Systole is the phase of the cardiac cycle when the heart muscles contract, pumping blood out of the heart. This process occurs in both the atria and ventricles, with the ventricles contracting to send blood to the lungs and the rest of the body. During systole, the heart valves close to prevent backflow, ensuring that blood moves efficiently. The duration and strength of systole are crucial for maintaining proper blood pressure and circulation, which are vital for overall health.