What do the heart valves do?

Prevent leak-back. The valves keep the blood moving forward in the circulatory system. The blood moves in starts and stops, with each heart beat. So to keep it moving forward and to prevent it from leaking back when the heart relaxes, there are one-way valves in place, much like the locks and dams in the panama canal.

Related Questions

Where are the heart valves?

4 locations. The first valve blood crosses is the tricuspid valve between the right atrium and right ventricle. Blood then flows across the pulmonic valve between the right ventricle and the lungs on its way to the lungs where it picks up oxygen and discards waste. It returns to the left atrium and crosses the mitral valve between the left atrium and left ventricle. It then exits across the aortic valve.
There are 4. The tricuspid valve sits between the right atrium and the right ventricle. The mitral valve sits between the left atrium and the left ventricle. (these are the "av" valves). The semi-lunar valves:the pulmonic valve sits in the right ventricular outflow tract at the root of the main pulmonary artery (or "conus"). The aortic valve sits at the root of the aorta separating it from the left ventricle.

Which heart valves usually need replacing?

Mitral and aortic. The valves in the left side of the heart.
Two. Aortic mitral (significant number can be repaired) tricuspid also can be repaired pulmonary uncommon does depend on cause congenital degenerative rheumatic bacterial endocarditis.