. Explain Counter Current Mechanism
Countercurrent mechanism, involves the nephron loops, particularly of the juxtamedullary nephrons, and reveals how the medullary interstitial fluid becomes so hypertonic. The descending and ascending limbs of the U-shaped structures lie parallel and are very close to one another. This mechanism is named partly for the fact that fluid moving down the descending limb creates a current that is counter to that of the fluid moving up in the ascending limb.
The different parts of the nephron loop have important functional differences. For example, the epithelial lining in the thick upper portion of the ascending limb thick segment is relatively impermeable to water. However, the epithelium does actively reabsorb sodium and chloride ions some potassium is actively reabsorbed as well. As these solutes accumulate in the interstitial fluid outside the ascending limb, it becomes hypertonic, while the tubular fluid inside becomes hypotonic because it is losing its solute.
In contrast, the epithelium of the descending limb thin segment is quite permeable to water, but relatively impermeable to solutes. Because this segment is surrounded by hypertonic fluid created by the ascending limb so water tends to leave the descending limb by osmosis. Thus, the contents of the descending limb become more and more concentrated, or hypertonic.
This very concentrated tubular fluid now moves into the ascending limb, and sodium chloride NaCl is again actively reabsorbed into the medullary interstitial fluid, raising the interstitial NaCl concentration even more. With the increased interstitial fluid solute concentration, even more water diffuses out of the descending limb, further increasing the salt concentration of the tubular fluid. Each time this circuit is completed, the concentration of NaCl increases, or multiplies. For this reason, the mechanism is called a countercurrent multiplier. In humans, this mechanism creates a tubular fluid solute concentration near the tip of the loop that is more than four times the solute concentration of plasma.