Diastole is the phase of the cardiac cycle when the heart muscles relax after contraction. During this time, the chambers of the heart, specifically the atria and ventricles, fill with blood. This process is essential for maintaining proper blood circulation throughout the body. In humans, diastole occurs after systole, which is when the heart pumps blood out. The duration of diastole can vary depending on heart rate, with slower rates allowing for longer filling times. Proper diastolic function is crucial for overall heart health and efficient blood flow.