Learn — Newborn care & neonatal resuscitation

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Fetal circulation and its closure in the newborn

Fetal circulation is the route blood takes while the placenta, not the lung, is the organ of gas exchange: oxygenated blood arrives in the single umbilical vein and three shunts — the ductus venosus, the foramen ovale and the ductus arteriosus — steer it past the liver and the unexpanded lungs to the heart, brain and upper body before the two umbilical arteries return it to the placenta. At birth the first breaths and the clamping of the cord reverse the pressures that held the shunts open, and each closes to leave a ligament; a shunt that stays open is a patent ductus arteriosus or a patent foramen ovale.

The placenta returns blood at a PO2 of only about 30 to 35 mmHg and a saturation near 80 per cent — the highest anywhere in the fetus — in the umbilical vein, the one vein in the body that carries oxygenated blood; fetal haemoglobin, with its higher oxygen affinity, is what makes that low tension sufficient.
The ductus venosus carries about half of the umbilical venous blood straight through the liver into the inferior vena cava; the rest perfuses the liver through the portal sinus and joins the same vena cava through the hepatic veins.
In the right atrium the crista dividens splits the inferior caval stream so that its better-oxygenated part crosses the foramen ovale into the left atrium, the left ventricle and the ascending aorta — the coronary arteries, brain and upper limbs therefore receive the freshest blood the fetus has.
Blood from the superior vena cava passes through the tricuspid valve to the right ventricle and the pulmonary trunk, but the collapsed, fluid-filled lungs offer a high resistance, so most of it crosses the ductus arteriosus into the descending aorta beyond the origin of the head and arm vessels; the right ventricle is the dominant ventricle before birth.
The descending aorta supplies the lower body and then the two umbilical arteries, which arise from the internal iliac arteries and carry deoxygenated blood (a saturation of about 55 to 60 per cent) back to the placenta — so the cord has one vein and two arteries, and a single umbilical artery is looked for at birth because it goes with renal and cardiac malformations.
The ductus arteriosus is held open before birth by the low oxygen tension and by prostaglandin E2 from the placenta, which is why prostaglandin E1 keeps it open in a newborn with a duct-dependent heart lesion and why indomethacin, ibuprofen or paracetamol closes it in a preterm baby.
At birth the first breaths expand the lungs and raise the oxygen tension, pulmonary vascular resistance falls and pulmonary blood flow rises many-fold, while clamping the cord removes the low-resistance placental circuit and raises systemic resistance: left atrial pressure now exceeds right and the flap of the foramen ovale (the septum primum) is pressed shut within hours, fusing over the first year — though a probe-patent foramen persists in about a quarter of adults.
The ductus arteriosus constricts as oxygen rises and prostaglandins fall — functionally closed within 10 to 15 hours in most term babies and in nearly all by 72 hours, anatomically by 2 to 3 weeks; the ductus venosus loses its flow when the cord is clamped and is obliterated over 1 to 3 weeks.
The remnants: foramen ovale becomes the fossa ovalis, the ductus arteriosus the ligamentum arteriosum, the ductus venosus the ligamentum venosum, the umbilical vein the ligamentum teres of the liver in the falciform ligament, and the umbilical arteries the medial umbilical ligaments, their proximal parts persisting as the superior vesical arteries.
When the transition fails — asphyxia, meconium aspiration, sepsis, diaphragmatic hernia — pulmonary resistance stays high and blood keeps shunting right to left across the foramen ovale and the ductus: persistent pulmonary hypertension of the newborn, seen as cyanosis that oxygen barely lifts and a pre-ductal (right hand) saturation higher than the post-ductal (foot) one; in a duct-dependent lesion such as coarctation or transposition the baby collapses on day 2 to 7 as the ductus closes.

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