General Changes at Birth

At birth, the interruption of placental blood flow results in an increase in carbon dioxide and a decrease in oxygen saturation in the fetal circulation. This causes the infant to inspire. The first breath inflates the lungs which lowers their resistance to blood flow. This increases the volume of blood flowing through them and thereby increases the amount of blood returning from the pulmonary trunk to the heart, all of which results in increased pressure in the left atrium. This increased pressure in the left atrium forces the flap covering the foramen ovale against the interatrial septum, blocking off the communication between the right and left atrium.

 Closure of the umbilical arteries occurs shortly after birth and is facilitated by smooth muscle contraction triggered by a change in oxygen tension. The actual obliteration of the vessels is accomplished by fibrous proliferation which occurs within two or three months.

 Closure of the umbilical vein and ductus venosus follows the closure of the umbilical arteries. This delayed closure allows blood to flow from the placenta into the newborn until the umbilical cord is cut. The ductus venosus forms the ligamentum venosum after being obliterated.

 Closure of the ductus arteriosus takes place almost immediately after birth due to muscular contraction. This can be mediated by a compound, bradykinin, which is released from the lungs after the newborn's first breath. In some instances, the ductus arteriosus does not obliterate within the first few days leaving a left to right shunt.

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