Countercurrent Multiplier and Exchanger Multiplication 1. Symporters in thick ascending limb cells of the loop of Henle cause a buildup of Na and Cl in the renal medulla In the thick ascending limb of the loop of Henle, the Na +/K+/2Clsymporters reabsorb Na and Cl from the tubular fluid. The symporters establish a 200mOsm gradient. Water is not reabsorbed in this segment because the cells are impermeable to water. Thus, there is a build-up of Na and Cl ions in the interstitum of the medulla. 2. Countercurrent flow through the descending and ascending limbs of the LoH establishes an osmotic gradient in the renal medulla. As urine continues to flow through the LoH, the TAL cells continue to produce the 200mOsm gradient by pumping Na and Cl into the interstitum. This eventually results in the production of an osmotic gradient it the medulla ranging from 300mOsm/L in the outer medulla to 1200mOsm/L in the inner medulla. 3. Cells in the collecting ducts reabsorb more water and urea. When ADH increases the water permeability of the principal cells, water quickly moves via osmosis out of the CD tubular fluid, into the interstitial fluid of the inner medulla, and then into the vasa recta. With loss of water, the urea left behind in the tubular fluid of the CD becomes increasingly concentrated. Because duct cells deep in the medulla are permeable to it, urea diffuses from the fluid in the duct into the interstitial fluid of the medulla 4. Urea recycling causes a build-up of urea in the renal medulla. Accumulating urea diffuses into the tubular fluid in the descending and thin ascending limbs of the LoH, which also are permeable to urea. However, while the fluid flows through the TAL, DCT, and cortical portion of the CD, urea remains in the lumen because cells in these segments are impermeable to it. As fluid flows along the CD, water reabsorption continues via osmosis because ADH is present. This water reabsorption further increases the concentration of urea in the tubular fluid, more urea diffuses into the interstitial fluid of the inner renal medulla, and the cycle repeats. The constant transfer of urea between segments of the renal tubule and the interstitial fluid of the medulla is termed urea recycling. In this way, reabsorption of water from the tubular fluid of the ducts promotes the build-up of urea in the interstitial fluid of the renal medulla, which in turn promotes water reabsorption. The solutes left behind in the lumen thus become very concentrated, and a small volume of concentrated urine is excreted. Exchanger Countercurrent exchange is the process by which solutes and water are passively exchanged between the blood of the vasa recta and interstitial fluid of the renal medulla as a result of countercurrent flow. Since countercurrent flow between the descending and ascending limbs of the vasa recta allows for exchange of solutes and water between the blood and interstitial fluid of the renal medulla, the vasa recta is said to function as a countercurrent exchanger. Blood entering the vasa recta has an osmolarity of about 300 mOsm/L. As it flows along the descending part into the renal medulla, where the interstitial fluid becomes increasingly concentrated, Na, Cl, and urea diffuse from interstitial fluid into the blood and water diffuses from the blood into the interstitial fluid. But after its osmolarity increases, the blood flows into the ascending part of the vasa recta. Here blood flows through a region where the interstitial fluid becomes increasingly less concentrated. As a result Na, Cl, and urea diffuse from the blood back into interstitial fluid, and water diffuses from interstitial fluid back into the vasa recta. The osmolarity of blood leaving the vasa recta is only slightly higher than the osmolarity of blood entering the vasa recta. Thus, the vasa recta provides oxygen and nutrients to the renal medulla without washing out or diminishing the osmotic gradient. The long loop of Henle establishes the osmotic gradient in the renal medulla by countercurrent multiplication, but the vasa recta maintains the osmotic gradient in the renal medulla by countercurrent exchange.