The atrial contraction is of little importance at rest but assumes much greater importance during exercise when diastole is shortened by up to 25%.
Note that during heavy exercise systole can shorten by up to 50% and diastole can shorten by up to 80%.

2. EJECTION STAGE

As atria relax and ventricles start to contract (QRS) ventricular pressure rises above atrial pressure and the mitral valve closes (AVO).