*Spoiler* If you haven't gotten that far in Doctor Who. But when I found out that he was The Face of Boe, I kind of figured it out before, but it was still a big surprise. The people of BBC really do know what they are doing.
Each of these sections has a main function in adjusting the amount and kind of solutes in the urine. Different drugs and diuretics work at distinct areas, some diuretics are potassium sparing while others (like Lasix/furosemide) are potassium wasting.