Last phase of the diastole, the atria is essentially empty but still a little blood to deliver. The atria contracts expelling the remainder of the blood into the ventricles. The ventricles now have a huge increase of pressure which forces the AV valve leaflets, which have been hanging down in the ventricles are now pushed up and slammed shut, ending the diastole. S1 is heard at this time. This kick provides 15%-30% ventricular filling and very important. Some heart rhythm abnormalities cause a loss of the atrial kick. This causes an immediate decrease in cardiac output.