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The heart has four valves: the pulmonic valve, the mitral valve, the aortic valve and the tricuspid valve. Each of the four valves has its own set of gates or flaps. In a normally functioning heart, these flaps swing open to let blood flow from one area to the next, then neatly close again until the next cycle begins.  Thus, blood should flow smoothly in only one direction and only when the valves are open.
Any restriction or narrowing (stenosis) of the valve opening limits blood flow. Any leaking of the blood back in the wrong direction (regurgitation) could lead to more serious problems (e.g., heart failure) if left untreated.
The routes traveled by oxygen-rich and oxygen-poor blood through the heart and its valves are described below:
- Oxygen-rich blood returns to left side of the heart from the lungs, via the pulmonary veins, and drains into the left atrium. From the left atrium, it passes through the mitral valve and into the left ventricle. From the left ventricle, it is pumped through the aortic valve and out the aorta to nourish the rest of the body.
- Oxygen-poor blood returns to the right side of the heart from the rest of the body, via the veins, and enters the right atrium. From the right atrium, it passes through the tricuspid valve into the right ventricle. From the right ventricle, it is pumped through the pulmonic valve and into the pulmonary artery, which carries the blood to the lungs for more oxygen.
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