The circulation of a foetus works differently to that of a newborn baby.
In the womb, the foetus is connected to the mother’s bloodstream through the placenta and the blood vessels in the umbilical cord. Blood rich in oxygen and nutrients is supplied directly from the mother to the foetus because the developing organs are unable to perform metabolic processes and the lungs are still inactive.
After reaching the foetus through the vessels in the umbilical cord, the oxygen-rich blood mixes together with the oxygen-poor blood from the vena cava veins (SVC, IVC) and reaches the heart where the foetal circulation begins. The blood available to the foetus is “mixed blood” which contains slightly less oxygen but is of sufficiently good quality for the foetus’ organs to thrive.
Every foetus has an opening or tunnel in the wall of tissue separating the right and left atriums (interatrial septum). This opening is known as the foramen ovale. Most of the blood that reaches the child’s right atrium (RA), flows through this opening into the left atrium (LA). It continues through the mitral valve into the left ventricle (LV) and flows into the aorta and to the organs. Only a small amount of blood flows through the tricuspid valve into the right ventricle (RV) and the pulmonary artery (PA). The foetal lungs are supplied with enough blood for the healthy development of the tissue and the pulmonary (lung) vessels which, until birth, are held in a “contracted” state by the muscles. For this reason, the pulmonary vascular resistance (resistance that must be overcome to push blood through the veins in the lungs) is particularly high. As a result, much of the blood in the pulmonary artery escapes through a connection between the trunk of the pulmonary artery and the proximal descending aorta; this vessel is known as the ductus.
At birth, a newborn baby takes its first breath and activates the lungs. This causes the pulmonary vessels to “relax”, lowering the pulmonary vascular resistance and allowing the blood to flow into the tiniest recesses of the pulmonary vascular tree (system of vessels in the lungs). With every breath, air is drawn into the bronchial tubes, allowing the process of gas exchange to begin. Now that more blood from the lungs is also flowing back through the pulmonary veins into the left atrium, the blood no longer needs to flow from right to left through the foramen ovale, which usually closes completely.
The child no longer depends on the blood supply from the placenta and the umbilical cord can be cut. The open ductus arteriosus is not needed anymore and it closes on its own within a matter of hours or days. In many heart defects, malformations can make it more difficult for the blood to reach the lungs or the aorta, sometimes blocking the blood flow altogether. The blood can no longer escape through the foramen ovale and ductus arteriosus as it did when the baby was still in the womb. Without medical intervention, these infants would die within hours or days of being born. In intensive care, the newborn is given prostaglandin, a highly effective form of medication that stops the ductus from closely spontaneously until corrective surgery can be performed in the first few days of the baby’s life. In the case of newborns who need an open foramen ovale, the connection may be widened in a cardiac catheterisation procedure (balloon atrioseptostomy or Rashkind manoeuvre). Cardiac catheterisation is when a long thin tube or catheter is inserted in an artery or vein and threaded through the blood vessels to the heart.
It usually takes a few weeks for the infant’s pulmonary vascular resistance (see above) to fall. This means it may be several weeks before the full extent of a heart problem becomes clear, e.g. in children with significant defects in the wall of tissue separating the right atrium from the left atrium (septum), causing large amounts of blood to flow from the left ventricle into the right ventricle and also into the lungs.
SVC – Superior vena cava
IVC – Inferior vena cava
RA – Right atrium
RV – Right ventricle
PA – Pulmonary artery
LA – Left atrium
LV – Left ventricle