Deoxygenated blood enters the heart from the body through the inferior and superior vena cava. It then enters the right atrium. From the right atrium, it moves through the tricuspid valve (tri before you bi) into the right ventricle. At this point, blood is now ready to move into the lungs. The blood passes through the pulmonary semilunar valve into the pulmonary arteries. This is the only time arteries carry blood with no oxygen. The pulmonary arteries deliver the blood to the lungs where the carbon dioxide/oxygen exchange takes place. Once this has taken place, the blood travels back through the pulmonary veins (the only time veins carry blood with oxygen) into the left atrium. It then moves through the mitral or bicuspid valve into the left ventricle. The blood, now oxygen rich, is ready to deliver the oxygen around the body. Blood is pumped through the aortic semilunar valve into the aorta, the largest artery in the body. From here blood travels through arteries, to arterioles, to capillaires (where oxygen is exchanged for carbon dioxide), to venules, to veins, and finally back to the inferior and superior vena cava.
in mammals, the function of the right side of the heart is to collect de-oxygenated blood, in the right atrium, from the body (via superior and inferior vena cavae) and pump it, through the tricuspid valve, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through the passive process of diffusion. The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle, through the bicuspid valve (mitral valve), which pumps it out to the body (via the aorta). On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, oxygenated blood flows back through the pulmonary vein to the left atrium. It then travels through the mitral valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta. The aorta forks and the blood is divided between major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles and then, finally, to the tiny capillaries which feed each cell. The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.