Determinants of the plasma sodium concentration

Among both normal subjects and patients with a variety of diseases, there is a very close correlation between the plasma water sodium concentration and the ratio of total body exchangeable solutes (primarily Na salts in the extracellular fluid + K salts in the cells) to the TBW. The exchangeable portion is used since approximately 30% of the body Na and a smaller fraction of the body K are bound in areas such as bone where they are "nonexchangeable" and therefore osmotically inactive.