Describe the countercurrent multiplication phenomenon in juxtamedullary nephrons and its contribution to water conservation in the kidneys
What will be an ideal response?
The descending limb (DL) of the loop of Henle is always permeable to water but not to solutes, whereas the ascending limb (AL) transports ions from the filtrate into the surrounding interstitial fluid (IF) but is not permeable to water. As the AL pumps more solutes into the IF, the IF becomes hypertonic, which causes water to leave the DL and enter the IF via osmosis. As a result, the tubular filtrate's osmolarity increases before entering the AL. Now, the higher osmolarity in the AL causes the AL to transport more solutes into the IF. This, in turn, further increases the osmolarity of the IF around the DL, causing even more water to leave the DL through osmosis. Now, the tubular filtrate's osmolarity increases even more, and the cycle continues, resulting in more water reabsorption from the DL; i.e., more water conservation.
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