Traditionally, the functional development, testing and verification of higher level automation, control and safety functions in extracorporeal circulations systems relies very much on the conduction of animal experiments. This especially applies to functions requiring interaction with the patients physiology. Besides ethical aspects, animal experiments have the disadvantage of higher complexity, limited reproducibility and certain pathological conditions can only be represented with difficulty. An alternative to this can be Hardware-in-the-loop (HIL) simulators.In this paper we present a HIL solution to simulate the hydraulic interaction between the cannula and the connected blood vessel. We derive a hydraulic model for an elastic vessel with particular emphasis on negative transmural pressure. In this case the resistance is mainly determined by collapse phenomena. The next section describes the design of an universal resistance actuator that can simulate vascular resistances in the expected range. Combined in the HIL simulator, the simulation model then generates the setpoint for the actuator while simultaneously receiving the resulting internal states of the hydraulic interface. This creates a truly interactive HIL simulator where the device under test interacts in the same way as with a physiological system.