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Fetal Circulation: Shunts & Transition at Birth

Fetal circulation bypasses non‑functioning lungs via three shunts; oxygenated blood comes from placenta.

Oxygenated Blood Path

  1. Umbilical vein (80% O₂ sat) → Ductus venosus (bypasses liver) → IVC.
  2. IVC blood streams preferentially across foramen ovale → Left atrium → Left ventricle → Ascending aorta → Coronary & carotid arteries (brain/heart get highest O₂).

Deoxygenated Blood Path

  1. SVC/IVC (lower body) → Right atrium → Right ventricle → Pulmonary artery.
  2. Most shunts via Ductus arteriosus → Descending aorta → Umbilical arteries → Placenta.
  3. Small amount to lungs (vascular resistance high).

Three Fetal Shunts

Transition at Birth

  • First breath → lung expansion → ↓PVR → ↑pulmonary blood flow.
  • Umbilical cord clamped → ↓IVC return → ↓RA pressure → foramen ovale flap closes.
  • ↑PaO₂ → ductus arteriosus constriction.
  • Adult series circulation established.

Clinical Relevance

  • Persistent shunts = congenital heart disease (PFO, PDA).
  • Preterm infants: immature transition → RDS, PDA.
  • Congenital heart lesions alter fetal flow patterns (e.g., HLHS).

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ShuntFrom → ToCloses
Foramen ovaleRA → LAFunctional: 1st breath (↑LA pressure); Anatomic: months‑years
Ductus arteriosusPA → Descending aortaFunctional: hours (↑O₂, ↓PGE₂); Anatomic: weeks (ligamentum arteriosum)
Ductus venosusUmbilical vein → IVCFunctional: hours; Anatomic: weeks (ligamentum venosum)