Step-by-step explanation:
Antidiuretic hormone or ADH is secreted from the posterior pituitary gland in response from the signals perceived by the osmoreceptors in the blood vessels.
When the water level drops below the optimal range, ADH is secreted by the gland which acts on the distal convoluted tubule and collecting duct to decrease the osmolarity of the blood plasma.
The process of maintaining osmolarity and water reabsorption are interconnected. The osmolarity of the plasma is maintained largely by the movement of sodium ions therefore sodium ion concentration is regulated by ADH.
ADH maintains the osmolarity by increasing the permeability of the tubules in the nephron which are usually impermeable. This allows the absorption of water which increases the osmolarity of the plasma.