The signals that originate from the human cardiovascular system were analysed using methods of both linear and nonlinear system theory. The analyses in the time domain and in the phase space revealed the deterministic and almost conservative nature of the cardiovascular control system on the time scale of minutes. Five characteristic frequencies were found in the signal of peripheral blood flow. Some of them had already been found in other cardiovascular functions, the breathing, blood pressure, ECG and HRV. Each characteristic peak reflects the periodic action of one of the subsystems, involved in the regulation of the blood flow. These systems are mutually dependent, via the couplings among them. Their strength plays an essential role in the performance of the system. In some cases their importance is indicated, thus pointing to practical applications in the diagnosis and prediction of cardiovascular functions.