Water is divided between the extracellular (ECF) and intracellular (ICF) compartments:
In an average 70kg person:
| Water (L) | Protein (kg) | Na+ (mmol) | K+ (mmol) | |
| Total body | 45 | 6 | 2550 | 4560 |
| ECF | 15 | 0.3 | 2250 | 60 |
| ICF | 30 | 5.7 | 300 | 4500 |
Most of the ECF is in the interstitial compartment.
Composition of the ECF:
| Compartment | Water (L) | Other constituents |
| Total ECF | 15 | Contains 230g of albumin and 2250mmol of Na+ |
| Interstitial volume | 12 | Contains 1/4 of the conc. of albumin in plasma |
| Plasma volume | 3 | Contains 120g of albumin |