Transition from Fetal to Adult Circulation

Transition from Fetal to Adult Circulation. The transition from fetal to adult circulation is a transformation from a parallel to a series circulation. In utero, there is right-to-left shunting of blood across the ductus arteriosus. After birth, as the lungs are expanded and alveolar oxygen tension rises, the PVR decreases. Simultaneously, the SVR increases in association with the loss of the low-resistance placental circulation. The net effect of the PVR falling beneath the SVR is a reversal of ductus flow. The ductus arteriosus will contract and functionally close in the first 10 to 15 hours of life. This is caused by a loss of placental-produced prostaglandins and an increase in neonatal blood oxygen tension. Accompanying the decrease in PVR is an increase in pulmonary blood flow, an improvement in RV compliance, and a decrease in right-sided pressures relative to the left. This drop in right atrial pressure results in the closure of the foramen ovale. With the closure of the ductus arteriosus and the foramen ovale, the circulation assumes an adult configuration. These changes in the neonatal period are tentative; however, and reversion to a fetal circulation can occur during periods of abnormal physiologic stress. Persistence of elements of fetal circulation is common in many cases of congenital heart disease (CHD) and can occasionally be lifesaving.