Quick Bio Question

[ajumobim]

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If urea is a waste product, then why would we want it to be permeable in the ascending loop of Henle? Wouldn't this mean that it would diffuse out and eventually be reabsorbed into the blood?

 

[sazerac]

Urea exits the collecting duct, travels through the interstitium, and then renters at the loop of henle, for another trip around the cycle. This concentrates the urea and helps push out other more valuable ions to be reclaimed, or at least to maintain the concentration gradients. You wind up peeing a very concentrated urea in a tiny amount of water.

This website explains it better than Wikipedia. Check out the section on urea recirculation.

http://en.wikivet.net/Reabsorption_and_Secretion_Along_the_Loop_of_Henle_-_Anatomy_&_Physiology

 

[ajumobim]

Thanks! This helps a lot! @sazerac