Heart
When the heart muscle contracts (pushes in) it is called systole. When the heart muscle relaxes (stops pushing in), this is called diastole. Both atria do systole together. Both ventricles do systole together. But the atria do systole before the ventricles. Even though the atrial systole comes before ventricular systole, all four chambers do diastole at the same time. This is called cardiac diastole