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In a normally functioning heart, each of the four valves has  its own set of “gates” or flaps that swing open to let blood flow from one area to the next. After swinging open, the flaps should neatly close until the next cycle begins. Thus, blood should flow smoothly only in one direction and only when the valves are open. Any restriction or narrowing (stenosis) of the valve opening limits blood flow.
The routes traveled by oxygen-rich and oxygen-poor blood are as follows:
- Oxygen-rich blood enters the left side of the heart from the lungs, via the pulmonary veins, and travels to 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 (sometimes referred to as the “gatekeeper” of the heart) and out the aorta to nourish the rest of the body.
- Oxygen-poor blood enters the right side of the heart from the rest of the body, via the veins, and travels to 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. Freshly oxygenated blood is carried back to the left side of the heart through the pulmonary veins, and the cycle starts over again.
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