Adaptation of fetal circulation at birth

The umbilical arteries become constricted at birth. They obliterate at the ventral abdominal wall on both sides and become the medial umbilical ligament. The umbilical vein also is closed to become the ligamentum teres of the liver. The ductus venosus is transformed into the ligamentum venosum. By the time respiration starts, resistance in the lungs to the blood flow decreases and the pulmonary flow increases. As a consequence, volume and relative pressure in the left atrium also increase. The septum primum gets pressed against the septum secundum. As a result, the foramen ovale is no longer functional. The structural closure usually occurs near the end of the baby's first year. Peripheral resistance increases in the systemic circulation with the suppression of the low-pressure zone of the placenta. Blood pressure becomes higher in the aorta than in the pulmonary artery. The prenatal right-left shunt in the ductus arteriosus (pulmonary artery => aorta) becomes a left-right shunt (aorta => pulmonary artery). The raised O2 pressure leads to contraction of the smooth musculature in the wall of the ductus arteriosus and, hence, to its functional closure. The structural closure occurs some weeks or months later: the ductus arteriosus becomes the ligamentum arteriosum.