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If the TAL is impermeable to water, then what is the meaning of this paragraph:
"Urea which remains in the loop creates a solute potential that prevents water completely osmosing out into the interstitial space. This means that while almost all the ions are reabsorbed, there will still be some water in the urine, and hence, the concentration of the filtrate in the loop is decreased here. (If only ions were present, and a certain amount of ions were reabsorbed, one would expect the same amount of the water to be reabsorbed too, and hence the concentration would remain the same, but this is not true.)" ?
What is the role of Urea here? Horia (talk) 20:10, 25 February 2008 (UTC)
Exactly! :-). That paragraph caught my eye as well, and I came to this discussion page just to see if anybody had asked about it. I can't explain that paragraph except as the result of some confusion, because as you correctly point out, and as the article itself states, the TAL is impermeable to water. I'm going to remove that paragraph. I think the author may have been thinking about the role of urea in maintaining the meduallary concentration gradient – in particular such that lumenal urea is balanced by interstitial urea, so that NaCl need only balance the concentration of solutes other than urea. Also... that paragraph said that "almost all the ions are reabsorbed", which is overstating it. The TAL absorbs about 25% of the filtered salt load, which it must take out of the 35% of filtered load delivered to it (if we assume the proximal tubule has absorbed 65%), which means that the TAL absorbs about 70% of the NaCl it sees. Mark Lundquist (talk) 17:09, 28 September 2008 (UTC)