A blood separation system (10) is described. The system comprises a device (14, 18) which, in use, operates to separate whole blood into red blood cells and plasma, a whole blood inlet line (290) for conveying a flow of whole blood into the device, a plasma collection line (292) for conveying a flow of plasma from the device, and a red blood cell collection line (294) for conveying a flow of red blood cells from the device. A first sensing assembly (334) including a first sensor in the plasma collection line is provided which in use operates to detect a characteristic of plasma in the flow of plasma and to generate a first output. A second sensing assembly (336) including a second sensor in the red blood cell collection line is provided which in use operates to detect a characteristic of red blood cells in the flow of red blood cells and to generate a second output. A controller (16) operates to control flows in the plasma collection line and the red blood cell collection line based upon the first and second outputs.