Dilute urine may be considered to have an isotonic portion and an electrolyte-free water portion. The proportions of electrolytes-free water and isotonic can be measured by the ratio of effective solutes (principally sodium and potassium, with associated anions) between the plasma and the urine [1,2]. Restriction of water intake to less than the amount of electrolyte-free water excreted will cause serum sodium to rise.
