. Intracellular volume, and plasma volume per body weight are the same in infants and adults.

When it comes to movement of fluid into and out of the body water most of the movement occurs via the extracellular fluid. Input may come via drinking and intravenous fluids and output via the lungs, skin, intestine and kidneys.