Transcription: During diastole, the heart muscle relaxes and the heart expands to allow blood to flow into the pumping chambers, ventricles, from the upper holding chambers, atria. During systole, the muscle of the ventricles contracts, squeezing blood out of the chambers. The blood from the right ventricle is propelled into the circulation of the lungs. The blood leaving the left ventricle goes to the rest of the body. Notice that blood begins to enter the atria while the ventricles are squeezing.