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Diagram of the heart, with the heart valves labeled, via Wikipedia
The heart valves are four valves that prevent blood from flowing in a direction opposite to the correct flow of blood through the heart and the arteries, as well as allowing a contraction of the heart to force the blood in one direction.
There are two types of valves: Atrialventricular, which control the flow of blood from atria (small chambers of the heart) to ventricles (large chambers of the heart). These are the mitral valve (from the left atrium to the left ventricle) and the tricuspid valve (from the right atrium to the right ventricle).
Semilunar, which control the flow of blood from the ventricles to the arteries. These are the pulmonary valve (to the pulmonary artery) and the aortic valve (to the aorta).
There are several pathologies of the heart valves. Rarely, an infection will attack a valve, but when this happens, it's very difficult to treat. More often, a valve will harden. This allows blood to flow back through the valve - regurgitation. In such cases, the valve must be replaced, either with an artificial valve or one taken from an animal, usually a pig. Regurgitation can be auscultated in some cases.