Countercurrent multiplier

A countercurrent mechanism system is a mechanism that expends energy to create a concentration gradient.

It is found widely in nature and especially in mammalian organs. For example, it can refer to the process that is underlying the process of urine concentration, that is, the production of hyperosmotic urine by the mammalian kidney. The ability to concentrate urine is also present in birds.

Countercurrent multiplication is frequently mistaken for countercurrent exchange, a similar but different mechanism where gradients are maintained, but not established.

The term derives from the form and function of the loop of Henle, which consists of two parallel limbs of renal tubules running in opposite directions, separated by the interstitial space of the renal medulla.

Countercurrent multiplication was originally studied as a mechanism whereby urine is concentrated in the nephron. Initially studied in the 1950s by Gottschalk and Mylle following Werner Kuhn's postulations, this mechanism gained popularity only after a series of complicated micropuncture experiments.

The proposed mechanism consists of pump, equilibration, and shift steps. In the proximal tubule, the osmolarity is isomolar to plasma (300 mOsm/L). In a hypothetical model where there was no equilibration or pump steps, the tubular fluid and interstitial osmolarity would be 300 mOsm/L as well.{Respicius Rwehumbiza, 2010}

Pump: The Na⁺/K⁺/2Cl⁻ transporter in the ascending limb of the loop of Henle helps to create a gradient by shifting Na⁺ into the medullary interstitium. The thick ascending limb of the loop of Henle is the only part of the nephron lacking in aquaporin—a common transporter protein for water channels. This makes the thick ascending limb impermeable to water. The action of the Na⁺/K⁺/2Cl⁻ transporter therefore creates a hypoosmolar solution in the tubular fluid and a hyperosmolar fluid in the interstitium, since water cannot follow the solutes to produce osmotic equilibrium.

...
Wikipedia