Countercurrent multiplication

The term countercurrent multiplication refers to the process underlying the process of urine concentration, that is, the production of hyperosmolar urine by the mammalian kidney. The ability to concentrate urine is also present in birds, but involves another mechanism which is not comparable.

Physiological principles
The term dervies from the form and function of Henle's loop, which consists of two parallel limbs of renal tubulus running in opposite directions, separated by the interstital space of the renal medulla.
 * Water flows from the tubular fluid of the descending limb of Henle's loop into the medullary space.
 * The ascending limb is impermeable to water, but here NaCl is actively reabsorbed into the medullary space, effectively diluting the tubular fluid. This constitutes the single effect of the countercurrent multiplication process.
 * Reabsorbtion of salt from the ascending loop creates an osmotic force drawing water from the descending limb into the hyperosmolar medullar space, further concentrating tubular fluid.
 * The countercurrent flow within the descending and ascending limb thus increases, or multiplies the osmotic gradient between tubular fluid and interstitial space.