With birth, a change from parallel flow through the heart to a serial one gradually takes place. The following changes must occur:
- The gas exchange takes place in the baby's lungs.
- By cutting the umbilical cord, the placental circulation system is switched off.
- The fetal heart shunts become closed.
With the activation of breathing the lungs becomes distended, the capillary network dilated and their resistance is reduced drastically so that a rich flow of blood can take place. As a consequence, the pressure in the right atrium sinks in comparison with that of the left one. This pressure turnaround in the atria causes the septum primum to be pressed against the septum secundum and the foramen secundum becomes functionally closed. Towards the end of the first year, it has also grown together in 99% of the babies --> The shunt between the left and right atrium is closed.
On the other hand, with the cutting of the umbilical cord following birth, the placental low resistance area also disappears and the peripheral resistance increases in the systemic circulation. The pressure in the aorta is now higher than that in the truncus pulmonalis and the right-left shunt via the ductus arteriosus that is present before birth is turned around into being a left-right shunt. The pO2 pressure in the aorta increases since the blood is now oxygenated directly in the baby's lungs. This increase in pO2 triggers a contraction of the smooth musculature in the wall of the ductus arteriosus and thereby to a functional seal.
After a few weeks or months this shunt via the ductus arteriosus is definitively obliterated and the remnant is known as the ligamentum arteriosus.
The severing of the baby's umbilical cord leads to the obliteration of the umbilical vessels, primarily through active constriction of their muscular layer. Only the proximal part of the umbilical arteries stays open as the superior vesical artery. The distal part forms the medial umbilical ligament on both sides in the anterior abdominal wall. The umbilical vein transforms itself into the ligamentum teres hepatis.