Collecting Tubule - Vertebrate Kidney
The fluid that reaches the collecting tubules may tie hypoosmotic or isosmotic depending on the water status of the animal. Final adjustments in urine composition are made in the collecting tubules, whose walls -are permeable only in the presence of ADH. In the absence of ADH, as it happens during water diuresis, the hypoosmotic fluid that enters the collecting tubule passes through the latter unchanged, so that dilute urine of relatively large volume is excreted.
Under situations demanding water conservation, ADH is secreted and the walls of the collecting tubule become permeable to water. The fluid entering the collecting tubules becomes progressively more concentrated as it passes down making the final urine practically isosmotic to the interstitial fluid and hyperosmotic to blood.