countercurrent multiplier

Process

a.  The active reabsorption of Na⁺ and Cl^- in the ascending thick limb of juxtamedullary nephrons combined with the thick limb impermeability to water results in an increased osmolality in the renal medulla.

b.  The increased medullary interstitial osmolality draws water from the descending thin limb, progressively concentrating the fluid remaining in the tubular lumen.

c.  As this fluid passes around the hairpin turn and flows into the ascending limb, it loses NaCl into the interstitium by passive diffusion in the ascending thin limb and by active transport in the ascending thick limb.

d.  As a result, interstitial fluid throughout the whole medulla becomes hyperosmotic and the fluid leaving the ascending thick limb and entering the distal tubule is hypoosmotic.

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