Thin Descending

At the boundary between the inner and outer stripes of the outer medulla, the proximal tubule empties into the thin descending limb of Henle s loop. Water is extracted from the descending limb of this loop by osmotic forces found in the hypertonic medullary interstitium. As in the proximal tubule, impermeant luminal solutes such as mannitol oppose this water extraction. The thin ascending limb is relatively water-impermeable. 

The thick ascending limb (TAL) of the loop of Henle actively reabsorbs NaCI from the lumen (about 25% of the filtered sodium), but unlike the proximal tubule and the thin descending limb of Henle s loop, it is nearly impermeable to water. Salt reabsorption in the TAL therefore dilutes the tubular fluid, and it is called a diluting segment. Medullary portions of the TAL contribute to medullary hypertonicity and thereby also play an important role in concentration of urine by the collecting duct. 

It is likely that transport also occurs in the highly lithium permeable thin descending limb along the osmotic gradient to the hyperosmolar inner medulla... 

Mannitol (IV only) inhibits water reabsorption in the proximal convoluted tubule (PCT) (main site), the thin descending limb of the loop of Henle, and the collecting ducts. It increases urine volume, preventing anuria in hemolysis and rhabdomyolysis, and facilitates elimination of toxic drugs (e.g cisplatin). Similar osmotic actions in the ECF of other tissues —>4- intraocular and intracerebral pressure. 

Aquaporins are a family of water channel proteins that allow water molecules to cross biological membranes. Of the 10 cloned mammalian aquaporins, at least 7 are found in the kidney. Aquaporin I is present in the apical and basolateral membrane of the proximal tubule and in the thin descending limb. Aquaporin 2 resides in the apical membrane and WCVs of the collecting-duct principal cells, whereas aquaporins 3 and 4 are present in the basolateral membrane of principal cells. Aquaporin 7 is in the apical brush border of the straight proximal tubule. Aquaporins 6 and 8 are... 

The U-shaped portion of a renal tubule lying between the proximal and distal convoluted portions. It consists of a thin-descending limb and a thicker ascending limb. 

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