5.2 Changes in the fetal circulation at birth
Since the oxygen in the fetua is in the plecenta, the system must necessarily undergo a change to adjust to gaseous exchange in the lungs at the point of birth. At birth, the plecental blood supply is removed and the pulmonary circuit changes from a high resistance to a low resistance circuit when it is exposed to oxygen by breathing. The systematic circulation now becomes the high pressure circuit and there is now no longer a pressure gradient to drive blood flow across the ductus arteriosus from the PA to the aorta. When exposed to oxygenated blood and prostaglandins released at birth, the ductus constricts and closes in the following 24 - 72 hours. This increases the amount of blood flowing through the lungs and results in an increase in left atrial pressure. This, in turn, pushes the atrial septum primum against the septum secundum, functionally closing the foramen ovale. The ductus venosus in the liver closes when plecental flow stops. Thus, at birth the system changes from a pulmonary and systemic system in a parallel to a double circulation in series.