Loop of Henle
The reduced volume of fluid from the proximal tubule, still isosmotic to blood, enters the descending limb of Henle's loop. As the fluid passes down the descending limb, it enters re3ions where the tubular walls are surrounded by interstitial fluid of increasing osmotic and Na+ concentration. The walls of the descending limb are permeable to water and salts. So, water leaves and sodium enters the tubular fluid so that as it descends the loop of Henle, the fl lid becomes progressively more concentrated and hyperosmotic to blood. There is also perhaps some inward diffusion of urea from a urea-rich interstitial space. As a result of these events, the tubular fluid, at any given level in the descending limb, maintains isosmotic relationship to the surrounding interstitium.
The tubular fluid from the descending loop of Henle passes through the hair-pin bend of the loop, and enters the ascending limb of Henle's loop. The walls of ascending loop do not permit diffusion of water and salts. However, the cells of these walls, particularly of the deeper parts of the ascending limb, actively transport Na+ outward. Recent studies reveal that it is Cl- which is actively transported out of the ascending limb and Na+ accompanies as the counter-ion. In this part there may also be some active transport of urea out of the tubular fluid. Because of this active transport out of the tubule, the fluid becomes progressively more dilute as it moves up the ascending limb. Since ascending limb is impermeable to water, there is no entry of water into the tubular fluid from the surrounding interstitium. Due to the net withdrawal of solute, the fluid at any given level in the ascending limb is hypoosmotic to the fluid in the interstitium as well as the descending limb.