The action of the heart includes contractions and relaxations of the atria and ventricles. The contraction of the heart is called systole and relaxation is called diastole. The dynamics of blood flow in blood vessels are no exception and blood flows through the blood vessels along a pressure gradient, always moving from higher to lower pressure areas.  The movement is also facilitated due to the development of suction pull, as a result of the pressure difference between the inferior vena cava and right atrium.