[3] Water, glucose, amino acids, vitamins, and inorganic salts freely diffuse across the placenta along with oxygen.

Its oxygen-hemoglobin dissociation curve is shifted to the left, meaning that it is able to absorb oxygen at lower concentrations than adult hemoglobin.

Oxygenated blood from the placenta is carried to the fetus by the umbilical vein, which will drain into the inferior vena cava (IVC) through the ductus venosus or the liver.

It also contains two additional structures, the foramen ovale and the ductus arteriosus, that function as shunts for oxygenated blood.

[2] The function of these shunts is to bypass the lungs and maintain proper circulation to important fetal tissue.

In the fetal stage, the lungs fill with fluid and collapse because the fetus is within the amniotic sac and the placenta is providing the oxygen it needs to grow.

[2] Because the aorta has lower pressure than the pulmonary artery, most of the blood flows across the ductus arteriosus away from the lungs.

This results in mixed blood oxygen saturation that supplies most of the structures of the lower half of the fetal body.

[6] As the umbilical vessels are obliterated and the infant starts breathing at birth, the source of oxygen changes from the placenta to the lungs.

The reduced blood flow through the umbilical vein at birth will collapse and close the ductus venosus.

While oxygen serves as a vasoconstrictor of the ductus arteriosus, prostaglandins can keep the ductus arteriosus open to maintain blood flow to the lower extremities in cases of hypoplastic left heart syndrome where the mitral valve is shut.

The removal of the placenta, a source of prostaglandin, is another mechanism by which the ductus arteriosus closes at birth.

In certain cases, the transition from fetal to postnatal circulation may not occur as described above due to complications leading to persistently high pulmonary vascular resistance.

Preterm infants are born without fully mature lungs lacking the surfactant compound that allows alveoli to remain open by overcoming the surface tension of water.

[10] In addition, occasionally during birth, infants may inhale remnants of their fecal matter known as meconium, preventing adequate breathing.

In both cases of infant respiratory distress syndrome and meconium aspiration syndrome, fetal shunts will remain open due to the high pulmonary vascular resistance until appropriate measures, such as administration of surfactant or mechanical ventilation, are taken to help the infant breathe on its own.