While "cleansing an organ" is hogwash, there is minor evidence that yoga may be better than no exercise for patients in dialysis. This study is really small, not replicated but at least a blind study. It does not compare yoga and conventional exercise, so I would be very careful and wait until further confirmation and better studies, but:
> Improvement of the variables in the yoga-based exercise program was found to be superior to that in the control group for all the variables except calcium, phosphorus, HDL-cholesterol and triglyceride levels.
--A modified yoga-based exercise program in hemodialysis patients: A randomized controlled study00073-2/abstract)
Most of the other variables are nervous system related, which is not surprising since Yoga has well known positive effects on it.
This full PDF article gives a good overview of the benefits of yoga.