A method comprising recirculating a patient's blood as a fluid through an extracorporeal circuit having a venous cannula line for fluid flow from the patient to an oxygenator and an arterial cannula line for fluid flow from the oxygenator to the patient； and adjusting the volume of fluid flow from the venous cannula line to the arterial cannula line by selectively redirecting fluid flow between the venous cannula line and the arterial cannula line into or out of a closed accumulator blood bag of substantially transparent, bio-compatible material, having a selectively open and closable accumulator inlet line from the venous cannula, and a selectively open and closable accumulator outlet line to the oxygenator.
