Systemic and Pulmonary Circulation
Systemic venous circulation
The venous system of the embryo develops from an irregular capillary system, in which single vessels go through transformation and/or atrophy in order to achieve their final classification and purpose. The embryonic vein system consists of three basic systems: 1) cardinal, 2) umbilical, and 3) omphalomesenteric. These all drain into the sinus venosus which initially overlies the common atrium from both the left and right side of the body. Due to the stochastic nature of development, those systems are not uniform among adults.
The cardinal system
By day 10 to 28 of human development, the primitive circulation is symmetric bilaterally with paired cardinal veins, which drain the two sides of the body. These pump to the body via right and left aortic arches and dorsal aortae. Two systems appear: 1) the superior cardinal vein delivers blood from the head region to the left and right common cardinal vein, and 2) the inferior cardinal vein brings blood from the lower half of the body into the two common cardinal veins. In addition, the subcardinal veins appear, forming anastomosis in front of the aorta dorsalis, and start to develop the urogenital system. Between day 15 and 33 of human development, the inferior cardinal veins slowly atrophy and the opening of the sinus venosus begins to shift toward the right atrium due to both cardiac looping and differential growth. Afterwards, the inferior cardinal vein only exists close to the heart and in a sacral region connected through a big intersubcardinal anastomosis net. During week 8, the proximal portion of the left cardinal vein forms the superior vena cava via anastomoses of left brachiocephalic and right anterior cardinal veins, whereas the inferior vena cava develops from all three vein systems (see below). The right posterior cardinal vein becomes the azygos vein. The left sinus horn receiving venous blood becomes the coronary sinus.
The umbilical system
The unpaired umbilical vein provides the embryo with nutrient- and oxygen-rich blood from the placenta and becomes connected to the two intraembryonic umbilical veins. They run lateral from the omphalomesenteric veins to the heart and open together in the sinus venosus. By day 36 of human development, the umbilical veins become involved in the development of the liver and connect to the liver's capillary plexus. This results in a division of the umbilical blood, one part going straight to the heart and the other part going to the liver via anastomosis. After atrophy of the extrahepatic part of the umbilical veins, the blood from the former umbilical veins enters the heart mixed with blood from the omphalomesenteric vein after passing through the liver. The ductus venosus, a parallel shunt, forms within the liver to ensure that nutrient- and oxygen-rich blood goes straight to the heart to supply the brain and other organs quickly. The right umbilical vein atrophies by day 44 of human development, whereas the left umbilical vein delivers the entire blood from the placenta to the liver. It follows atrophy after birth and the ductus venos becomes the ligamentum venosus.
The omphalomesenteric system
The development of the duodenum and liver happens parallel to the implementation of an additional venous system right below the heart. It forms a big hepatic plexus and anastomoses with the umbilical veins. Due to a number of transformations and atrophy, the vena portae originates. Again, the ductus venosus ensures the direct forwarding of nutrient- and oxygen-rich blood to the heart, whereas the rest of the blood goes to the liver for further metabolization. In the process of early cardiac development, the left omphalomesenteric vein atrophies and all the blood from the liver is transported by the right omphalomesenteric vein to the sinus venosus.
Development of the pulmonary circulation occurs concomitantly with the shift in sinus venosus toward the right side and is independent from the three other venous systems just mentioned. Growth and branching of the lung buds, combined with the surrounding mesoderm, form the lung parenchyma and pulmonary blood vessels. The pulmonary arteries are thick-walled and muscular (similar to the aorta) in utero when pulmonary resistance is high. Thinning of the walls occurs postnatally with the onset of breathing, increased oxygenation, and decreased pulmonary resistance. The proximal portion of the main pulmonary artery develops from the truncus arteriosus, while the distal portion arises from the ventral sixth aortic arch. The right pulmonary artery develops from the right sixth aortic arch (proximal portion) and right branchial artery, and the left develops from the left branchial artery.
The primitive pulmonary vein sprouts from the left atrium before bifurcating twice to form four pulmonary veins which grow toward the developing lungs (see http://www.vhlab.umn.edu/atlas/left-atrium/pulmonary-vein-ostia/index.shtml). All four pulmonary veins are incorporated into the posterior aspect of the growing left atrium, resulting in formation of a smooth-walled area in the posterior aspect of the left atrium. The pulmonary valve forms via cavitation of the truncoconal ridge tissue and develops three triangular valve leaflets in the outflow vessel.