A control system is provided to restrict the flow rate of blood in a venous line 12 of a perfusion system 1 comprising a reservoir 10 supplied by the venous line 12 and supplying an outgoing line 22. The control system comprises an outgoing flow sensor 32 configured to determine an outgoing flow value indicative of the outgoing flow rate in the outgoing line 22, and a controller configured to process the outgoing flow value and to determine if the outgoing flow value exceeds a pre-set pairing threshold. The system further comprises an adjustable restriction 28 for restricting the flow rate in the venous line 12 to maintain a venous flow rate that does not exceed a venous restriction threshold, wherein the adjustable restriction 28 is responsive to the controller and wherein the controller comprises a configuration allowing the controller to set the adjustable restriction at the level of the outgoing flow rate, to change the venous restriction threshold with the outgoing flow rate, if the outgoing flow value is above the pre-set pairing threshold.