<P> The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae . These muscles prevent the valves from falling too far back when they close . During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight . As the heart chambers contract, so do the papillary muscles . This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria . </P> <P> Two additional semilunar valves sit at the exit of each of the ventricles . The pulmonary valve is located at the base of the pulmonary artery . This has three cusps which are not attached to any papillary muscles . When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket - like valve, pressing against the cusps which close to seal the valve . The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles . This too has three cusps which close with the pressure of the blood flowing back from the aorta . </P> <P> The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve . </P> <P> The right atrium receives blood almost continuously from the body's two major veins, the superior and inferior venae cavae . A small amount of blood from the coronary circulation also drains into the right atrium via the coronary sinus, which is immediately above and to the middle of the opening of the inferior vena cava . In the wall of the right atrium is an oval - shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale . Most of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface has prominent ridges of pectinate muscles, which are also present in the right atrial appendage . </P>

Position of the human heart in the body