[Embryogenesis of the venous portion of the pulmonary circulation]

The wall of the cardiac venous sinus and splanchnic plexus participate in the formation of the venous part of the pulmonary circulation. At the 4th week of the intrauterine life, splanchnic and sinusal parts come into contact. At this stage, the venous part of the pulmonary circulation is represented only by the primary pulmonary vein. Further, from the splanchnic plexus, common and lobar pulmonary veins are formed. At the 6th week of the intrauterine development, assimilation of the primary and common pulmonary veins by the heart is started--the process resulting in the formation of the primary interatrial septa at the expense of the sinusal wall and the walls of the assimilated vessels. By the end of the 8th week of the intrauterine development, the common atrium is separated into the right and left halves up to the auricular canal, the primary and common pulmonary veins are completely assimilated and the venous part of the pulmonary circulation is represented by four pulmonary veins discharging into the left atrium.