Heart valve 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. 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
Heart development into the ascending aorta and pulmonary artery. The bulbus cordis forms part of the ventricles. The sinus venosus connects to the fetal circulation. The heart tube elongates on the right side, looping and becoming the first visual sign of left-right asymmetry of the body. Septa form within the atria and ventricles to separate the left and right sides of the heart. The heart derives from embryonic mesodermal germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels, develop from endothelium.