Why is pulmonary circulation reduced in the unborn fetus?
After birth, with the first breath the newborn infant takes, the blood circulation suddenly changes. There is decreased resistance in the lungs now that the infant is surrounded by air instead of amniotic fluid. The lowered resistance allows more blood to flow into the pulmonary arteries from the right atrium and ventricle, and less to flow through the foramen ovale into the left atrium. Blood now travels to the lungs through the pulmonary arteries and then back to the heart through the pulmonary veins to the left atrium. This produces an increase in pressure in the left atrium that forces the foramen ovale to close. Once the foramen ovale closes, blood can no longer flow through it and bypass the pulmonary circulation. The ductus arteriosus is no longer needed to shunt blood away from the lungs, and it normally closes within a day or two of birth. The ductus venosus usually closes within another couple of days.