Systems and methods are provided for controlling the diameter of a mammalian hybrid coronary bypass graft. The system includes a controller having at least one input for receiving information and feedback information and an output for outputting control signals, including at least one steady flow system control signal; and a pressure/flow loop subsystem coupled to the controller. The pressure/flow loop subsystem includes a specimen holder, an external flow loop system coupled to the specimen holder, a steady flow system, and an output for outputting the feedback information. The pressure/flow loop subsystem receives the control signals and is capable of adjusting a diameter of a specimen in accordance with the control signals, when the specimen holder contains the specimen.