Blood returning from the lungs arrives in the left atrium, which pushes it downward to the left ventricle. Click Photo to Enlarge 3. The left ventricle pumps blood out to rest of the body, taking the oxygen out where it will be used by muscles and other tissues to do their work. Blood returning from various parts of the body arrives in the right atrium. From there it goes to the right ventricle for another trip through the lungs so it can get more oxygen, and the cycle continues.
Blood circulates in two loops as it flows through the heart.
Normally, the right side pumps blood only to the lungs. The left side pumps blood to the rest of the body. Heart defects like ASDs, VSDs, or PDAs allow blood to escape from the left to the right side through an abnormal hole or passageway.