Ventricular systole. Artwork showing a section through a human heart and its associated blood vessels during ventricular systole. The heart is composed of four chambers. Two upper chambers, the atria, and two lower chambers, the ventricles. The muscular ventricular walls are shown in red. The left and right side of the heart are separated by the septum. Between each atrium and ventricle, and between the chambers and the major blood vessels, are valves that prevent the backflow of blood. Here, ventricular contraction (ventricular systole) was preceded by ventricular relaxation (diastole, see C016/2873).