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In a healthy human, osmoregulation keeps the sodium (Na+) concentration in bodily fluids from becoming too dilute or too concentrated. Water and sodium balance are closely interdependent. The homeostasis of sodium, the major extracellular cation, is vital to the normal physiologic function of cells.

Because many different disease states can potentially disrupt the finely balanced mechanisms that control sodium and water balance, hyponatremia, an abnormally low concentration of sodium in the blood, is one of the most commonly encountered problems in the practice of clinical medicine. Too little sodium can cause cells to malfunction and extremely low sodium can be fatal.