Explanation

Antidiuretic hormone (ADH or vasopressin) is secreted from the posterior lobe of pituitary. The injection of extremely minute quantity of ADH (as small as 2 nanograms) can cause decreased excretion of water in the urine. In the absence of ADH, the collecting tubules and ducts become almost impermeable to water which prevent significant reabsorption of water and therefore allow extreme loss of water in urine.