Abstract: A flow circuit for recirculating dialyzing liquid through a blood dialyzer includes an airtight bag sealed to conduits for liquid flow between the dialyzer and the bag which is enclosed in a closed container. The container is associated with scale means for weighing the liquid content of the bag and thus indicating the amount of ultrafiltrate extracted from the dialyzed blood and received by the dialyzing liquid. Further, the volume of the container outside the bag is connected with a suction pump operatively disposed to provide suitably reduced pressure within the bag and thus the complete recirculation circuit, with the purpose of achieving predetermined regulation of the extracted amount of ultrafiltrate. The bag and the associated flow conduits may be made of inexpensive plastic materials, which may be discarded after single usage. Thus, sterilization will be superfluous and uninhibited bacterial growth in the recirculation system will be avoided.