So they allow the passive movement of water across the membrane of the descending loop of Henley. However, the cells are impermeable to ions, so the ions such as sodium and chloride do not flow across the membrane in the descending loop of Penne. Now, as the filtrate moves down, because the membrane of the descending loop of Penne is permeable of water, that means it will either flow out of the tubule or into the tubule. The question is, where exactly will it flow? Well, let's go back to the thick ascending loop of Henley. Recall that the thick ascending loop of Henley, what it basically did is it created a hypertonic environment in the surrounding tissue, in the interstituum.