What is fetal circulation and why does it need shunts?
In the fetus the placenta, not the lung, is the organ of gas exchange. Oxygen and nutrients cross the placental membrane and return to the fetus in the single umbilical vein; deoxygenated blood goes back to the placenta in the two umbilical arteries. Because the lungs are fluid-filled and their vascular resistance is high, and the liver does not need the whole venous return, the fetal circulation uses three shunts: the ductus venosus (bypasses the liver), the foramen ovale (right atrium → left atrium) and the ductus arteriosus (pulmonary trunk → aorta).
The fetal heart starts beating at about 22 days, marking the start of fetal circulation; gas exchange shifts from the yolk sac to the placenta at about 10 weeks. Because maternal blood mixes with placental blood, the fetus is relatively hypoxic compared with the mother. Several adaptations protect oxygen delivery: fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, and hypoxia drives renal erythropoietin. Normal fetal heart rate is 110–160 beats per minute.
What path does oxygenated blood take through the fetus?
- Placenta → umbilical vein: the most oxygenated blood in the fetus.
- At the liver the umbilical vein divides into a large ductus venosus and a small portal sinus; most blood bypasses the liver through the ductus venosus into the inferior vena cava, while the portal sinus supplies the liver itself.
- IVC → right atrium: the well-oxygenated stream stays medial and is directed at the medially placed foramen ovale, crossing into the left atrium.
- Left atrium → left ventricle → ascending aorta: the coronary arteries and the head and neck vessels receive the best-oxygenated blood — the brain and heart are prioritised.
- SVC blood (deoxygenated, from head and arms) and the lateral hepatic-vein stream miss the foramen ovale and pass to the right ventricle → pulmonary trunk.
- Because pulmonary resistance is high, most pulmonary-trunk blood crosses the ductus arteriosus into the descending aorta, joining blood from the left heart beyond the head-and-neck branches.
- Descending aorta → umbilical arteries → placenta for re-oxygenation.
Embryology of the portal system. The portal vein develops from the vitelline (omphalomesenteric) veins together with the umbilical veins, between about the 4th and 12th week of gestation. The two vitelline veins are remodelled: part of the left vitelline vein and its anastomoses regress, and the dorsal and cranio-ventral anastomoses form the main portal vein and the left portal vein.
What are the oxygen saturations in different fetal vessels?
Exact values differ between textbooks and between species studied (much of the original physiology came from fetal sheep). The figures below are those given in StatPearls — Physiology, Fetal Circulation — and preserve the order that exams test.
| Site | Estimated SaO₂ | Comment |
|---|---|---|
| Umbilical vein / ductus venosus | 70–80% | Highest in the fetus — fresh from the placenta |
| Left atrium | About 65% | Receives the foramen ovale stream |
| Right atrium | About 55% | Mixture of IVC and SVC blood |
| Descending aorta (after the ductus) | About 60% | Partially oxygenated blood joins via the ductus arteriosus |
| Umbilical arteries | About 40% | Lowest — returning to the placenta |
What changes happen in the circulation at birth?
Two events drive the transition: the first breath and clamping of the cord. With breathing and oxygen in the alveoli, pulmonary vascular resistance falls sharply. With removal of the low-resistance placenta, systemic vascular resistance rises. Left-sided pressures now exceed right-sided pressures, and the shunts reverse or close.
| Structure | Trigger for closure | Functional closure | Anatomical closure / remnant |
|---|---|---|---|
| Ductus arteriosus | Rising PaO₂ constricts ductal smooth muscle; fall in prostaglandin E2 after loss of the placenta | 12–24 hours in healthy term babies; virtually all by 72 hours | Anatomic closure over 2–3 weeks; becomes ligamentum arteriosum |
| Foramen ovale | Left atrial pressure exceeds right; septum primum pressed against septum secundum | Shortly after birth (flap closure) | Fusion forms the fossa ovalis; fails in about 25% → patent foramen ovale |
| Ductus venosus | Loss of umbilical venous flow; fall in circulating prostaglandins | Typically 3–7 days | Fibrous closure completes over 1–3 months; becomes ligamentum venosum |
| Umbilical arteries | Loss of placental flow | Within minutes of delivery | Distal parts → medial umbilical ligaments; proximal parts stay open as superior vesical arteries |
| Umbilical vein | Loss of placental flow | After birth | Ligamentum teres hepatis (round ligament), in the lower free edge of the falciform ligament |
The ductus arteriosus has a left-to-right flow within about 10 minutes of birth. Oxygen constricts the ductal smooth muscle through increased calcium-channel activity, while the drop in circulating prostaglandin E2 (the placenta was its main source, and the now-perfused lungs break it down) removes the main signal keeping it open.
