2.73 Understand that water is reabsorbed into the blood from the collecting duct

Water is re-absorbed in the collecting duct one last time into the medulla because it is so salty. 

Following the rules of osmosis, water travels from a high concentration to a low concentration through a semi permeable membrane - that is exactly what happens, because the concentration of water in the medulla is low compared with the filtrate. However, it depends on how permeable the membrane(s) are/is: the more permeable, the more water is absorbed. I.e. if your body lacks water, the membrane will be permeable so there is no unnecessary water loss.

More about this on post 2.71