Heart valves function to heart valves function to control the amount of blood pumped by the heart keep blood moving forward through the heart mix blood thoroughly as it passes through the heart slow blood down as it passes through the heart

The answer to this question would be: keep blood moving forward through the heart

When the ventricle contracts, the valve will plug the hole to atrium so the blood will be pump to the body. The valve in heart will prevent backflow of blood from ventricle to atrium which makes the blood keep moving forward. 
A defect in valve could reduce heart deficiency as a part of the blood might be going back to the atrium, called regurgitation.

Heart valves function to keep blood moving forward through the heart. The heart has four valves. These four valves control  control the flow of blood through the heart( the mitral valve and tricuspid valve,control blood flow from the atria to the ventricles and the aortic valve and pulmonary valve control blood flow out of the ventricles)