The theory is based on the fact of existence of natural conditions in large blood vessels which are responsible for formation of a specific pattern of blood flow, where alternating ring-shaped layers of blood elements and plasma are found. This specific blood pattern is capable of significantly reducing friction losses in blood flow both inside the blood vessels and at their walls. The predetermined anatomy of the heart and its associated blood vessels, and, in particular, the predetermined function of aorta expanding, allows maintaining the said blood pattern generated in the ascending aorta. The generation and maintenance of such flow pattern is provided by the phase operation of the heart. It should be noted that every heart cycle contains ten phases. Every phase can be recorded on an ECG curve. In doing so, it is possible to calculate blood volumes in every heart cycle phase by measuring durations of the respective phases in a heart cycle and using the actually measured times in the equations by Poyedintsev - Voronova. If a Rheogram is recorded simultaneously with an ECG, then, in addition, actual functioning of some individual segments of the cardiovascular system can be evaluated.