A i.e. Proximal tubuleVasopressing acts on the collecting ducts and enhances their permeability to water.When vasopressing is absent- The collecting duct epithelium is relatively impermeable to water and only 2% of water is reabsorbed.- However by the time water reaches collecting duct, a maximum of 60-70% has already been re- absorbed in the proximal tubules. % of water reabsorbed inabsence of vasopressin% of water reabsorbed inpresence of vasopressinProximal tubule60-70%Q60-70%0Loop of Henle15%15%Distal tubule5%5%Collecting duct2%10%QThus the greatest fraction of filtered water (60-70%) is reabsorbed in proximal tubule irrespective of presence or absence of vasopressinQ.