= Loop of Henle
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The <Loop of Henle> is the hairpin portion of a <nephron>'s <renal tubule>, carrying fluid from the <proximal tubule> toward the <renal medulla> and back toward the <renal cortex>. Its descending and ascending portions have different transport properties. In particular, the <thin descending limb> can be highly water-permeable, while the <thick ascending limb> reabsorbs <sodium chloride> with little accompanying <water>. These differences support <countercurrent multiplication> rather than mere circulation around a bend.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Sections25.4 and25.6: nephron loop and tubular transport]{ref}
In a 1970 experiment, isolated rabbit <thin descending limb> segments were perfused inside a separate surrounding bath. Raising bath <osmolality> concentrated the collected tubular fluid mainly through water leaving, with little solute entry. A separate 1973 study of isolated rabbit <thick ascending limb> segments found salt removal with very low osmotic water transport. The experimental comparison connects different functions to distinct parts of one continuous tubule.<Sodium chloride and water transport in the descending limb of Henle by Juha P. Kokko>[Abstract: isolated segments, imposed bath gradients and water efflux]{ref} <Sodium chloride and water transport in the medullary thick ascending limb of Henle. Evidence for active chloride transport by Antonino S. Rocha and Juha P. Kokko>[Abstract: salt removal and low water transport]{ref}
A simple <mass balance> clarifies why both observations matter. If a fluid initially contains 30 arbitrary units of <solute> in 10 units of <water>, removing 5 water units raises its concentration from 3 to 6. Removing 15 solute units at unchanged water content instead lowers it from 3 to 1.5. These are illustrative limiting cases, not complete measurements of a nephron. Concentrating and diluting fluid need not require the same kind of transport in reverse.
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