After the proximal tubule in the nephron. Filtrate moves down the thin descending limb, which is permeable to water but less so to sodium and urea; water leaves by osmosis to the salty medulla. Filtrate moves up thin ascending limb into the thick ascending limb, where Na+ followed by Cl- filters out; water cannot follow as it is impenetrable to water (making the medulla salty). Thin segments of the loop are lined with squamous epithelium. It dips down into the medulla. The bottom of the loop is highly concentrated with salt, as water has left.