definition of hypo-osmolality and hyponatremia
By Mayo Clinic Staff. Hyponatremia is a condition that occurs when the level of sodium in your blood is abnormally low. Sodium is an electrolyte, and it helps regulate the amount of water that's in and around your cells. In hyponatremia, one or more factors â ranging from an underlying medical condition to drinking too much water during endurance sports â causes the sodium in your body to become diluted. When this happens, your body's water levels rise, and your cells begin to swell.