PROBLEM TO BE SOLVED: To provide a blood processing system for collecting and exchanging blood components, the blood processing system creating no lag times when returning unwanted blood components to the donor, thereby minimizing the total processing time.SOLUTION: A blood processing system comprises: a venous access device for drawing blood from a subject and returning blood components to the subject; a blood draw line for fluidly connecting the venous access device to a blood component separation device; and a return line separate from an anticoagulant line and the blood draw line. A draw pump, an anticoagulant pump, and a return pump control the flow through the blood draw line, the anticoagulant line, and the return line, respectively. The blood component separation device can separate the drawn blood into a first blood component and a second blood component, and send the first blood component to a bag. The return line is used for returning uncollected blood component to the donor, and fluidly connected to the venous access device and an inflow port of the blood component separation device.