what is counter current mechanism in loop of henle?

In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. The loop has a hairpin bend in the renal medulla. The main function of this structure is to reabsorb water and ions from the urine. To do this, it uses a countercurrent multiplier mechanism in the medulla. It is named after its discoverer, F. G. J. Henle.

COUNTERCURRENT SYSTEM and the LOOP OF HENLE

1.  The Loop of Henle establishes medullary hyperosmolarity

The ascending limb of the loop of Henle transports solutes (NaCl) out of the tubule lumen with little or no water, generating an hyperosmotic medullary interstitium and delivering an hyposmotic tubule fluid to the distal tubule. This is called the "single effect".

The osmolarity of the interstitium rises progressively from cortex to medulla and papilla through multiplication of the "single effect" by countercurrent flow in the branches of the loop: The single effect in fluid processed by loop segments located near the tip of the papilla occurs in fluid already subject to the single effect when the fluid was in loop segments located closer to the cortex.

Countercurrent exchange of solutes between ascending and descending vasa recta (the renal medullary capillaries) minimizes solute washout from the medullary interstitium.


2.    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.


3.  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.

Here is the list of some good info on this topic
http://members.aol.com/Bio50/LecNotes/lecnot38a.html
http://www.nda.ox.ac.uk/wfsa/html/u09/u09_017.htm
http://en.wikipedia.org/wiki/Loop_of_Henle

Hope this helps you

Filtrate from the proximal tubule flows into the descending limb of the loop of Henle, which is permeable to water but does not transport ions.  As the loop dips into the medulla, water moves by osmosis from the descending limb into the progressively more concentrated interstitial fluid.  As a result, solutes are left behind in the proximal tubule, and the filtrate also becomes progressively more concentrated.  When the fluid flow reverses direction (when it enters the ascending limb), the properties of the tubule epithelium change: the tubule becomes impermeable to water, but actively transports Na+, K+, and Cl- out of the tubule and into the interstitial fluid.  The osmolarity of the filtrate is about 10X greater at the bottom of the loop than at the top.  The purpose of this countercurrent exchange system is to produce hyperosmotic interstitial fluid in the medulla and hyposmotic filtrate leaving the loop of Henle (the water that leaves the descending loop of Henle is absorbed into the vasa recta, which prevents dilution of the interstitial fluid).

http://www.acbrown.com/renal/OutLoop.htm
 (the website from which the post above me was copy & pasted, i might add)

http://www.acbrown.com/kidney/Lectures/RnWatr/RnWatrLpHeCntr.htm

http://en.wikipedia.org/wiki/Loop_of_Henle