Particles such as urea cross cell membranes rapidly and distribute equally in ICF and ECF. Ions which are regulated by transporters and active pumps and therefore have different concentrations in the ECF and ICF are the ones able to exert an osmotic effect on the distribution of water between ECF and ICF. These are:
| ECF | ICF (skeletal muscle) | |
| Na+ (mmol/l) | 141 | 10 |
| K+ (mmol/l) | 4.1 | 120-150 |
| Cl- (mmol/l) | 113 | 3 |
| HCO3- (mmol/l) | 26 | 10 |
| Phosphate (mmol/l) | 2.0 | 140 (organic phosphates) |