There are three fundamental processes that produce concentrated urine.
Filtration occurs in the renal corpuscle .
If this fluid were not modified before being secreted you would lose your entire plasma volume within 30 minutes. Near the end of the PCT two-thirds of the water and virtually all of the nutrients have been returned to the blood. Further reduction of filtrate volume occurs in the loop of Henle . This reduces the osmotic concentration of the filtrate without changing its volume. As the fluid descends through the loop it passes through the osmotic gradient set up by the ascending loop.