Osmolality is the measure of the osmotic concentration of a solution or fluid, a measure of the ionic concentration of dissolved substances per unit of solvent. Osmolality can be measured in fecal material, serum, plasma, or urine.
The plasma (blood) osmotic concentration is normally between 275-295 mOsm/kg. If plasma osmolality increases above normal, ADH/antidiuretic hormone is released from the posterior pituitary gland (neurohypophysis). ADH conserves fluid in the renal system (prevents the loss) this will decrease the plasma osmolality.
When you are thirsty the osmolality of your plasma is increased, when you drink fluids this decreases the plasma osmolality. The cells of your body maintain osmotic balance by selectively allowing water and ions, specifically bicarbonate and chloride, into and out of the cell.