The distribution of all the water in the body into the different compartments takes place through the cell membrane by the osmotic pressure. Water can pass through the cell membrane unrestricted and aims for the same ion concentration everywhere. If large electrolyte losses disturb this balance, serious diseases can follow.
Serum osmolality is mainly influenced by the sodium concentration and the metabolites glucose and urea. It is a measure of all particles in solution in the serum and can be used as a criterion for assessing kidney function.