Concentration of Urine: The Countercurrent Mechanism

Under conditions such as low water intake or heavy sweating, the human kidney can concentrate urine to almost four times the osmolarity of blood — up to about 1200 mOsm/L, against blood's normal 300 mOsm/L. This feat depends on a countercurrent mechanism operating jointly in the loops of Henle of the juxtamedullary nephrons and in their accompanying vasa recta.

The loop of Henle runs the countercurrent multiplier. Its descending limb is thin-walled and freely permeable to water, so as filtrate flows down through it, water diffuses out into the increasingly salty surrounding tissue fluid and the filtrate becomes progressively more concentrated. Its ascending limb, by contrast, is thick-walled and impermeable to water, but its cells actively pump Na+ and Cl− out of the filtrate into the surrounding tissue fluid, so as filtrate flows up through it, its concentration progressively falls. Because fluid flows in opposite directions through the two adjacent limbs — hence 'counter-current' — the Na+/Cl− pumped out of the ascending limb raises the salt concentration of the surrounding tissue fluid, which in turn pulls even more water out of the neighbouring descending limb; repeated over and over along the length of the loop, this builds up a steep osmotic gradient in the medulla, becoming progressively more concentrated from the cortex-medulla boundary down to the tip of the pyramid.

The vasa recta, which loop alongside the juxtamedullary nephrons' loops of Henle, run a parallel countercurrent exchanger that preserves this gradient rather than washing it away. Blood entering the descending part of the vasa recta starts at the normal blood osmolarity of about 300 mOsm/L; as it descends into the increasingly concentrated medullary tissue fluid, Na+, Cl− and urea diffuse into the blood while water diffuses out of it, so the blood becomes progressively more concentrated. This same blood then flows back up through the ascending part of the vasa recta, now passing through regions of progressively less concentrated tissue fluid, so the process reverses — Na+, Cl− and urea diffuse back out of the blood while water diffuses back in. Because blood exchanges solutes and water with the surrounding tissue at every level on the way down and again on the way up, it leaves the medulla close to normal osmolarity without ever flushing away the gradient the loop of Henle worked to build. …