The main reason for this lies in the embryonic development. During the embryonic development blood flows from the right atrium to left atrium through foramen ovale and from atrium to ventricle via atrioventricular channel. The main cause of this situation lies in embryonic development. During embryonic development, blood flows from the right atrium to the left atrium through the foramen ovale, and from the atrium to the ventricle through the atrioventricular channel.
The wall of atrioventricular channel contains endocardial cushion. This endocardial cushion have the primordial cells which forms the cusps of atrioventricular valves. The wall of the atrioventricular channel contains the endocardial cushion. This endocardial cushion contains primordial cells that form the cusps of the atrioventricular valves.
During developmental stage blood flows from right atrium to left atrium due to pressure gradient. This pressure gradient is created due to following reasons: 1) The right atrium receives blood from both the venacava as well as Umblical vein(which is obliterated in adults). 2) Left atrium receives blood only from pulmonary veins. The amount of blood in pulmonary circuit is low during intrauterine life due to very high vascular resistance as the lungs have not yet expanded. Due to this more pressure develops in right side than the left. During the developmental stage, blood flows from the right atrium to the left atrium due to a pressure gradient. This pressure gradient occurs for the following reasons: 1) The right atrium receives blood from both the vena cava and the umbilical vein (which is obliterated in adults). 2) The left atrium receives blood only from the pulmonary veins. The amount of blood in the pulmonary circuit is low during intrauterine life due to very high vascular resistance because the lungs have not yet expanded. Because of this, higher pressure develops on the right side than on the left.
The flow of blood from right atrium to left atrium leads to bulging of central endocardial cushion towards left. Due to this bulging, the third cusp of mitral valve that would have developed gets fused. Blood flow from the right atrium to the left atrium causes the central endocardial cushion to bulge toward the left. Due to this bulging, the third cusp of the mitral valve that would have developed is fused.