Counter-current Mechanism — Loop of Henle Multiplier, Vasa Recta Exchanger and Urea Recycling

The kidney can excrete urine as dilute as 50–100 mOsm/kg or as concentrated as about 1200 mOsm/kg. The ability to concentrate depends on a hyperosmolar renal medulla, and that gradient is generated and preserved by structures that run in counter-current — two limbs of a hairpin carrying fluid in opposite directions side by side.

• Counter-current multiplier — the loop of Henle, which creates the corticomedullary osmotic gradient.
• Counter-current exchanger — the vasa recta, which preserves the gradient while still supplying blood to the medulla.
• Urea recycling — urea reabsorbed from the inner medullary collecting duct adds to the inner medullary osmolality.
• ADH (vasopressin) — makes the collecting duct permeable to water so that urine equilibrates with the hypertonic interstitium.

The concentrating mechanism makes sense only against the background of what each segment does to the filtrate. The proximal tubule does the bulk work; the loop sets up the gradient; the distal nephron fine-tunes; the collecting duct decides the final concentration under ADH.

The multiplier depends on two opposite properties of the limbs. The thick ascending limb actively pumps NaCl into the interstitium through NKCC2 but cannot let water follow because it has no aquaporins. The thin descending limb is the reverse: water leaves freely through AQP1 while solutes stay inside. So the fluid going down equilibrates with an interstitium that the fluid coming up keeps loading with salt.

1. Single effect — at any one horizontal level, active NaCl transport out of the thick ascending limb creates a small osmotic difference (about 20 mOsm/kg in the classic model) between the ascending limb and the surrounding interstitium/descending limb.
2. Flow — fresh isotonic fluid from the proximal tubule pushes the now-concentrated descending-limb fluid deeper around the bend.
3. Repetition — the single effect is repeated at every level as the fluid keeps moving. Because the limbs run in opposite directions, the small transverse difference is multiplied axially along the length of the loop.
4. Result — a gradient from about 300 mOsm/kg at the corticomedullary junction to about 1200 mOsm/kg at the inner medulla (papillary tip).

The outer medullary gradient is made mostly of NaCl, driven by active transport in the thick ascending limb. In the inner medulla there are no thick limbs, and the gradient is made of NaCl plus urea. How the inner medulla generates its single effect (the 'passive mechanism' of Kokko-Rector and Stephenson) is still debated, but the exam answer is simple: the thick ascending limb is the engine, and urea is the inner-medullary helper.

The medulla needs blood, but ordinary capillaries would simply wash the solute gradient away. The vasa recta solve this by running as hairpin loops (descending and ascending vasa recta) parallel to the loops of Henle. They are freely permeable to water, urea and sodium, so at each level the blood equilibrates with the interstitium.

Solute is therefore recirculated within the medulla rather than carried away, and blood returning to the cortex has an osmolality close to plasma. The exchanger is passive — it does not create the gradient, it only prevents it from being dissipated. It also removes the water reabsorbed from the descending limb and collecting duct.

In the inner medulla, NaCl and urea are the two major constituents of the osmotic gradient. As water is reabsorbed under ADH, urea is concentrated inside the collecting duct lumen. When the fluid reaches the terminal inner medullary collecting duct (IMCD), urea leaves through the urea transporters UT-A1 (apical) and UT-A3, raising interstitial urea. Urea transporters are also found in the thin descending limb (UT-A2) and the descending vasa recta (UT-B), so urea is handed back and forth within the medulla instead of being lost — hence 'recycling'.

• Vasopressin stimulates UT-A1 — it increases phosphorylation and apical membrane accumulation of UT-A1 and UT-A3, so ADH raises both water and urea permeability of the IMCD.
• Hyperosmolality also increases urea permeability (via PKC and calcium).
• Mice lacking UT-A1/UT-A3 have reduced urine concentrating ability and lower inner medullary urea — proof that urea recycling is part of the mechanism.
• Urea is normally the predominant urinary solute during strong antidiuresis.

Arginine vasopressin acts on V2 receptors on the basolateral membrane of principal cells in the late distal tubule and the whole collecting duct. V2 is coupled through a G protein to cAMP, which drives insertion of aquaporin-2 (AQP2) water channels into the apical membrane of this otherwise water-impermeable segment. Water then moves out into the hypertonic medulla and the urine becomes concentrated.

In the absence of vasopressin, all collecting-duct segments are nearly water-impermeable (the terminal IMCD keeps a modest basal permeability), so the dilute fluid leaving the thick ascending limb and distal tubule is excreted as dilute urine. Vasopressin also increases NKCC2 transcription in the thick ascending limb, strengthening the gradient itself.

Because NKCC2 is the engine of the multiplier, blocking it with a loop diuretic (furosemide, bumetanide, torsemide) stops Na+, K+ and 2Cl- reabsorption in the thick ascending limb, reduces interstitial tonicity and thwarts the counter-current mechanism. The kidney can then neither concentrate nor fully dilute urine.

• Which limb is water-permeable? Only the thin descending limb. Both ascending limbs and the distal convoluted tubule are water-impermeable.
• Where is tubular fluid most dilute? In the thick ascending limb and early distal tubule — not in the collecting duct, whose final concentration depends on ADH.
• Where is it most concentrated? At the bend of a long loop and, under ADH, in the final urine of the papillary collecting duct.
• Exchanger vs multiplier — vasa recta exchange passively and do not create the gradient; the loop creates it with active transport.
• Maximum urine osmolality in humans is about 1200 mOsm/kg; minimum is about 50–100 mOsm/kg.
• Disorders that damage the medulla or reduce NaCl reabsorption in the medullary thick ascending limb impair the counter-current mechanism and can cause nephrogenic DI.