Blood which is poor in oxygen but rich in carbon dioxide flows into the right atrium and from there into the right ventricle. The tricuspid valve separates the right atrium from the right ventricle. The tricuspid valve, which is so-called because it has three cusps, is one of the two atrioventricular valves.
From the right ventricle, the blood passes through the pulmonary valve into the lungs, where carbon dioxide is expelled and the blood is oxygenated. The pulmonary valve is a semilunar valve.
After oxygenation the blood goes from the lungs into the left atrium and is pumped into the left ventricle through the mitral valve. The mitral valve has two cusps and is the other atrioventricular valve.
From the left ventricle the blood goes through the aortic valve into systemic circulation. The left ventricle has to do the most work because it pumps the blood into the systemic circulation. Like the pulmonary valve, the aortic valve is a semilunar valve.