Counter Current Exchange

Counter Current Exchange (multiplier)    The countercurrent mechanism depends on the special anatomical arrangement of the loops of Henle and the vasa recta, the specialised peritubular capillaries of the renal medulla Paralleling the long loops of Henle are the vasa recta, which also loop down into the medulla before returning to the renal cortex The collecting ducts also play a critical role in the countercurrent mechanism Countercurrent multiplier system in the loop of Henle produces a hyperosmotic renal medulla: (Numerical values are in milliosmoles per litre) 1) Assume that the loop of Henle is filled with fluid with a concentration of 300 mOsm/L, the same as that leaving the proximal tubule 2) The active pump (uses ATP to transport Na/K/Cl) of the thick ascending limb on the loop of Henle is turned on, reducing the concentration inside the tubule and raising the interstitial (medulla) concentration; this pump establishes a 200-mOsm/L concentration gradient between tubular fluid and interstitial fluid 3) Tubular fluid in the descending limb of the loop of Henle and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. 4) Additional flow of fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb. 5) Once this fluid is in the ascending limb, additional ions are pumped into interstitium, leaving water behind 6) The fluid in the descending limb again reaches equilibrium with hyperosmotic medullary interstitial fluid 7) This gradually traps solutes in the medulla and multiplies the concentration gradient established by active pumping of ions out of the thick ascending loop of Henle, eventually raising the interstitial fluid osmolarity Countercurrent exchange in the vasa recta    The countercurrent exchange in the vasa recta preserves hyperosmolality of the renal medulla o Blood flow must be provided to the renal medulla to supply the metabolic needs of the cells in this part of the kidney. Without a special medullary blood flow system, the solutes pumped into the renal medulla by the countercurrent multiplier system would be rapidly dissipated Blood enters and leaves medulla by way of the vasa recta at the boundary of the cortex and renal medulla. Vasa recta, like other capillaries, are highly permeable to solutes in the blood, except for plasma proteins. As blood descends into the medulla toward the papillae, it becomes progressively more concentrated, partly by solute entry from interstitium and partly by loss of water into the interstitium. By the time blood reaches the tips of the vasa recta; its concentration is about 1200 mOsm/L, the same as that of the medullary interstitium. As blood ascends back toward the cortex, it becomes progressively less concentrated as solutes diffuse into the medullary interstitium and water moves into the vasa recta (essentially opposite movements of the descending part of this loop). Thus, the vasa recta do not create the medullary hyperosmolality, but they do prevent it from being dissipated They prevent it from being dissipated because normally a capillary would just take in the solute due to the concentration gradient, which is what is happening in the descending part of this loop. However, due to the countercurrent exchange, it puts the solute back into the interstitium due to the concentration gradient (there is now a higher osmolality in the ascending part of the loop than the interstitium) and thus does not disrupt the hyperosmotic medulla, which is necessary for other functions  Note however that tiny imperfections of the system cause the final osmolality in the vasa recta to be slightly higher than the initial osmolality coming into the loop