A control system to restrict the flow rate of blood in a blood line 12 in which blood is permitted to flow from an inlet towards an outlet, wherein the control system comprises a first flow sensor 26,32 to determine a first flow value indicative of the flow rate 18 in the blood online 12. A controller configured to process the first flow rate, and an adjustable restriction 28 responsive to the controller, wherein the adjustable restriction 28 is configured to reduce the flow rate 18 in the blood line 12 to maintain a flow rate that does not exceed a restriction threshold. The adjustable restriction comprises a gradually actuatable occlusive device e.g. clamp.
