Countercurrent multiplier mechanism
The countercurrent system permits forming a concentrated urine

In the presence of ADH, which increases water permeability, the hyposmotic fluid that enters the distal tubule (DT) from the thick ascending limb (TAL) looses most of its water by osmotic equilibration with the surrounding cortical interstitium along the CNT and cortical collecting duct (CCD). It also continues loosing NaCl through reabsorptive transport along DT, CNT and CCD, until the tubule fluid becomes isoosmotic with plasma, by the end of the CCD.

The relatively small amount of isoosmotic fluid that flows into the medullary collecting ducts losses progressively more and more water to the hyperosmotic medullary and papillary interstitia and is finally excreted as hyperosmotic, highly concentrated urine.

The countercurrent system permits forming a dilute urine

In the absence of ADH, the hyposmotic fluid that enters the DT from the loop of Henle, continues to be diluted by transport of NaCl via NaCl (thiazide sensitive) cotransporters into DT cells and via Na channels (amiloride sensitive) along the CD. Water reabsorption is limited so that the tubule fluid becomes more and more dilute along DT, CNT and collecting ducts (CCD, OMCD and IMCD), until it is excreted as a large volume of hyposmotic urine.

Sources: 


Barac-Nieto, Prof. Mario. "Countercurrent System and the Loop of Henle." Countercurrent System and the Loop of Henle. Kuwait University. Web. 08 Feb. 2016.
Wikipedia. Wikimedia Foundation. Web. 08 Feb. 2016. https://en.wikipedia.org/wiki/Countercurrent_multiplication.
"7. Urinary Concentration (countercurrent Mechanism)." 7. Urinary Concentration (countercurrent Mechanism). Web. 08 Feb. 2016.