Sure your going to be recirculating the basic blood stream derived replenisment fluid and if your system is full of any toxin then that will also continue to be readded with each draught.
But read up on this from the advised required readings(R) or from all the literature on UT and youll find that toxins are designed to leave via the colon not the bladder.
Also many feel that the recircualtion informs the body of certain facts (im personally not so sure about that w re to metal tox, due to the qualities of the toxin and the interface they have w our system and its ability to relieve such toxins vs storeing them) and this is why some medical researchers feel ut is so effective there.
BTW one informative text advises no grains, no greens, no dairy when doing UT and ive felt re the greens part of this advise may be on account of the their ability to remove toxins too fast.