Physiology · Reproduction, pregnancy and the fetus
Fetal circulation and the transition at birth !
- Mechanism
- Exam priority: high
Before birth the placenta does the work of the lungs, so blood bypasses them through three shortcuts that close in the first hours and days of life.
Updated 2026-11-01
Also called: Fetal shunts, Ductus arteriosus, foramen ovale, ductus venosus
See it in 3D: Interatrial septum · Ligamentum arteriosum · Pulmonary trunk · Aortic arch · Right atrium · Left atrium · Inferior vena cava · Liver
What it is
In the fetus, oxygen comes from the placenta through the umbilical vein, and three shunts (ductus venosus, foramen ovale, ductus arteriosus) send the best-oxygenated blood to the heart and brain while bypassing the liver and the fluid-filled lungs. At birth the first breaths drop pulmonary resistance, clamping the cord raises systemic resistance, and the pressures reverse: the shunts close and the two circulations separate.
How it works, step by step
- Oxygenated placental blood travels in the umbilical vein; about half bypasses the liver through the ductus venosus into the IVC.
- In the right atrium the crista dividens and Eustachian valve direct this stream through the foramen ovale into the left atrium.
- Left ventricle sends it to the coronary arteries and the brain, which receive the best-oxygenated blood.
- SVC blood (deoxygenated) goes to the right ventricle and pulmonary trunk; high lung resistance diverts most of it through the ductus arteriosus.
- The descending aorta carries the mixed blood to the lower body and back to the placenta via the two umbilical arteries.
- First breaths inflate the lungs and raise alveolar PO₂: pulmonary arterioles dilate and pulmonary resistance falls sharply.
- Cord clamping removes the low-resistance placenta: systemic resistance and left-sided pressures rise.
- Left atrial pressure now exceeds right: the septum primum flap presses on the septum secundum and closes the foramen ovale functionally.
- Higher PO₂ and falling prostaglandin E₂ constrict the ductus arteriosus; functional closure usually in 1–3 days, fibrous closure in weeks.
Oxygen–hemoglobin dissociation curve
P50 19.0 mmHgArterial blood 99 %Mixed venous blood 88 %O₂ given up to the tissues 11 %
HbF binds little 2,3-BPG: its P50 is lower, so it takes O₂ from maternal blood in the placenta.
Left shift (P50 19.0 mmHg): Hb holds O₂ more tightly and gives up less in the tissues.
- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues
Typical values (term fetus and newborn)
| Parameter | Value | Note |
|---|---|---|
| Umbilical vein PO₂ | ≈30–35 mmHg | saturation ≈80% |
| Fetal hemoglobin P50 | ≈19–20 mmHg | adult HbA ≈27 mmHg |
| Share of RV output through the ductus | ≈90% | only ≈10% reaches the fetal lungs |
| Functional closure of the ductus arteriosus | ≈12–72 h | later in preterm infants |
| Functional closure of the foramen ovale | minutes to hours | probe-patent in ≈25% of adults |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
The three fetal shunts
| Shunt | Connects | Closes because | Remnant |
|---|---|---|---|
| Ductus venosus | Umbilical vein → IVC | Umbilical flow stops | Ligamentum venosum |
| Foramen ovale | Right atrium → left atrium | Left atrial pressure exceeds right | Fossa ovalis |
| Ductus arteriosus | Pulmonary trunk → aorta | PO₂ rises, prostaglandin E₂ falls | Ligamentum arteriosum |
Clinical notes
- Patent ductus arteriosus (common in preterm infants and congenital rubella): continuous machinery murmur and bounding pulses.
- In duct-dependent lesions (for example transposition, critical coarctation) closure of the ductus causes sudden collapse or cyanosis; keeping it open with prostaglandin E is life-saving.
- Persistent pulmonary hypertension of the newborn: pulmonary resistance stays high, shunts stay right to left and the baby remains cyanotic despite oxygen.
Test yourself
Think of the answer, then tap the question to see it.
Why does blood in the fetal ductus arteriosus flow from the pulmonary trunk to the aorta, and why does it reverse if the ductus stays open after birth?
In the fetus pulmonary resistance is higher than systemic, so flow goes to the aorta. After birth pulmonary resistance falls and systemic rises, so a patent ductus shunts left to right (aorta to pulmonary artery).
What closes the foramen ovale at birth?
A pressure reversal: lung expansion lowers right-sided pressure and cord clamping raises left atrial pressure (more pulmonary venous return), pushing the flap of septum primum against septum secundum.
Why do prostaglandin inhibitors close a patent ductus in a preterm baby?
Ductal patency depends on prostaglandin E₂; blocking its synthesis removes that dilator and lets the ductus constrict.
How does fetal hemoglobin help the fetus load oxygen from maternal blood?
HbF binds 2,3-BPG poorly, so its affinity for oxygen is higher (P50 ≈19 mmHg): at the low PO₂ of the placenta it still saturates well.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Interatrial septum
- Ligamentum arteriosum
- Pulmonary trunk
- Aortic arch
- Right atrium
- Left atrium
- Inferior vena cava
- Liver
Related topics
- Newborn physiology and fetal maturation
- Physiology of congenital shunts
- The pulmonary circulation and hypoxic vasoconstriction
- Oxygen transport and the dissociation curve
- Surface tension, surfactant and Laplace's law
Related conditions
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 28.5 Adjustments of the Infant at Birth and Postnatal Stages (OpenStax (Rice University), 2022); 19.5 Development of the Heart (OpenStax (Rice University), 2022); Patent Ductus Arteriosus (StatPearls (NCBI Bookshelf), 2023); Anatomy, Thorax, Heart Foramen Ovale (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 84). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.