Blood flow direction
The next two examples are a way to remember that the blood flows first through the tricuspid and then through the bicuspid.
You learn to ride a tricycle before a bicycle.
Flow of unoxygenated blood
Unoxygenated blood returning from the body flows into the right atrium of the heart from the superior and inferior vena cava. It is then squeezed through the tricuspid valve into the right ventricle of the heart. Then the blood is forced out of the right ventricle through the pulmonary valve into the pulmonary artery which takes the blood to the lungs.
Flow of oxygenated blood
The unoxygenated blood exchanges carbon dioxide for oxygen in the lungs and returns as oxygenated blood to the heart via the pulmonary veins. The oxygenated blood enters the left atrium and is then squeezed through the mitral valve into the left ventricle. The heart then pumps the oxygenated blood out of the left ventricle through the aortic valve, into the aorta. The oxygenated blood is then pumped around the rest of the body before returning as unoxygenated blood to the heart via the superior and inferior vena cavas.