The initial event at birth is the taking of the first breath, which inflates the lungs (Figure 3.07)(Table 3.09). This triggers a sharp drop in pulmonary vascular resistance (PVR) due to pulmonary arteriolar smooth muscle relaxation, increasing blood flow and decreasing pulmonary artery pressure. The resulting surge of blood to the left atrium raises left atrial pressure above right atrial pressure and causes the foramen ovale to close functionally.
Within a few minutes the umbilical cord is either tied off or closes naturally. The latter process is stimulated by rising PO2, cold, etc.
Next, the ductus venosus closes and the resistance of the systemic circuit rises. With loss of the placental circulation, venous return to the heart decreases, decreasing both atrial pressures. As pulmonary vascular resistance continues to decline, pulmonary artery pressure eventually falls below aortic pressure, at which time, flow through the ductus arteriosus is reversed. This causes pulmonary blood flow and pressure to temporarily increase.
Within hours to days after birth, the ductus arteriosus closes due to rising arterial O2 saturation. Over the next weeks to months, pulmonary vascular resistance continues to fall because of the atrophy of pulmonary arteriolar smooth muscle.
Anatomical closure of the foramen ovale and ductus arteriosus take longer, some times months or years, and in a fraction of individuals these conduits never completely close.
Table 3.11 shows the re-routing of the various fetal vascular circuits at birth, the time required for the events, and the probable underlying mechanisms.
Go to Next Page
Return to Previous Page
Return to Index