Answer:
ADH(antidiuretic hormone) is a hormone which is secreted by the hypothalamus gland and stored in the pituitary gland. It regulates the amount of reabsorption of water by the kidney into the blood by controlling the permeability of late distal convoluted tubule and collecting duct.
When a high level of ADH is present in the blood it binds to these tubular section and make them permeable to water. This change in permeability helps in reabsorption of water in the blood and results in concentrated urine.
When ADH is absent in blood then these section of nephron remains impermeable to water and results in diluted urine.