Venous blood leaves both the superior and inferior vena cava and enters the right atrium .
The atrium contracts forcing blood through the tricuspid valve, into the right ventricle . Once the right ventricle fills, it then contracts, forcing the tricuspid valve to close and the pulmonary valve to open. This permits the blood to enter the pulmonary artery and travel to the lungs. As the right ventricle relaxes, the pulmonary valve closes to prevent backflow. In the lungs , carbon dioxide is released and oxygen is picked up by the hemoglobin in the red blood cells.