Osmoregulation and the Counter-current Mechanism

Osmoregulation is the maintenance, within a narrow and fairly constant range, of the osmotic concentration (the total concentration of dissolved solutes) of the body's fluids, despite the fact that the amount of water and salt taken in with food and drink, and lost through sweat, breath and other routes, varies considerably from hour to hour and day to day. In humans this task falls chiefly to the kidney, which achieves it by continuously adjusting exactly how much water, and how much of various dissolved ions, it reabsorbs back into the blood versus how much it allows to be excreted in the urine — producing urine that may be either dilute (when the body has surplus water to get rid of) or strongly concentrated (when water must be conserved), while keeping the osmotic concentration of the blood itself essentially unchanged either way. The kidney's remarkable ability to produce urine considerably more concentrated than blood plasma — something the filtration process alone, described earlier, cannot by itself explain — depends on a special structural arrangement in the medulla known as the counter-current mechanism.

The counter-current mechanism operates through the combined action of the loop of Henle and the vasa recta, both of which run as long hairpin-shaped structures deep into the medulla and then back out again, so that within each structure fluid flows in one direction going in and in the exact opposite direction coming back out — hence 'counter-current'. It works because the two limbs of the loop of Henle are immediately adjacent to each other yet have very different properties, as described in the previous section: the thick ascending limb actively pumps Na+ and Cl- ions out of the tubular fluid into the surrounding interstitial tissue while remaining impermeable to water, so with each unit of salt pumped out, the tissue fluid around it becomes a little saltier without being diluted again by any accompanying water. The descending limb, running immediately alongside, is freely permeable to water but not to salt, so as fluid moves down through this increasingly salty surrounding tissue, water is drawn passively out of the descending limb into the interstitium, concentrating the fluid remaining inside the tubule. Because the two limbs run in opposite directions rather than the same direction, a small, repeatable difference in concentration set up between them at every level along the loop is progressively multiplied along its whole length — a process called counter-current multiplication — so that a very steep overall osmotic gradient builds up, from roughly 300 milliosmoles per litre near the cortex (close to the concentration of ordinary blood plasma) to as much as 1200-1400 milliosmoles per litre deep in the inner medulla, several times more concentrated than blood.

This steep gradient would quickly be washed out and lost if the blood supplying the medulla simply flowed straight through it in one direction, carrying the accumulated salt away; the vasa recta prevents this by running as a similar hairpin loop immediately alongside the loop of Henle, with blood flowing down into the medulla in one limb and back up out of it in the other, in each case running counter to (opposite to) the flow in the adjacent limb of the loop of Henle. As blood in the descending part of the vasa recta moves deeper into the increasingly salty medulla, it picks up salt and loses water into the tissue, but as it then turns and moves back up through the ascending part, it re-absorbs a corresponding amount of water and loses salt back into the tissue — so that blood leaving the vasa recta carries away the water and solutes the loop of Henle and collecting duct have reclaimed from the filtrate, but does so without significantly diluting or flushing out the gradient itself, acting as a 'counter-current exchanger' rather than a multiplier. …