**4. Fetal and neonatal circulation**

The circulatory system evolved as the simple process of diffusion of nutrients from amniotic sac is no longer able to meet the metabolic needs of the growing embryo.

The placenta does the function of oxygenation in the fetus as the lungs are bypassed by presence of ductus. The arrangement of circulation in the fetus is such that the more oxygen‐ ated blood goes to the head and the less oxygenated blood goes through the ductus into descending aorta and into the umbilical arteries.

The umbilical vein brings the oxygenated blood which bypasses the liver through the ductus venosus and this blood coming into the IVC is preferentially streamed by the Eustachian valve into the left atrium through the foramen in the atrial septum and this oxygenated blood enters the left ventricle and from there to the aorta and arch vessels.

The more deoxygenated blood from the SVC goes into the right ventricle which is pumped by the RV into the PA and as the fetal lungs are collapsed with very high resistance in the pulmonary circulation, the blood bypasses the lungs and goes into the descending aorta through the ductus.

A few lesions provide insights, which can evolve into more complicated congenital heart pathologies. Absence of foramen is one such lesion, which has been linked to the development of hypoplastic left heart syndrome, the development of which can be prevented by dilating the foramen ovale.

Atresia of the pulmonary and aortic valve can also be intervened by ballooning to prevent the development of its sequel which can be hypoplastic left heart and RV dependent coronary circulation in cases of pulmonary atresia with intact ventricular septum.

Absence of ductus arteriosus is a condition which is associated with absent pulmonary valve The absence of ductus could be the primary condition which leads to RV output regurgitating back into ventricle due to high fetal pulmonary vascular resistance, with the VSD partially decompressing the right ventricle. The increased right ventricular output causes the main and the branch pulmonary arteries to dilate. This could extend into the lungs, compressing the airways, and thereby causing severe respiratory compromise in neonatal period as seen in extreme cases of TOF with absent pulmonary valve.
