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The loop of Henle is a part of the nephron in the kidney that is responsible for reabsorbing water and minerals from the urine. The loop consists of a thin section of renal tubule that wraps around a larger diameter section called the medulla. Recall that in the cortex of the kidneys, there is a low solute concentration, however, from the cortex to the medulla, the solute concentration in the kidney tissues increases. This solute concentration gradient is produced by the loop of Henle through the single effect and the flow of fluid; together, these two effects are known as the countercurrent multiplication system.
The single effect refers to how the Na+-K+-2Cl– cotransporters located in the thick ascending limb of the nephron reabsorb ions into the interstitial fluid. These cotransporters remove Na+, Cl−, and K+ from the filtrate as it passes through the thick ascending limb of the Loop of Henle. Therefore, the solute concentration of the filtrate at the beginning of the ascending limb is higher than it is at the end, as solutes are actively moved into the interstitial fluid. In this way, the interstitial fluid concentration increases.
Furthermore, recall that the descending limb is water permeable but impermeable to solutes. Therefore, its concentration will always equilibrate with the interstitial fluid. As the interstitial fluid concentration increases because of the active transport of Na+, Cl−, and K+ from the thick ascending limb, the concentration in the descending limb will increase as well, as water is flowing out of it to equilibrate with the interstitial fluid.
The other factor that contributes to the concentration gradient is the flow of fluid, referring to fluid moving through the renal tubule. Due to the single effect, the filtrate found in the renal medulla is more highly concentrated with solutes than the fluid that initially enters the descending loop of Henle. And, as we’ve already described, the concentration of the fluid decreases as the fluid ascends into the cortex via the ascending loop of Henle. The new fluid entering the descending loop of Henle then equilibrates to the concentration of the interstitial fluid. Concentrating the interstitial fluid will, in turn, cause more water to flow out of the descending limb, concentrating the new fluid in the renal tubule. Again, this concentrated fluid moves to the ascending limb, and this process continues to repeat itself, creating the concentration gradient of the loop of Henle.
In Figure 1, you can see the initial state of the loop of Henle without a concentration gradient. By simply repeating the single effect and the flow of fluid, you can see how the concentration gradient gradually begins to form.