A biomedical apparatus for pumping blood of a human or an animal patient through a secondary blood circuit is provided, comprising a blood pump (10), an inlet duct (11) and an outlet duct (12) for guiding blood of the patient to the blood pump (10) and back to the patient. The device further comprises a measuring device (14) for measuring a physical parameter of the heart (2) or of a blood vessel, and a controller (13) for regulating the power of the blood pump (10). The measuring device (14) is adapted to be arranged inside the heart (2) or the blood vessel, and the controller is configured to determine an estimate for an inner volume of the heart (2) or of the blood vessel based on the physical parameter and is configured to regulate the power of the blood pump (10) depending on this estimate.
