Now, at the top of our descending loop of Henley, the inside filtrate is isotonic with respect to our interstituent. Now, what exactly will happen as the filter travels downward? Now, the epithelial cells of the descending loop of Henley are permeable to water. 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.