In the presence of ADH, greatest fraction of the filtered water is reabsorbed from:

65%-70% of the filtered water is reabsorbed from the PCT. This is called obligatory water reabsorption and occurs irrespective of the osmolarity of ECF. In the complete absence of ADH action, 88% of the filtered water is reabsorbed from various segments of nephron. 10%-12% water is reabsorbed from collecting duct under the maximum action of ADH. This is called facultative water reabsorption. Water reabsorption from the collecting duct is determined by osmolarity of ECF.