Loop of Henle

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.^[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.^{[ref] [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.