A method for operating a biomedical apparatus for pumping blood of a human or an animal patient through a secondary intra- or extracorporeal blood circuit, wherein the biomedical apparatus comprises a blood pump for pumping blood, an inlet duct connected to the blood pump, for being inserted into a patient's circulatory system, in order to guide blood of the patient to the blood pump, and an outlet duct connected to the blood pump, for being inserted into the patient's circulatory system, in order to guide blood from the blood pump back to the patient's circulatory system, and wherein the method comprises at least the following steps:
- measuring at least one physical parameter of the heart or of a blood vessel of the patient by means of electromagnetic or mechanical waves sent out by a measuring device arranged inside the heart or the blood vessel,
- determining an estimate for an inner volume of the heart or of the blood vessel based on the measured physical parameter, and
- regulating the power of the blood pump depending on the determined estimate.