Human heart valves are remarkable structures. These tissue-paper thin membranes attached to the heart wall constantly open and close to regulate blood flow (causing the sound of a heartbeat). This flexing of the tissue occurs day after day, year after year. In fact, the tissue withstands about 80 million beats a year, or 5 to 6 billion beats in an average lifetime. Each beat is an amazing display of strength and flexibility.
The heart has 4 valves:
The mitral valve and tricuspid valve, which control blood flow from the atria to the ventricles
The aortic valve and pulmonary valve, which control blood flow out of the ventricles