The heart is a muscular structure whose purpose is to pump blood through the body. The heart valves serve to direct the flow of blood in one direction, and they are made of sheets of connective tissue called leaflets, which as a flap. The valves open and close due to differences in pressure, so when the pressure behind them is greater than the pressure in front, the valves open and blood flows in. When pressure is greater in the other direction, the valves shut so as to prevent blood from flowing in the wrong direction. There are a number of different valves connecting the four chambers of the the heart, and which valves are open or closed depends on the contraction of the left and right ventricles. When the ventricles contract and blood is pushed through, there are tendinous structures called the chordae tendinae that help pull the atria-ventricular flaps close, like strings pulling a lid shut.