Cardiac Embryology & Fetal Circulation

The heart tube and its derivatives

Origin. The heart arises from splanchnic (lateral plate) mesoderm as a single primitive heart tube that beats spontaneously by week 4, making it the first functional organ.

Five swellings. From cranial to caudal the tube forms the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus, each with a defined adult fate.

Looping. In week 4 the tube folds to establish left–right polarity. Defective left–right dynein disrupts looping and can produce dextrocardia, as in Kartagener syndrome.

Septation of the atria, AV canal, and ventricles

Failure of the septa to fuse leaves a patent foramen ovale in about a quarter of adults — a route for paradoxical emboli.

Endocardial cushions. These form the membranous interventricular septum and part of the atrial septum and divide the AV canal into the mitral and tricuspid orifices and valves. Complete AV canal (endocardial cushion) defects are classic in Down syndrome.

Ventricular septum. A muscular septum grows up from the ventricular floor; the remaining interventricular foramen is sealed by the membranous septum, where the aorticopulmonary septum meets the cushions and muscular septum. This complex junction is why most VSDs are membranous.

The outflow tract and conotruncal defects

Neural crest and the spiral septum. Neural crest cells migrate into the truncal and conal ridges, which fuse and spiral to form the aorticopulmonary septum dividing the truncus into the aorta and pulmonary trunk. Faulty septation causes conotruncal anomalies, which is why they cluster with neural crest disorders such as DiGeorge syndrome (22q11.2 deletion).

Transposition. The parallel circuits are incompatible with life unless a shunt (PDA, PFO, or VSD) mixes blood, so prostaglandin E1 keeps the ductus open until surgery.

Tetralogy. The severity of pulmonary stenosis drives cyanosis. Squatting raises systemic vascular resistance, reducing the right-to-left shunt and relieving tet spells.

Aortic arch derivatives

• 1st arch: part of the maxillary artery
• 2nd arch: stapedial and hyoid arteries
• 3rd arch: common carotid and proximal internal carotid arteries
• 4th arch: on the left, part of the aortic arch; on the right, the proximal right subclavian artery
• 6th arch: proximal pulmonary arteries and, on the left, the ductus arteriosus

The recurrent laryngeal nerves hook around arch derivatives: the left around the ligamentum arteriosum (6th arch) near the aortic arch, the right around the right subclavian artery (4th arch). This explains the longer course on the left.

Fetal circulation and the transition at birth

The placenta is the oxygenator. The single umbilical vein carries oxygenated blood to the fetus; its PO2 is only about 30 mmHg, yet it is roughly 80% saturated thanks to fetal hemoglobin. The two umbilical arteries return deoxygenated blood. A single umbilical artery is associated with congenital and chromosomal anomalies.

At birth. Lung expansion drops pulmonary vascular resistance, and loss of the placenta raises systemic resistance; left atrial pressure now exceeds right, closing the foramen ovale. Rising arterial O2 and falling PGE2 constrict the ductus arteriosus.

Indomethacin, which lowers prostaglandins, closes a patent ductus arteriosus; prostaglandin (alprostadil, PGE1) keeps it open.

Fetal hemoglobin. HbF (two α and two γ chains) binds 2,3-BPG poorly, giving higher O2 affinity and a left-shifted curve, so it pulls O2 from maternal HbA across the placenta. After birth β chains replace γ chains and HbF falls over roughly the first 6 months, which is when β-globin disorders such as sickle cell disease begin to cause symptoms.

Adult remnants of fetal structures