It is important to understand fetal circulation before you can begin to learn about the various complex heart defects that preemies and neonates can be born with.
Here is a quick (and hopefully helpful) description of the Fetal Cardiovascular System:
The blood from the placenta that has been enriched with oxygen and nutrients gets passed via the umbilical vein to the liver. Some of this blood flows through it and some bypasses it via the ductus venosus and travels to the inferior vena cava and then the right atrium (RA) of the heart.
The blood flowing from the superior vena cava into the RA is partially mixed with the oxygen-rich blood from the placenta.
The largest part of the blood from the RA bypasses the right ventricle (RV) and instead flows through the foramen ovale into the left atrium (LA) and then down through the mitral/bicuspid valve into the left ventricle (LV). From the LV, the blood goes into the aorta and into the right and left pulmonary artery (aortic arch), then the descending part of the aorta, and then to systemic circulation. This blood gets returned back to the placenta via the umbilical arteries for gas exchange, waste removal, and nutrients.
The remaining part of the blood travels from the RA into the RV via the tricuspid valve and into the pulmonary artery (PA). Due to the high pressure in the lungs (vasoconstriction of the pulmonary bed), a large part flows through the ductus arteriosus and goes into the descending aorta and directly into the large systemic circulation.
Neonatal Cardiac Circulation:
Oxygen-poor blood enters the RA and passes through the tricuspid valve into the RV where it is pumped through the PA to the lungs. As the oxygen flows through the lungs, it gives up carbon dioxide and gains oxygen. Oxygen-rich blood returns from the lungs through the pulmonary veins. It enters the LA and then passes through the mitral valve into the LV, which pumps it through the aortic valve and into the aorta.
PVR decreases and SVR increases:
When the baby takes his first breath, the arterial PO2 increases. Oxygen dilates the pulmonary vasculature causing pulmonary resistance to fall. When the cord is clamped and the placenta is removed, this results in increased systemic vascular resistance.
Clear? Questions? Comments? Concerns?