Fetal Circulation

The placenta does the breathing

Before birth, the lungs are collapsed, fluid-filled, and useless for gas exchange. Every molecule of oxygen and every nutrient the fetus needs crosses the placenta, where maternal and fetal blood run close enough for diffusion but never actually mix. Carbon dioxide and waste travel back the other way. The job of fetal circulation is therefore not to push blood through the lungs — it is to get placental blood, the most oxygen-rich blood in the body, to the organs that matter most: the heart and the brain.

The constraint that shapes the entire layout is resistance. The unexpanded fetal lungs have extremely high pulmonary vascular resistance, partly from hypoxic pulmonary vasoconstriction and partly from the simple fact that the vessels are squeezed shut in collapsed tissue. The placenta, by contrast, is a vast low-resistance bed that receives about 40% of the combined ventricular output. Blood always takes the path of least resistance, so the fetal heart works in parallel: both ventricles pump into the aorta, and most blood detours around the lungs through a set of three shunts.

The three shunts, in order of flow

Trace a red cell from the placenta and the logic of fetal circulation becomes a simple road map of three shortcuts.

1. The ductus venosus. Oxygenated blood leaves the placenta in the single umbilical vein at roughly 80-85% saturation. About half of it is shunted through the ductus venosus, a vessel that tunnels straight past the liver into the inferior vena cava (IVC). This protects the precious oxygen from being extracted by the metabolically busy liver and delivers it, still well saturated, to the heart. The rest of the umbilical flow does perfuse the liver through the portal sinus.

2. The foramen ovale. Blood arriving from the IVC is streamed — by the shape of the right atrium and a small ridge called the crista dividens — preferentially across the foramen ovale, a flap valve in the interatrial septum. This sends the best-oxygenated blood directly into the left atrium, then the left ventricle, then the ascending aorta, where it feeds the coronary arteries and the carotids. The brain and heart thus get the highest available oxygen. Deoxygenated blood returning from the head in the superior vena cava is instead directed toward the right ventricle.

3. The ductus arteriosus. The right ventricle pumps into the pulmonary artery, but because pulmonary resistance is so high, only 10-15% of that blood enters the lungs (just enough to nourish growing lung tissue). The remaining ~85% is diverted through the ductus arteriosus, a muscular connection between the pulmonary artery and the descending aorta. This blood, slightly less oxygenated, supplies the lower body and ultimately returns to the placenta through the two umbilical arteries to pick up oxygen again.

Note the naming inversion that trips up every student: in the fetus the umbilical vein carries oxygenated blood and the umbilical arteries carry deoxygenated blood, because the convention is direction relative to the heart, not oxygen content.

The first breath rewires everything in minutes

Birth flips the system from parallel to series in a remarkably choreographed sequence, collectively called the transitional circulation:

• Lungs expand. The first breaths inflate the alveoli and oxygen replaces fluid. Pulmonary vascular resistance falls roughly tenfold within minutes, so blood now rushes into the lungs instead of around them.
• Foramen ovale closes. Pulmonary venous return floods the left atrium, raising left-atrial pressure above right-atrial pressure. The pressure gradient pushes the septum primum flap against the septum secundum, sealing the foramen ovale functionally within minutes. Anatomical fusion takes months.
• Cord clamping raises systemic resistance. Removing the low-resistance placenta abruptly increases systemic vascular resistance and aortic pressure, reinforcing the new left-to-right pressure relationships.
• Ductus arteriosus constricts. Two signals converge: rising arterial oxygen tension directly constricts the ductal smooth muscle, and loss of placental prostaglandin E2 (plus increased metabolism of circulating prostaglandins by the now-functioning lungs) removes the dilating stimulus. Functional closure occurs in 24-72 hours; fibrosis into the ligamentum arteriosum takes about three weeks.
• Ductus venosus closes. With the umbilical flow gone, the ductus venosus closes over 1-2 weeks, becoming the ligamentum venosum. The umbilical vein becomes the ligamentum teres of the liver, and the umbilical arteries become the medial umbilical ligaments.

Fetal versus neonatal circulation

The single most useful summary is a side-by-side of the same plumbing before and after the first breath.

Feature	Fetal circulation	Neonatal / adult circulation
Organ of gas exchange	Placenta	Lungs
Circuit arrangement	Parallel (both ventricles feed the aorta)	Series (right then left in sequence)
Pulmonary vascular resistance	High (collapsed, hypoxic lungs)	Low (inflated, oxygenated lungs)
Pulmonary blood flow	~10-15% of cardiac output	100% of right-ventricular output
Foramen ovale	Open right-to-left shunt	Closed → fossa ovalis
Ductus arteriosus	Open, right-to-left (PA → aorta)	Closed → ligamentum arteriosum
Highest O₂ saturation	Umbilical vein (~80-85%)	Pulmonary veins / left heart (~98%)
Dominant oxygen carrier	Fetal hemoglobin (HbF), high O₂ affinity	Adult hemoglobin (HbA)

Fetal hemoglobin deserves a line of its own. HbF binds 2,3-BPG poorly, shifting its oxygen-dissociation curve to the left so it grabs oxygen avidly at the low partial pressures found at the placenta — about 30-35 mmHg. This higher affinity lets the fetus extract oxygen from maternal blood across the placental membrane, and it is gradually replaced by adult HbA over the first six months of life.

When the transition fails

Most clinical problems in this domain are failures of the shunts to close — or, paradoxically, treatments designed to keep them open.

• Patent ductus arteriosus (PDA). Common in premature infants, whose ductal tissue is less responsive to oxygen. After birth the shunt reverses to left-to-right (aorta → pulmonary artery), causing pulmonary overcirculation, a continuous "machinery" murmur, and a widened pulse pressure. Treatment uses prostaglandin synthesis inhibitors — indomethacin, ibuprofen, or acetaminophen — to constrict the duct; resistant cases need catheter device closure or ligation.
• Patent foramen ovale (PFO). The foramen ovale remains probe-patent in about 25% of adults. Usually silent, it can transiently reopen when right-atrial pressure spikes (coughing, Valsalva, pulmonary embolism), allowing a venous clot to cross into the arterial side — a paradoxical embolism and a recognized cause of cryptogenic stroke in younger patients.
• Persistent pulmonary hypertension of the newborn (PPHN). If pulmonary vascular resistance fails to fall after birth, blood keeps shunting right-to-left through a persistent ductus and foramen ovale, bypassing the lungs and causing severe hypoxemia. Treatment targets the pulmonary vessels: oxygen, inhaled nitric oxide, and sometimes extracorporeal membrane oxygenation (ECMO).
• Duct-dependent congenital heart disease. In lesions such as transposition of the great arteries, critical coarctation, hypoplastic left heart, or pulmonary atresia, survival depends on a shunt staying open to mix the circulations. Here clinicians deliberately keep the duct open with an intravenous prostaglandin E1 infusion until corrective surgery — the opposite of treating a PDA.

These conditions are why the obstetric and neonatal teams watch the transition so closely: a low-resistance placenta and three open shunts are perfect for life in the womb and instantly dangerous outside it.

This article is educational and is not medical advice. For any concern about pregnancy, a newborn, or congenital heart disease, consult a qualified clinician.