The human heart is divided into four chambers, two atria and two ventricles. The heart acts as a double pump, with the right side of the heart dealing with used, deoxygenated blood and the left side of the heart dealing with freshly oxygenated blood. The used, deoxygenated blood from the veins passes through the inferior and superior vena cavas into the heart via the right atrium. The blood then flows through the tricuspid valve into the right ventricle. From here is passes through the pulmonary semilunar valve out of the heart and through an artery to the lungs where it loses CO2 and gains oxygen. The blood then re-enters the heart in the left atrium and passes through the bicuspid valve to the left ventricle. From there, it passes through the aortic semilunar valve out of the heart and into the aorta where it ultimately travels throughout the body. These various valves prevent blood from backflowing and keep it moving in a singular direction through the heart.