This is a colligative property and is proportional to the number of particles per kg of solvent - in the case of plasma, per kg of plasma water.
The normal osmolality of extracellular fluid is 280-295 mOsmol/kg. The osmolality is monitored through osmoreceptors in the hypothalamus which can detect a 1% change in osmolality. ADH is secreted from the posterior pituitary and acts on the collecting duct increasing resorption of water. The level of ADH is maximal at levels of osmolality above 295 mOsmol/kg, and undetectable at less than 280 mOsmol/kg.
Last reviewed 01/2018