<P> During diastole, a normally - functioning mitral valve opens as a result of increased pressure from the left atrium as it fills with blood (preloading). As atrial pressure increases above that of the left ventricle, the mitral valve opens . Opening facilitates the passive flow of blood into the left ventricle . Diastole ends with atrial contraction, which ejects the final 30% of blood that is transferred from the left atrium to the left ventricle . This amount of blood is known as the end diastolic volume (EDV), and the mitral valve closes at the end of atrial contraction to prevent a reversal of blood flow . </P> <P> The tricuspid valve has three leaflets or cusps and is on the right side of the heart . It is between the right atrium and the right ventricle, and stops the backflow of blood between the two . </P> <P> The aortic and pulmonary valves are located at the base of the aorta and the pulmonary trunk respectively . These are also called the "semilunar valves". These two arteries receive blood from the ventricles and their semilunar valves permit blood to be forced into the arteries, and prevent backflow from the arteries into the ventricles . These valves do not have chordae tendineae, and are more similar to the valves in veins than they are to the atrioventricular valves . The closure of the semilunar valves causes the second heart sound . </P> <P> The aortic valve, which has three cusps, lies between the left ventricle and the aorta . During ventricular systole, pressure rises in the left ventricle and when it is greater than the pressure in the aorta, the aortic valve opens, allowing blood to exit the left ventricle into the aorta . When ventricular systole ends, pressure in the left ventricle rapidly drops and the pressure in the aorta forces aortic valve to close . The closure of the aortic valve contributes the A2 component of the second heart sound . </P>

Where are the semilunar valves located in the aorta