Valves are structures that ensure that fluids will pass through in one direction only, that is a valve will open to allow fluids to pass in one direction but will close to prevent fluids from passing in the other direction there are two sets of cardiac valves, the atrioventricular (AV) valves, and the semilunar valves, although the two sets of valves are quite different in design, they both function passively in response to the flow of blood. The AV valves are found between the atria and the ventricles, the av valves consist of flaps known as cusps, the outer margin of each flap is attached to the inner surface of the fibrous ring. the inner edge of each flap is free. On the right side is the tricuspid valve , on the left side is the mitral valve(might is never on the right) Thus the tricuspid valve is between the right atrium and the left ventricle, due to having two cusp's it is sometimes known as the 'bicuspid valve. The contraction of the atrial walls forces the blood from the atria through the av valves into the ventricles (atrial systole). When the atria relax (atrial diastole) and the ventricles contract, the pressure would tend to drive the blood back into the atria, however each opening is sealed when the cusp of each av valve meet in the valve center. This prevents blood from flowing further back into the atria. Chordae Tendineae are a special anatomic arrangement which also helps prevent backward flow into the atria. Chordae tendineae are fibrous cords attached to the ventricular side of the cusp, Since these cords are of dense FCT and have a fixed length that can not be stretched or shortened. The other ends of the cords are attached to the papillary muscles. The papillary muscles are special extensions of the muscular walls of the ventricles. As the ventricles contract and become smaller the muscles take up the slack in the cords. The bases of the two great arteries( the pulmonary arch and the aortic arch) begin at their respective ventricles as short cylinders of the fibrous skeleton of the heart. Within each of these cylinders are three cuplike cusps, which make up the pulmonary semilunar valve and the aortic semilunar valve. When the ventricles contract (ventricular systole) and the av valves have closed, the blood moves out into the great arteries through the semilunar valves. When ventricles relax (ventricular diastole) the back pressure of blood in the great arteries forces the cusps of the semilunar valves to the centre and seals off each opening.