The osmotic concentration of plasma. Normally the ionic concentration in the plasma is maintained within a narrow range: 275 to 295 mOsm/kg. When plasma osmolality increases above normal, antidiuretic hormone (ADH) [also called vasopressin] is released. ADH prevents loss of water by the kidney and thus decreases plasma osmolality. An increase in plasma osmolality also produces the sensation of thirst, which stimulates the person to drink fluids; this, too, serves to decrease plasma osmolality.