What fetal cardiac shunt closes after birth?

The ductus arteriosus is a critical fetal structure that connects the pulmonary artery to the descending aorta, allowing blood to bypass the non-functioning fetal lungs. In a fetus, this shunt is essential because it directs blood away from the lungs, which are filled with fluid and not yet engaged in gas exchange. After birth, as the newborn takes its first breaths, the lungs expand, and the increase in oxygen levels triggers a series of physiological changes. This includes the closure of the ductus arteriosus, which happens within the first few days of life, usually within the first few hours to days as it constricts and eventually forms a fibrous ligament known as the ligamentum arteriosum.

The other structures mentioned have different closure mechanisms or functions. The foramen ovale also closes after birth due to the increased pressure in the left atrium once the lungs become functional, but it operates differently from the ductus arteriosus. The ductus venosus, which allows a portion of the oxygenated blood from the placenta to bypass the liver, also closes after birth but serves a different purpose. The pulmonary veins, on the other hand, do not represent a shunt that closes after birth, but rather they become the vessels