Water loss in excess of salt deficit. Hypernatremia is usually due to insufficient water (primarily in patients who either do not experience thirst normally, or cannot act on it). Hypernatraemia occurs less commonly associated with excess salt
Sodium is the major positively charged Cation in ECF. It determines ECF volume. Major intravascular ion to affect osmolality…..an acute increase or decrease in serum sodium will produce an increase or decrease in serum osmolality.