In vasopressin, the greatest fraction of filtered water is reabsorbed by:

In the presence of vasopressin, the greatest fraction of filtered water is reabsorbed by the 

A. Proximal tubule 
B. Distal tubule 
C. Loop of Henle 
D. Collecting duct

Ans. A. Proximal tubule
• Vasopressin acts on the collecting duct to cause water reabsorption. The level of vasopressin and its action of water reabsorption is determined by the body fluid osmolality.
• About 67% of the total water filtered will be absorbed in the PCT. This is obligatory and irrespective of the body fluid osmolality or vasopressin action.
• 15% will be absorbed in the loop of Henle. Approximately 5% will be absorbed in the DCI. In the absence of vasopressin, only about 2% will be absorbed from collecting duct. Thus, about 88—89% of filtered water will be reabsorbed even in the absence of vasopressin.
• In the presence of vasopressin, collecting duct absorption will be about 10—11% fraction of the total filtered water. In addition, 99% of the filtered water will be reabsorbed. Thus, greatest fraction of filtered water will always be absorbed from PCT, irrespective of vasopressin action.