Blood enters the Right Atrium from the Superior Vena Cava (Vein from the top of body) and the Inferior Vena Cava. (Vein from the lower part of body) The right atrium contracts pushing the blood through the Tricuspid Valve into the Right Ventricle where another contraction pushes the blood through the Pulmonary Valve into the Pulmonary Artery that splits into the Right and Left Pulmonary Arteries (They go to the right and left lungs). The lungs remove the CO2 from the blood through the capillaries (These are porous to gases) and replace it with Oxygen. The oxygenated blood returns to the Left Atrium of the Heart through the four Pulmonary Veins. A contraction in the Left Atrium pushes the oxygenated blood through the Bicuspid Valve into the Left Ventricle. The Left Ventricle then contracts to push the blood into the Aorta for transport around the body.