A sensing and storage system for fluid balance during dialysis is provided. The sensing and storage system has flow sensors on either side of a dialyzer in a controlled volume dialysate flow path. The sensors are positioned so that no fluid can be added to or removed from the dialysate flow path between the sensors except for that which is added or removed by action of a control pump. The sensing and storage system can have a fluid removal line for the removal of fluid from the dialysate flow loop.