In the presence of vasopressin, the greatest fraction of filtered water is absorbed in:

Water absorption in different segment of nephron Segment % absorption Through Proximal tubule 60-70% AQP 1 Descending loop of Henle 15% AQP 1 Ascending LOH (Diluting segment) Impermeable to water Distal Tubule Relatively impermeable to water (~5% in ADH presence in late DT) Collecting Duct 1. Coical 2. Medullary In absence of ADH, CD is relatively impermeable to water (~2%) 10% (in presence of ADH) ~5% (in presence of ADH) AQP 2 on ocal and AQP 3,4 on basolatreral membrane ADH dependent absorption is 20% only, even in presence of ADH maximum absorption is occurring from PCT Obligatory water absorption Facultative water absorption Approx, 80% (65% in PCT, 15% in loop of henle) Approx, 20% (from collecting duct and late distal tubule) Water is obligated to follow solutes, irrespective of blood osmolality Depends on blood osmolality Independent of ADH Depends on ADH