I have included the distal convoluted tubule and the collecting duct on this page. Anatomically, the collecting duct is not a part of the nephron. But physiologically, it works with the nephron.
The distal convoluted tubule, or DCT, has a lot to do. It carries out both tubular reabsorption and secretion:
Both the DCT and the collecting duct are involved in reabsorption of water. You see, if they only reabsorb a tiny bit of water, then around 97% of all the water that was in the filtrate is reabsorbed. If they hardly reabsorb any water, then only around 95% of all the water that was in the filtrate is reabsorbed. But if they are very active in water reabsorption, then we can reabsorb 99% of all the water that was in our filtrate.
Since our kidneys handle 180 liters of fluid a day, 5% loss versus 1% loss is a big deal! I took these numbers from books, but 5% of 180 is 9 liters. I can't believe a person can ever urinate 9 liters in one day!
Water reabsorption can be regulated
Depending on our condition, we may want to reabsorb as much water as possible in our kidneys or urinate as much water as possible. If you are in an arid climate and haven't had much food or water, you would be better off if you didn't lose much water in your urine. And if you are drinking tons of liquids, you would be better off excreting plenty of water in your urine.
The only place where we regulate our water reabsorption is in the DCT and collecting duct. Here, the permeability of the epithelia to water depends on hormonal influence. ADH (from the posterior pituitary) causes these epithelia to do a lot of water reabsorption. Lack of ADH prevents water reabsorption. This is also mentioned in the water balance web page.
© 2011 STCC Foundation Press