The countercurrent multiplier system is a sophisticate apparatus that evolved in mammals and birds to con serve water. It forms a longitudinal concentration gradient in the medullary interstitium that increases in strength toward the papilla. This gradient is crucial for water reabsorption from the renal tubules, which is a passive process that depends on osmotic pressure from the interstitium.

The creation and maintenance of this gradient is best understood by first considering a simplified model of the loop of Henle. In this model, a tube of fluid is divided by a membrane in all but its most inferior aspect. The left side represents the entire descending limb, whereas the right side represents the entire ascending limb. Fluid enters at the top of the left-sided column, travels beneath the membrane, and then exits at the top of the right-sided column. The dividing membrane is impermeable to water but contains active transporters, which pump solute from the ascending limb to the descending limb. These transporters are powerful enough to establish a transmembrane gradient of about 200 milliosmoles (mOsm).

In Panel 1, the entire tube is filled with fluid concentrated at 285 mOsm, which is roughly equal to the osmolality of filtrate as it enters the descending limb. A transmembrane gradient is established as the transporters pump solute across the membrane.

In Panel 2, fluid begins to move through the circuit. Thus, at the hairpin turn, concentrated fluid from the descending limb mixes with less concentrated fluid from the ascending limb. As a result, a fluid of average concentration is formed. Because the active transporters can establish a 200 mOsm gradient, the last part of the descending limb becomes correspondingly more concentrated.

In Panel 3, the flow process continues, and as concentrated fluid continues to rise in the ascending limb, reestablishment of the 200 mOsm transmembrane gradient causes a corresponding rise in the concentration of fluid in the descending limb. At this stage, solute is still being retained within the system, and thus the outgoing fluid is less concentrated than the incoming fluid.