The permeability of the wall of the collecting duct varies under the influence of antidiuretic hormone, ADH.  ADH is released by the posterior pituitary in response to increased osmotic pressure (decreased water or increased solutes in blood).  Its effect is to increase the wall’s permeability to water so that more water leaves the collecting duct and returns to the bloodstream.  If osmotic pressure of the blood decreases, ADH is not released and water stays in the collecting duct, thus leaving in urine.