<P> In the developing heart, the valves between the atria and ventricles, the bicuspid and the tricuspid valves, develop on either side of the atrioventricular canals . The upward extension of the bases of the ventricles causes the canal to become invaginated into the ventricle cavities . The invaginated margins form the rudiments of the lateral cusps of the AV valves . The middle and septal cusps develop from the downward extension of the septum intermedium . </P> <P> The semilunar valves (the pulmonary and aortic valves) are formed from four thickenings at the cardiac end of the truncus arteriosus . These thickenings are called endocardial cushions . The truncus arteriosus is originally a single outflow tract from the embryonic heart that will later split to become the ascending aorta and pulmonary trunk . Before it has split, four thickenings occur . There are anterior, posterior, and two lateral thickenings . A septum begins to form between what will later become the ascending aorta and pulmonary tract . As the septum forms, the two lateral thickenings are split, so that the ascending aorta and pulmonary trunk have three thickenings each (an anterior or posterior, and half of each of the lateral thickenings). The thickenings are the origins of the three cusps of the semilunar valves . The valves are visible as unique structures by the ninth week . As they mature, they rotate slightly as the outward vessels spiral, and move slightly closer to the heart . </P> <P> In general, the motion of the heart valves is determined using the Navier--Stokes equation, using boundary conditions of the blood pressures, pericardial fluid, and external loading as the constraints . The motion of the heart valves is used as a boundary condition in the Navier--Stokes equation in determining the fluid dynamics of blood ejection from the left and right ventricles into the aorta and the lung . </P> <Dl> <Dt> Relationship between pressure and flow in open valves </Dt> </Dl>

When are the tricuspid and bicuspid valves closed