Systemic venous circulation
- The primitive circulation that is present at day 22, is symmetric bilaterally with paired cardinal veins draining the two sides of the body and with blood pumped to the body via right and left aortic arches and dorsal aortae.
- The sinus venosus, which receives blood from the vitelline, common cardinal, and umbilical veins of both the right and left sides of the body, initially overlies the common atrium.
- During the 4th week of gestation, the paired dorsal aortae fuse to form a single midline dorsal aorta and also the venous system begins the process of remodeling.
- During week 7, the opening of the sinus venosus begins to shift toward right atrium due to both cardiac looping and differential growth.
- During week 8:
- The distal end of left cardinal, left posterior cardinal, and left umbilical veins degenerate.
- The proximal portion of left cardinal vein forms superior vena cava via anastamoses of left brachiocephalic and right anterior cardinal veins.
- The left sinus horn receiving venous blood becomes coronary sinus.
- The right vitelline vein becomes inferior vena cava.
- The right posterior cardinal vein becomes azygos vein.
- The right umbilical vein connects to vitelline system via ductus venosus.
- Development of the pulmonary circulation occurs concomitantly with the shift in sinus venosus toward the right side.
- Growth and branching of 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 aorta, in utero when pulmonary resistance is high. Thinning of the walls occurs post-natally with the onset of breathing, increased oxygenation, and decreased pulmonary resistance.
- The proximal portion of the main pulmonary artery develops from the truncus arteriosis, while the distal portion arises from ventral sixth aortic arch.
- The right pulmonary artery develops from right sixth aortic arch (proximal portion) and right branchial artery and the left 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 growing left atrium, resulting in formation of a smooth-walled area in the posterior aspect of the left atrium.