Editing Nephron (section)

===Loop of Henle===
The [[loop of Henle]] is a U-shaped tube that extends from the proximal tubule. It consists of a descending limb and an ascending limb. It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal convoluted tubule. The primary role of the loop of Henle is to enable an organism to produce concentrated urine, not by increasing the tubular concentration, but by rendering the interstitial fluid hypertonic.<ref name="lote" />{{rp|67}}

Considerable differences aid in distinguishing the descending and ascending limbs of the loop of Henle. The [[Descending limb of loop of Henle|descending limb]] is permeable to water and noticeably less permeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the [[Hypertonicity|hypertonic]] [[interstitium]] of the renal medulla, water flows freely out of the descending limb by [[osmosis]] until the tonicity of the filtrate and interstitium equilibrate. The hypertonicity of the medulla (and therefore concentration of urine) is determined in part by the size of the loops of Henle.<ref name="lote" />{{rp|76}}

Unlike the descending limb, the [[Ascending limb of loop of Henle|thick ascending limb]] is impermeable to water, a critical feature of the [[countercurrent exchange]] mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows [[Hypotonicity|hypotonic]] since it has lost much of its sodium content. This hypotonic filtrate is passed to the [[distal convoluted tubule]] in the renal cortex.<ref name="lote" />{{rp|72}}