What do the fetal structures become in the adult?
| Fetal structure | Adult remnant | Where to find it |
|---|---|---|
| Umbilical vein | Ligamentum teres hepatis | Free lower edge of the falciform ligament, liver to anterior abdominal wall |
| Ductus venosus | Ligamentum venosum | Fibrous band running from the ligamentum teres to the IVC |
| Foramen ovale | Fossa ovalis | Depression on the right atrial side of the interatrial septum; its rim is the limbus (from septum secundum) |
| Ductus arteriosus | Ligamentum arteriosum | Tethers the pulmonary artery to the aorta |
| Umbilical arteries (distal) | Medial umbilical ligaments | Anterior abdominal wall |
| Umbilical arteries (proximal) | Superior vesical arteries | Still patent, supply the bladder |
What happens if the ductus arteriosus stays open?
A patent ductus arteriosus (PDA) is a ductus that fails to close. After birth the flow reverses to left-to-right (aorta → pulmonary artery). The incidence is inversely proportional to gestational age — in extremely premature infants, especially with respiratory distress syndrome, up to 80% have a PDA at 3 days. Prostaglandin E2 keeps the ductus open, which is the basis of both drug strategies below.
- Sign: a continuous 'machinery' murmur below the left clavicle, radiating to the back (may be only systolic early on).
- Conservative management first in more mature preterm infants — many close spontaneously.
- Drugs to close it (preterm, symptomatic): indomethacin, ibuprofen or paracetamol (acetaminophen). Indomethacin is given as 3 doses 12 hours apart; ibuprofen as 3 doses 24 hours apart. Indomethacin and ibuprofen have similar efficacy (about 66–70%); indomethacin carries more concern for gut perfusion and necrotising enterocolitis.
- Surgery or catheter closure if drugs fail. Untreated large shunts cause pulmonary hypertension and eventually Eisenmenger syndrome (shunt reversal with cyanosis).
| Goal | Drug | Used in |
|---|---|---|
| Close the ductus | COX inhibitors — indomethacin, ibuprofen; or paracetamol | Symptomatic PDA in preterm infants |
| Keep open the ductus | Prostaglandin E1 (alprostadil) IV infusion | Ductal-dependent lesions until surgery — e.g. transposition, tetralogy of Fallot, tricuspid atresia, pulmonary stenosis, coarctation, interrupted aortic arch |
What is persistent pulmonary hypertension of the newborn?
Persistent pulmonary hypertension of the newborn (PPHN) is a failure of the normal fall in pulmonary vascular resistance after birth — in effect, the fetal circulation persists. Blood keeps shunting right-to-left across the patent foramen ovale and ductus arteriosus, producing hypoxaemia. It is often secondary to parenchymal lung disease such as meconium aspiration syndrome, to lung hypoplasia with congenital diaphragmatic hernia, or is idiopathic. Reported incidence is about 1.9 per 1000 live births in the United States.
- Labile hypoxaemia — large swings in saturation with little change in ventilator settings — is characteristic, unlike the fixed hypoxaemia of cyanotic heart disease.
- Differential cyanosis: compare pre-ductal saturation from the right arm (the right subclavian artery is always pre-ductal) with post-ductal saturation from a leg; a lower leg reading indicates right-to-left ductal shunting.
- Confirm with echocardiography (also excludes structural heart disease).
- Treat: optimal ventilation and surfactant where lung disease is present, target pre-ductal saturation in the low-to-mid 90s, inhaled nitric oxide (a selective pulmonary vasodilator that reduces the need for ECMO in term babies), sildenafil where iNO is not available, and ECMO for refractory hypoxaemia.
Why does a patent foramen ovale matter?
In about 25% of people the septum primum never fuses with the septum secundum, leaving a patent foramen ovale (PFO). It is different from an atrial septal defect, which is a true deficiency of septal tissue; a PFO is a flap that stays shut while left atrial pressure exceeds right. Most PFOs never cause symptoms, but a rise in right atrial pressure can open the flap and allow a paradoxical embolus — a venous clot crossing to the systemic side and causing a stroke.
How is fetal circulation asked in NEET PG and INI-CET?
- Vessel with the highest oxygen saturation in the fetus → umbilical vein.
- Remnant matching — ductus venosus → ligamentum venosum; umbilical vein → ligamentum teres; umbilical arteries → medial umbilical ligaments; foramen ovale → fossa ovalis; ductus arteriosus → ligamentum arteriosum.
- Which shunt bypasses the liver? → ductus venosus.
- Main stimulus for ductal closure → rise in oxygen tension and fall in prostaglandin E2.
- Drug to keep the ductus open in a cyanotic neonate → prostaglandin E1 (alprostadil); key side effect apnoea.
- Drug to close a PDA in a preterm baby → indomethacin or ibuprofen (or paracetamol).
- Embryological origin of the ductus arteriosus → 6th aortic arch.