Reabsorption of water takes place in renal tubule. Other wise the body will get dehydrated. So during reabsorption water balance of blood and body fluid is maintained. The hormomes from pituitary called ADH or antidiuretic hormone or vasopressin is released when the water content of the body fluids is less than normal. So the effect of the hoemone is to increase is to increase the permeability for water DCT and collecting duct.. Large amount of water is reabsorbed and is retained. On the other hand higher amount of water in blood suppresses ADH production resulting in decreased permeability for water. Therefore water is not reabsorbed and body fluid tend to return to their regular level of concentration. Thus according to needs of body the urine produced may be hypotonic or hypertonic in comparison to the body fluids. This type of absorption is called facultative absorption.
Low secretion of ADH causes diabetes insipidus. Very dilute
urine is excreted out and person feels thirsty.