An emergency alertable gravity feed cardiopulmonary bypass system is disclosed in which a blood volume responsive transducer is utilized in returning oxygenated blood to a human circulatory system at a pumping rate corresponding to venous drainage or selected norms. The transducer is coupled to a standpipe and is responsive to a confined gas volume therein related to the blood volume in a first air evacuable, gravity fed, collapsible bag and is coupled to a rate setting control. Blood flow from the first bag is directed by a control responsive oxygenation pump through a membrane oxygenator and heat exchanger and air evacuable collapsible bag and a main pump before return to the patient. The oxygenation pump rate slaved to the main pump is greater than the gravity feed rate, and a pressure relieving conduit recirculates excess blood flow from the second bag to the first bag. Supervisory control of the flow rate of the main pump may be exercised by manual adjustment of the flow rate.