The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It includes two main phases: systole, when the heart muscles contract to pump blood out of the heart, and diastole, when the heart muscles relax and fill with blood. This cycle ensures that oxygen-rich blood is delivered to the body and that carbon dioxide is removed. During systole, the atria contract first, pushing blood into the ventricles. Then, the ventricles contract, sending blood to the lungs and the rest of the body. In diastole, the heart chambers fill with blood, preparing for the next heartbeat. This rhythmic process is essential for maintaining proper circulation.