Kidney

Can someone please explain in english how the loop of Henle concentrates urine. I have listened to the AO portion of this several times and I am still confused.
I understand that water leaves the loop in the thin descending limb thereby concentrating the solute. But if the ascending limbs are impermable to water and actively reabsorbes solute how does it get more dilute?

Gabe - this may help - give it a read
http://www.liv.ac.uk/~petesmif/teaching/1bds_mb/notes/kidney/loop.htm

Thanks!!! I am sure that it will.