This photograph of a human heart valve, the mitral or bicuspid valve, shows the thin strands called chordae tendinae that attach the underside of the valve to the wall of the chamber below. The strands slacken to allow the valve to open and blood flow into the chamber. Then, when the chamber contracts to pump blood out of the heart via a different valve, the pressure pushes the mitral valve, the pressure pushes the mitral valve closed. The taut chords prevent it from opening the other way, thus stopping blood from leaking out the way it came in.