Final answer:
Systole is the phase of the cardiac cycle where the ventricles contract and build enough pressure to pump blood out of the heart. It includes an initial isovolumic contraction phase followed by the ventricular ejection phase resulting in the stroke volume.
Step-by-step explanation:
Systole refers to the contraction of the ventricles when pressure is high. During the cardiac cycle, systole is the phase where the ventricles contract, raising the pressure within the heart to a point that is greater than that in the pulmonary trunk and the aorta, allowing blood to be pumped out of the heart. There is an initial phase known as isovolumic contraction where the pressure rises but the blood volume in the ventricles remains constant because it is not sufficient to open the semilunar valves. Eventually, during the ventricular ejection phase, enough pressure is built up to open these valves, and blood is ejected, which is then referred to as the stroke volume.