Cardiac Embryology, Fetal Circulation & Anatomy — USMLE Step 1 Notes

Heart morphogenesis

• Cardiac looping: the primitive heart tube loops to the right (dextral looping) in week 4, placing the future left ventricle to the left and establishing the atria behind the ventricles. Failed/reversed looping gives dextrocardia (as in Kartagener syndrome).
• Outflow tract formation: neural crest cells migrate in and the truncus arteriosus is divided by a spiralling aorticopulmonary septum into the aorta and pulmonary trunk. Failure to divide = persistent truncus arteriosus; failure to spiral = transposition of the great arteries; unequal division = tetralogy of Fallot.
• Atrial separation: the septum primum grows down (leaving the ostium primum, then ostium secundum), then the thicker septum secundum grows beside it, leaving the foramen ovale as a flap valve. Failure to fuse after birth = patent foramen ovale; defective septation = atrial septal defect.
• Ventricular separation: a muscular interventricular septum grows up and the membranous septum (from endocardial cushions/conotruncal ridges) closes the remaining gap — the membranous VSD is the commonest ventricular septal defect.
• Valve development: the AV (mitral, tricuspid) valves and the membranous septum derive from the endocardial cushions; the semilunar (aortic, pulmonary) valves form from outflow-tract swellings. Cushion defects give AV septal (endocardial cushion) defects, classically in Down syndrome.

Embryonic structure → adult derivative

• Truncus arteriosus → ascending aorta and pulmonary trunk.
• Bulbus cordis → smooth outflow parts of both ventricles.
• Primitive ventricle → trabeculated parts of the left and right ventricles.
• Primitive atria → trabeculated parts (the atrial appendages).
• Sinus venosus → smooth part of the right atrium (sinus venarum) and the coronary sinus.
• Right common cardinal vein / right anterior cardinal vein → superior vena cava.
• Primitive pulmonary vein → smooth part of the left atrium.

Fetal circulation

• Oxygenated blood returns from the placenta in the umbilical vein (the highest oxygen saturation in the fetus, ~80%).
• The ductus venosus shunts it past the liver into the IVC.
• The foramen ovale shunts blood right atrium → left atrium, delivering the best-oxygenated blood to the brain and heart.
• The ductus arteriosus shunts pulmonary artery blood into the aorta, bypassing the fluid-filled, high-resistance lungs (kept patent by prostaglandin E2 and low O2).
• At birth: the first breath drops pulmonary vascular resistance and raises left atrial pressure, closing the foramen ovale; rising O2 and falling prostaglandins close the ductus arteriosus. Indometacin closes a patent ductus; prostaglandin E1 keeps it open when a duct-dependent lesion needs it.

Fetal → postnatal derivatives

• Umbilical vein → ligamentum teres hepatis (in the falciform ligament).
• Ductus venosus → ligamentum venosum.
• Ductus arteriosus → ligamentum arteriosum.
• Foramen ovale → fossa ovalis.
• Umbilical arteries → medial umbilical ligaments; urachus → median umbilical ligament.
• Notochord → nucleus pulposus (a classic paired-derivative question).

Heart anatomy & pericardium

• The right ventricle is the most anterior chamber (so it is injured first in penetrating trauma); the left atrium is the most posterior (enlargement causes dysphagia or hoarseness from recurrent laryngeal nerve stretch).
• Pericardium: an outer tough fibrous layer and an inner serous layer (parietal and visceral, with ~15-50 mL of fluid between). It is innervated by the phrenic nerve (C3-C5), which is why pericarditis pain refers to the shoulder/trapezius ridge.
• Pericarditis gives pleuritic pain relieved by sitting forward with a friction rub; tamponade gives Beck triad (hypotension, raised JVP, muffled sounds) with pulsus paradoxus.

Cardiac blood supply

• Left anterior descending (LAD): anterior wall, anterior two-thirds of the septum and apex — the most commonly occluded artery (leads V1-V4).
• Left circumflex (LCx): lateral wall (leads I, aVL, V5-V6).
• Right coronary artery (RCA): right ventricle and, via the posterior descending artery (PDA), the inferior wall (leads II, III, aVF). The RCA supplies the SA node in ~60% and the AV node in ~90% — hence bradycardia and heart block with inferior infarcts.
• Dominance is defined by which artery gives the PDA: right-dominant ~85%, left-dominant ~8%, co-dominant ~7%.
• Coronary flow occurs mainly in diastole (the left ventricle squeezes its own vessels shut in systole), so tachycardia shortens filling and worsens ischaemia.

High-yield revision notes and original diagrams; verify against a primary source before clinical use.