Sodium in biology mEq/liter (135 - 145 mmol/L). A serum sodium level of less than 135 mEq/L qualifies as hyponatremia, which is considered severe when the serum sodium level is below 125 mEq/L. The renin–angiotensin system and the atrial natriuretic peptide indirectly regulate the amount of signal transduction in the human central nervous system, which depends on sodium ion motion across the nerve cell membrane, in all nerves. Sodium is thus important in neuron function and osmoregulation between cells and the extracellular fluid; the distribution of sodium ions are mediated in all animals by sodium–potassium pumps, which are active transporter solute pumps, pumping
Hypotonic hyponatremia ADH secretion. As a result, the kidneys of such patients recover water and produce a fairly concentrated urine. Treatment is simple (if not without risk) — simply restore the patient's blood volume, thereby turning off the stimulus for ongoing ADH release and water retention. It is worth considering separately, the hyponatremia that occurs in the setting of diuretic use. Patients taking diuretic medications such as furosemide (Lasix), hydrochlorothiazide, chlorthalidone, etc., become volume depleted. That is to say that their diuretic medicine, by design, has caused their kidneys to produce more urine than they would otherwise make. This extra urine represents