The adult body (70 kg) is composed of approximately 42 litres of water (approx 60 %). Looking at the composition of the various compartments can help in understanding why some fluids are better at staying in some compartments than others.

All values in mmol/l	Plasma 3/42 =7 %	Interstitial 11/42=26 %	Intracellular 28/42=67 %
Sodium	135-145	145	10
Potassium	4.2	4	157
Calcium	2.4	2.4	<1.0
Magnesium	1.1	1	40
Chloride	112	117	3
Bicarbonate	25	27	7
Protein	15	<0.1	45
Other	8.1	8.7	154 (phosphates)

Table 1 The Composition of the Compartments