Question

Explain why ADH reduces fluid volume in the collecting ducts.

one-two sentence answers, please.

Answer 1

ADH, a peptide hormone, causes the insertion of Aquaporins (which are water channels) into the membranes of cells lining the collecting ducts, which will promotes reabsorption of water back into the circulation, this movement of water from the collecting duct back into the circulation reduces the fluid volume in the collecting duct.  In the absense of antidiuretic hormone, the collecting ducts are virtually impermiable to water, and it flows out as urine.