Learning Objective

By the end of this topic, the learner should be able to:

• Describe the three major fetal circulatory shunts and their flow patterns.
• Explain the physiologic changes at birth that close these shunts.
• Identify pharmacologic agents that can open or close the ductus arteriosus.
• Recognize the postnatal derivatives of fetal vessels.

Fetal Circulation: Three Key Shunts

1. Umbilical Vein → Ductus Venosus → IVC

• Highly oxygenated blood from the placenta enters via the umbilical vein.
• Bypasses the liver through the ductus venosus.
• Drains into the inferior vena cava (IVC).

2. Foramen Ovale

• Most oxygenated blood from the IVC is directed through the foramen ovale into the left atrium (LA).
• Ensures oxygen-rich blood reaches the brain and heart.

3. Ductus Arteriosus

• Deoxygenated blood from the superior vena cava (SVC) enters the right atrium (RA) → right ventricle (RV) → main pulmonary artery.
• Blood is shunted through the ductus arteriosus into the descending aorta.
• Shunting occurs due to high fetal pulmonary vascular resistance.

Changes at Birth

• First breath → pulmonary vasculature resistance ↓
• LA pressure > RA pressure → foramen ovale closes → becomes fossa ovalis
• Increase in oxygen + decrease in placental prostaglandins → ductus arteriosus closes → forms ligamentum arteriosum

Pharmacologic Modulation of Ductus Arteriosus

• Closure of PDA: NSAIDs (e.g., indomethacin, ibuprofen) or acetaminophen
• Maintenance of PDA: Prostaglandins E1 and E2

Tip: “Endomethacin ends the PDA” (mnemonic).

Postnatal Derivatives of Fetal Vessels