Diastole is the period of time when the heart fills with blood after systole. Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing. The term diastole originates from Greek and means dilation.
During ventricular diastole, the pressure in the left and right ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, causing accumulated blood from the atrium to flow into the ventricle.