Systole is the period of time when the left and right ventricles contract and discharge blood into the aorta and pulmonary artery, respectively. The aortic and pulmonic valves open during systole to allow ejection into the aorta and pulmonary arteries. Ejection occurs because blood pressure is greater in the chambers of the heart than outside of the heart. Thus, the walls of the heart have to stretch to allow the blood to flow out. Diastole is the period between heartbeats when the ventricles are relaxed. The atria relax to allow blood into the ventricles from the lungs and the brain and other tissues receive oxygenated blood. The ventricles then contract to pump this blood out to the body. Diastole also includes the time before the next contraction (systole) begins. The cycle of the heart starts with diastole. During diastole, the muscles of the heart relax so that blood can fill the ventricles; this is where the word "diastolic" comes from. At the end of diastole, the muscles of the heart tighten up again, causing the ventricles to squeeze during systole. This is how blood is pumped out of the heart and into the aorta and other vessels supplying the body. Heart failure is the final stage of many different heart problems.