Final answer:
During systole, the heart contracts, particularly the ventricles, to pump blood out into the body. Systole is part of the cardiac cycle, alternating with diastole when the heart muscle relaxes and fills with blood.
Step-by-step explanation:
During systole, the heart experiences a period of contraction where the ventricles contract to pump blood out into the circulation. This phase contrasts with diastole, a period of relaxation when the heart chambers fill with blood.
The cardiac cycle includes both systole and diastole, encompassing the rhythmic contraction and relaxation of the heart muscle. Specifically, after the atria relax during atrial diastole, the ventricular systole commences, pushing blood out of the heart and into the body and lungs. This process is driven by the electrical stimulation of the heart, starting from the sinoatrial (SA) node, passing through the atrioventricular (AV) node, and down the bundle of His into the Purkinje fibers, ultimately causing the powerful contraction of the ventricles.