Water is absorbed in the descending loop of Henle via aquaporin channels. These channel proteins are constantly open and allow for the movement of water molecules in response to the concentration gradient of the surrounding fluids.
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In the collecting duct, membrane permeability is variable. Aquaporins are also present, and osmosis spurred by concentration gradients still takes place. The gradient itself is altered by the variable membrane. It changes in response to hormones that are released either to expel or conserve more water, altering the concentration gradient and therefore the amount of water that will move through the Aquaporins.