The numerical model of one-loop circulation exploits viscous deformation as mechanism of ventricular filling. Mathematical advantage of viscous deforming is a possibility to present the ventricular filling as the function of two independent variables (stress and time); two-dimensional argument frames a table which permits to calculate end-diastolic ventricular volume due to information about measured venous pressure and duration of ventricular diastole. The equation was deduced which balances the system while varying of such parameters as arterial resistance, the value of normal rhythm and volume flow rate. The model explains the phenomenon of asymmetrical position of normal value of rhythm, i.e. the model explains why the range of working brady-rhythms is much narrower than the range of working tachy-rhythms.