A blood extraction and reinfusion flow control system includes a blood pump coupled to pump intravenous fluid to or from a subject, a pressure sensor coupled to sense extra veinous pressure at the subject, and a microprocessor based flow control subsystem. The flow control subsystem optimizes the flow rate by sensing pressure at at least two different fluid flow test points and extrapolating and translating the test point data to generate a flow control curve. The blood pump is then operated at a maximum speed that is limited by a desired maximum flow command and the flow control curve to maximize flow without vein collapse or damage during both blood extraction and reinfusion.