Loop of Henle
Loop of Henle
In the loop of Henle mostly Na+ and Cl- ions and water are reabsorbed. The process of exchanging water and ions is called countercurrent multiplication. This efficient process is possible because of the anatomy of the loop of Henle with a descending and an ascending limb in each other's close vicinity. Na+ and Cl- ions are pumped out of the ascending limb into the peritubular fluid. This leads to a higher osmotic concentration around the descending limb. As a result, water flows out of the descending limb (by osmosis) into the peritubular fluid and thus the solute concentration within the descending limb increases. When this fluid arrives in the ascending limb, Na+ and Cl- ions are pumped into the peritubular fluid.
The carrier in the ascending limb takes Na+, K+ and 2 Cl- ions from the tubular fluid via a co-transporter in the apical membrane. A Cl-/K+-co-transporter and a Na+/K+-ATPase pump transport Na+, K+ and Cl- ions further over the basal membrane into the peritubular fluid. The Na+/K+-ATPase pump also brings K+ back into the cell. Via leak channels, the K+ ions flow back into the tubular fluid. Thus the net result is that Na+ and Cl- ions enter the peritubular fluid. Because the descending limb is permeable to water, but not to solutes and the ascending limb is impermeable to water but active in transport of solutes, a highly efficient concentration gradient is built (see figure above).
The main function of the loop of Henle: