Figure 49.4 The Cardiac Cycle (A) The rhythmic contraction (systole) and relaxation (diastole) of the ventricles is called the cardiac cycle. (B) During diastole, the ventricles fill and pressure is low. During systole, the ventricular pressure rises and the ventricles empty. The representation here shows pressure and volume changes for the left ventricle only. The dashed lines represent the maximum and minimum volumes of blood in the left ventricle.