Fetal Circulation
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Direct answer
Fetal circulation is a parallel system built around three shunts that keep blood away from the lungs and liver: the ductus venosus, the foramen ovale and the ductus arteriosus. Oxygenated blood returns through the umbilical vein, partially bypasses the liver via the ductus venosus, crosses from right to left atrium through the foramen ovale, and most right ventricular output is diverted from the pulmonary artery into the descending aorta through the ductus arteriosus. At birth, lung expansion and the stoppage of umbilical flow close these shunts in sequence, converting the circuit to the adult pattern.
What you must remember
- Route of blood: umbilical vein to inferior vena cava (about half the flow bypasses the liver through the ductus venosus), right atrium to left atrium through the foramen ovale, left ventricle to the brain and coronaries, while desaturated blood reaches the descending aorta through the ductus arteriosus and returns via the hypogastric and umbilical arteries.
- Lungs are largely bypassed: only about a tenth of right ventricular output perfuses the fetal lungs because pulmonary vascular resistance is high.
- Oxygen values: umbilical vein saturation is around 70-80 per cent and mixed fetal arterial saturation roughly 60 per cent — adequate only because fetal haemoglobin carries more oxygen at a given tension.
- Fetal haemoglobin: HbF (two alpha and two gamma chains) has a higher oxygen affinity than adult haemoglobin (a left-shifted dissociation curve), with haemoglobin concentration of about 16-17 g/dL at term.
- Closure at birth: umbilical vessels shut first with cord clamping; the foramen ovale closes functionally with the first breaths; the ductus arteriosus closes functionally within 24-48 hours as oxygen rises and prostaglandins fall.
- Ligaments: ductus venosum from the ductus venosus, ligamentum arteriosum from the ductus arteriosus, fossa ovalis from the foramen ovale, ligamentum teres of the liver from the umbilical vein, and medial umbilical ligaments from the umbilical arteries (the proximal parts persist as superior vesical arteries).
- Clinical links: prostaglandin E2 keeps the ductus open — maternal NSAIDs after about 32 weeks constrict it, while prostaglandin infusion maintains flow in duct-dependent lesions postnatally.
Common confusion
Two confusions dominate. First, vessel content: the umbilical vein carries the most oxygenated fetal blood and the arteries the least — the reverse of postnatal instinct. Second, the ligaments are mixed up in MCQs; pairing each shunt with its remnant (ductus venosus with ligamentum venosum, ductus arteriosus with ligamentum arteriosum) resolves most questions.
Exam-focused takeaway
NEET-PG tests this as a fact-and-logic topic: name the three shunts, trace the path of a drop of blood, state which structure closes first after birth, and match fetal structures to their adult remnants. Physiology stems ask why only a fraction of output reaches the lungs and why indomethacin is unsafe late in pregnancy while prostaglandins are used in duct-dependent lesions. Draw the circuit mentally before answering sequence questions.
Frequently asked questions
What are the three shunts of fetal circulation?
The ductus venosus (liver bypass), the foramen ovale (right-to-left atrial shunt) and the ductus arteriosus (pulmonary artery to descending aorta).
Which vessel carries the most oxygenated fetal blood?
The umbilical vein, with saturation of roughly 70-80 per cent.
Why does fetal haemoglobin hold more oxygen?
Its gamma chains give higher affinity than adult haemoglobin, loading fully at the low oxygen tension of placental blood.
When does the ductus arteriosus close?
Functionally within 24-48 hours as oxygen rises and prostaglandins fall; anatomical closure over two to three weeks forms the ligamentum arteriosum.
What do the umbilical vessels become after birth?
The umbilical vein becomes the ligamentum teres of the liver; the umbilical arteries become the medial umbilical ligaments with proximal parts as superior vesical arteries.