Q.Describe the counter-current mechanism operating in the loop of Henle and the vasa recta, and explain how it helps the kidney produce concentrated urine.

[!TLDR]

The ascending limb pumps out salt (impermeable to water); the descending limb loses water into that same salty tissue; opposite-direction flow multiplies this into a steep gradient; the vasa recta preserves it; the collecting duct then uses it (with ADH) to concentrate urine. …

The counter-current mechanism depends on the loop of Henle's two limbs running immediately alongside each other but flowing in opposite directions, combined with their sharply different permeabilities. The thick ascending limb actively pumps Na+ and Cl- out of the tubular fluid into the surrounding medullary interstitium while remaining impermeable to water, so with each unit of salt pumped out, the tissue fluid around it becomes progressively saltier without being diluted 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 on its way to the hairpin bend, water is drawn passively out into the interstitium, concentrating the fluid remaining inside the descending limb.

Because the two limbs carry fluid in opposite directions (down the descending limb, up the ascending limb) 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 the loop's whole length -- a process called counter-current multiplication -- building a very steep overall osmotic gradient from roughly 300 milliosmoles per litre near the cortex up to as much as 1200-1400 milliosmoles per litre deep in the inner medulla.

This steep gradient would ordinarily be washed away by the blood supplying the medulla, but the vasa recta -- a similar hairpin-shaped capillary loop running alongside the loop of Henle, with blood flowing down into the medulla in one limb and back up in the other, in each case counter to the flow in the adjacent limb of the loop of Henle -- picks up the water and solutes reabsorbed from the tubule without significantly diluting or flushing out the gradient, acting as a counter-current exchanger rather than a multiplier. …

Explain the mechanism in three linked stages: (1) the two limbs' contrasting permeabilities set up a small concentration difference at every level, (2) opposite-direction flow multiplies this small difference into a large overall gradient along the loop's length, (3) the counter-flow …

Describing only ONE limb's action (e.g. only the ascending limb pumping salt) without explaining how the opposite-direction flow of the adjacent limb MULTIPLIES this into a large gradient -- the counter-c …