1. A blood processing system comprising: a blood separation device for separating whole blood into a red blood cell component and a plasma component; an inlet flowpath including a pump operable at a prescribed rate Q WB to convey whole blood from a patient having a whole blood volume V, a whole blood hematocrit value H T into the separation device; a return flowpath to convey fluid to the patient; a first outlet flowpath including a pump operable at a prescribed flow rate Q P to convey separated plasma from the separation device; a second outlet flowpath to convey separated red blood cells from the separator; a source of replacement red blood cells having a hematocrit H T1 in fluid communication with the return flowpath; a third flowpath including a pump operable at a prescribed flow rate Q RF to convey replacement red blood cells to the patient; and a controller including a first stage operable to accept or derive input values relating to the patient condition and clinical objectives; a second stage operable to derive operating values for the for Q WB , Q P , and Q RF such that the clinical objective are achieved simultaneously at the end of the procedure; and a third stage operable to maintain and or adjust these values in accordance with changing procedural conditions.