 Thin descending limb

The thin descending limb is the thin-walled descending portion of the Loop of Henle, carrying tubular fluid deeper into the renal medulla. In water-permeable portions, outward osmosis can raise the tubular fluid's osmolality as it encounters more concentrated interstitial fluid. The term identifies a segment and direction of travel; it does not imply that every point in every descending limb has identical transport properties.

A 1970 experiment separated two possible causes of concentration: losing water or gaining solute. Isolated rabbit thin descending limb segments were perfused in a controlled bath. Increasing bath osmolality with sodium chloride or another solute concentrated the collected fluid chiefly through water leaving, without substantial salt entering. The preparation thus supplied direct evidence for selective permeability rather than inferring transport solely from the final concentration.^[ref]

A simple example makes the distinction quantitative. Starting with 6 solute units in 2 volume units gives a concentration of 3. Losing one volume unit at unchanged solute content raises it to 6; adding 6 solute units at unchanged volume also raises it to 6. Concentration alone cannot distinguish these processes. Measuring fluid movement and solute movement separately is therefore essential to explaining this segment's role in countercurrent multiplication.