The heart has four valves which, by opening and closing in a rhythmic way, serve to keep blood blowing through the heart in just one direction. Two of the valves – the mitral valve and the tricuspid value – are known as the atrioventricular (AV) valves because they are located between the atria and the ventricles. The other two valves – the aortic valve and the pulmonary valve – are called semilunar (SL) valves because they are located in the arteries leaving the heart.