Fetal circulation
16 named structures.
Draft — not yet clinically reviewed. The structure of this map is checked automatically, but its wording has not been fact-checked against a textbook. Do not rely on it for an exam answer yet.
Hover or tab a structure to trace what it connects to. Some structures reveal further branches.
Clinical detail
- PlacentaLow resistance Fetal 'lung'Placental insufficiency (pre-eclampsia, smoking, chronic hypertension)2 questions
- Umbilical veinSaO2 ~80% PO2 30-35 mmHgUmbilical venous catheter in a sick neonate
- Ductus venosusSaO2 ~80% Bypasses liverFetal hypoxaemia or growth restriction on Doppler surveillance
- Hepatic sinusoids (portal and umbilical mixing)SaO2 ~70-80%Chronic fetal hypoxaemia (placental insufficiency)
- Foramen ovale to left atrium and left ventricleSaO2 ~65% R → L onlyTransposition of the great arteries at birth
- Right atrium to right ventricle and pulmonary trunkSaO2 ~50-55% SVC ~40%Neonatal ECG in the first week1 question
- Ascending aorta and arch (pre-ductal)SaO2 ~65% Pre-ductalNewborn pulse oximetry screening (right hand and either foot at 24 h)1 question
- Ductus arteriosusSaO2 ~50-55% PGE2 keeps openPatent ductus arteriosus (prematurity, congenital rubella, high altitude, trisomy 21)
- Pulmonary circulation (fluid-filled lungs)~10-25% of CVO PVR highElective caesarean section without labour1 question
- Coronary, cerebral and upper-limb circulation (pre-ductal)SaO2 ~65% Pre-ductalMiddle cerebral artery Doppler in a growth-restricted fetus1 question
- Descending aorta to lower body (post-ductal)SaO2 ~55-60% Post-ductalUmbilical arterial catheter in the neonate
- Umbilical arteriesSaO2 ~55-58% PO2 20-25 mmHgSingle umbilical artery on the morphology scan (~1% of pregnancies)
- Ligamentum teres hepatis (round ligament of the liver)Was: umbilical veinPortal hypertension in cirrhosis
- Ligamentum venosumWas: ductus venosusDuctus venosus that fails to close (congenital portosystemic shunt)
- Fossa ovalis and patent foramen ovaleWas: foramen ovale PFO ~25%Deep vein thrombosis with a PFO and a transient rise in right atrial pressure (Valsalva, pulmonary embolism)
- Ligamentum arteriosumWas: ductus arteriosusHigh-speed deceleration (car crash, fall from height)3 questions
Common questions
Why do the two fetal ventricles work in parallel rather than in series?
Because the lungs are bypassed. The right ventricle pumps into the pulmonary trunk but most of that blood crosses the ductus arteriosus into the descending aorta, and the left ventricle pumps the foramen ovale stream into the ascending aorta. Both therefore feed one systemic-placental circuit, which is why fetal output is quoted as a combined ventricular output (~450 mL/kg/min) and why pulmonary artery and aortic pressures are nearly equal.
Why does the head get better-oxygenated blood than the legs?
Streaming. The fast ductus venosus jet in the IVC is directed by the eustachian valve and crista dividens across the foramen ovale into the left heart and ascending aorta (~65%), whose branches supply the coronaries, brain and arms. Desaturated SVC blood goes to the right ventricle and, via the duct, to the descending aorta (~55-60%) and the legs and placenta.
What closes the foramen ovale at birth?
A reversal of the atrial pressure gradient. The first breaths drop pulmonary vascular resistance, so pulmonary venous return and left atrial pressure rise; cord clamping removes the umbilical venous inflow, so IVC return and right atrial pressure fall. Septum primum is pressed against septum secundum within minutes, and the two fuse over the first year. About a quarter of adults keep a probe-patent foramen.
What closes the ductus arteriosus, and how do drugs manipulate it?
Rising arterial PO2 constricts ductal smooth muscle, and PGE2 falls because the placental source is gone and the now-perfused lungs metabolise it. Functional closure takes 10-15 hours, anatomical closure 2-3 weeks. Indomethacin, ibuprofen or paracetamol (COX inhibitors) close a patent duct in the preterm; alprostadil (PGE1) keeps it open in duct-dependent lesions such as transposition, pulmonary atresia and hypoplastic left heart.
What does each fetal structure become after birth?
Umbilical vein: ligamentum teres hepatis. Ductus venosus: ligamentum venosum. Foramen ovale: fossa ovalis. Ductus arteriosus: ligamentum arteriosum. Umbilical arteries: medial umbilical ligaments (distal) and superior vesical arteries (proximal). Note the urachus, not a vessel, becomes the median umbilical ligament.
Why is umbilical venous blood only ~80% saturated when maternal arterial blood is ~98%?
The placenta is a venous equilibrator: fetal blood leaves at a PO2 close to the maternal intervillous or uterine venous PO2 (~30-40 mmHg), not the arterial value, and the placenta itself consumes oxygen. Fetal haemoglobin compensates with a left-shifted curve (P50 ~19 mmHg vs 27 mmHg for HbA), so 80% saturation is reached at a PO2 of only 30-35 mmHg.