The osmotic equilibration of the collecting duct luminal fluid with the hypertonic medullary interstitium results in the formation of concentrated urine. The inner medulla uses countercurrent multiplication to achieve urinary concentration. Active NaCI absorption from the thick ascending limb is the only action in the outer medulla. While the existence of a single effect in the inner medulla has yet to be proven, the majority of experimental evidence points to passive NaCI absorption from the thin ascending limb. To properly understand the process of urine concentration, more experimental studies in inner medullary nephron segments will be required.
Concurrent flow
The solutions in both tubes flow in the same direction in this case. If one begins with a concentration of 0 percent while the other begins with a concentration of 100 percent. As illustrated in the diagram, by the time they reach the opposite end of the tubes, the concentrations in each tube will be around 50%.
Countercurrent flow
In this case, the two tubes’ solutions flow in opposing directions. The solution in one tube begins to flow at 0% concentration from one end, while the solution in the second tube begins to flow at 100% concentration from the opposite end. Because the contents freely circulate between the two tubes, by the time the solutions reach the tube’s end it will have acquired a concentration equal to the other tube at that end. This will become clear from the figure.
MECHANISM:
Step 1: Assume that the Henle loop is filled with a concentration of 300mOsm/L, which is the same as the concentration leaving the proximal tubules.
Step 2: The thick ascending limb of the Henle loop’s active ion pump lowers the concentration inside the tubule and raises the interstitial concentration.
Step 3: Due to osmosis of water out of the descending limb, the tubular fluid in the descending limb and the interstitial fluid quickly approach osmotic equilibrium.
Step 4: Additional fluid flow from the proximal tubule into the Henle loop, causing hyperosmotic fluid previously generated in the descending limb to flow into the ascending limb.
Step 5: More ions are pushed into the interstitium while water remains in the tubular fluid, resulting in a 200-mOsm/L osmotic gradient.
Step 6: As the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, additional solute is constantly pushed out of the tubules and deposited into the medullary interstitium.
Step 7: These stages are repeated over and over, with the net result of bringing more and more solute to the medulla in excess of water. Over time, this process traps solutes in the medulla and magnifies the concentration gradient generated by active pumping of ions out of the thick ascending limb. Eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L .
COUNTERCURRENT EXCHANGER IN VASA RECTA:
In the Vasa recta, there is a countercurrent exchanger.
Because of the diffusion of water molecules out of the blood and solutes from the renal interstitial fluid into the blood, plasma travelling down the descending limb of the vasa recta becomes increasingly hyperosmotic. Solute molecules diffuse back into the interstitial fluid in the ascending limb of the vasa recta, and water diffuses back into the vasa recta. Because of the U-shape of the vasa recta capillaries, which operate as a countercurrent exchanger, there is a small scale net dilution of the concentration of the interstitial fluid volume at each level of the renal medulla, despite the vast amount of fluid and solute exchange across the vasa recta. As a result, while the vasa recta does not cause hyperosmolarity in the medulla, it does prevent it from dissipating.
HENLE’S LOOP:
Henle’s Loop is a loop created by Henle.
The most important factor in the generation of concentrated urine is the presence of a steep osmotic gradient in the renal medullary interstitium. Counterflow systems are formed by the architectural organisation of the renal tubules and blood arteries in the medulla, which are necessary for both creating and sustaining a high osmotic pressure in the renal medulla.While it is widely accepted that active NaCl transport in Henle’s loop’s thick ascending limb plays the most important role in the operation of the countercurrent multiplication system in the renal medulla, whether the thin ascending limb (tAL) also has an active salt transport system to provide the “single effect” required for the countercurrent multiplication system’s operation is still a point of contention.
ROLE OF UREA:
Several studies have mentioned the idea of urea recycling. Urea is absorbed from the inner medullary collecting duct and secreted into the thin ascending limb, where it stays until it returns to the inner medullary collecting duct through the tubule lumen. In protein-deficient animals, maximum concentrating ability is reduced, but urea restores it. As a result, the passive process, which is dependent on adequate urea supply to the inner medulla, explains the well-known relevance of urea in concentrating ability. We now have a better grasp of urea absorption across the inner medullary collecting duct thanks to a number of recent research. In terminal inner medullary collecting ducts, urea transport is facilitated by a vasopressin-stimulated mechanism. As the urea concentration in the terminal lumen rises, When the concentration gradient in the inner medullary collecting ducts exceeds the concentration gradient in the vasa recta, urea is rapidly absorbed into the inner medullary interstitium, down the concentration gradient.
CONCLUSION:
A countercurrent mechanism is a system that uses energy to generate a concentration gradient. It can, for example, refer to the process that underpins urine concentration, namely, the mammalian kidney’s creation of hyperosmotic urine. In this chapter we have discussed counter current mechanism, Henle’s loop , role of Urea etc.