Fetal Circulation: Unique Shunts and Postnatal Transition

Placental Gas Exchange

Umbilical vein (80% O₂ saturation) → ductus venosus (bypasses liver) → IVC → right atrium.

Three Critical Shunts

Shunt	From → To	Function	Postnatal Closure
Ductus venosus	Umbilical vein → IVC	Directs oxygenated blood to heart	Functional: hours (sphincter); Anatomic: 1–2 weeks → ligamentum venosum
Foramen ovale	Right atrium → Left atrium	Bypasses non-ventilated lungs	Functional: minutes (↑ LA pressure); Anatomic: months–years → fossa ovalis (25% probe patent)
Ductus arteriosus	Pulmonary artery → Aorta	Shunts blood away from lungs	Functional: 12–24 h (↑ O₂, ↓ PGE2); Anatomic: 2–3 weeks → ligamentum arteriosum

Oxygen Saturation Gradient

• Umbilical vein: ~80%
• Ductus venosus/IVC: ~65–70%
• Right atrium: mixed ~55%
• Left atrium (via foramen): ~65%
• Ascending aorta (coronaries, brain): ~65%
• Descending aorta (lower body): ~55%
• Umbilical arteries: ~45% (deoxygenated → placenta)

Transition at Birth

1. Lung expansion → ↓ PVR → ↑ pulmonary blood flow.
2. Cord clamping → ↑ systemic vascular resistance → ↑ LA pressure → foramen ovale closure.
3. ↑ PaO₂ → ductus arteriosus constriction.

Clinical Relevance

Persistent shunts → PDA, PFO, portosystemic shunt. Prostaglandin E1 maintains ductal patency in ductal-dependent lesions.