After the introduction of ballistocardiography (BCG), there were various approaches and procedures with partly very different results in measurements and in the evaluation of the measurement results. Thus, BCG could not prevail against ECG. In the meantime, the focus has turned away from diagnostic to monitoring approaches and thus BCG, which derives mass shifts from cardiac and respiratory activity, is used in many new systems. To support the development of BCG based systems, a dedicated approach to modeling the cardiovascular and respiratory system including numerical values for the parameters and evaluation methods will be presented.