Transition of Fetal Circulation to Neonatal Circulation

Fetal Circulation - Overview

1. Ductus venosus - bypasses the liver
2. Foramen ovale - bypasses the pulmonary circuit (right-to-left atrial shunt)
3. Ductus arteriosus - bypasses the lungs (right-to-left, pulmonary trunk to aorta)

• Highly oxygenated blood from placenta enters via the umbilical vein
• ~50% bypasses the liver via the ductus venosus directly into the IVC; the other 50% flows through liver sinusoids
• Blood from the IVC enters the right atrium; most is directed by the crista dividens through the foramen ovale into the left atrium
• From the left atrium → left ventricle → aorta → preferentially perfuses the brain and upper body
• Blood from the SVC and right atrium enters the right ventricle → pulmonary trunk
• Since pulmonary vascular resistance is HIGH (lungs collapsed, vessels vasoconstricted), most of this blood crosses the ductus arteriosus into the descending aorta
• Deoxygenated blood returns to placenta via two umbilical arteries (branches of internal iliac arteries)

Triggering Events at Birth

1. Clamping of the umbilical cord / placental separation - removes the large low-resistance placental circuit, which doubles systemic vascular resistance, raising aortic, left ventricular, and left atrial pressures.
2. Initiation of breathing / lung expansion - the lungs expand, pulmonary vessels are no longer compressed, and hypoxic pulmonary vasoconstriction is relieved. Pulmonary vascular resistance falls by as much as 5-fold, causing a massive increase in pulmonary blood flow.

Closure of the Three Shunts

1. Ductus Venosus

• With cord clamping, flow in the umbilical vein ceases
• The ductus venosus collapses due to loss of blood flow
• Functional closure: within hours of birth
• Anatomic closure: over the next few weeks
• Adult remnant: Ligamentum venosum (on the visceral surface of the liver)

2. Foramen Ovale

• Increased pulmonary blood flow returns to the left atrium via pulmonary veins → raises left atrial pressure
• Simultaneously, reduced venous return to the right atrium (cord clamping) → lowers right atrial pressure
• This pressure reversal (LA > RA by 2-4 mmHg) pushes the flap-valve of the foramen ovale (derived from septum primum) against the septum secundum, causing functional closure

• Functional closure: with the first few breaths
• During a transitional phase, intermittent right-to-left flow may still occur
• Anatomical closure (fibrous fusion): over months to years; in 2/3 of people, permanent fusion occurs within months to a few years
• In ~20% of adults, the foramen ovale fails to fuse completely (patent foramen ovale, PFO), but remains functionally closed as long as LA pressure exceeds RA pressure
• Adult remnant: Fossa ovalis in the interatrial septum

3. Ductus Arteriosus

• After birth, systemic vascular resistance rises (aortic pressure rises) while pulmonary vascular resistance falls (pulmonary artery pressure falls)
• Blood flow through the ductus reverses direction: from right-to-left (fetal) to left-to-right (aorta → pulmonary artery) transiently
• The key trigger for constriction is a rise in PaO₂: fetal ductal PO₂ is only 15-20 mmHg; after lung inflation, it rises to ~100 mmHg
• Increased O₂ directly contracts smooth muscle in the ductal wall, and also reduces production of prostaglandin E₂ (PGE₂) and prostacyclin (PGI₂), which were keeping the ductus open in fetal life
• Bradykinin released from inflated lungs also contributes to ductal smooth muscle contraction
• TGF-β mediates anatomic fibrotic closure

• Functional closure: By 24 hours - 20% closed; by 48 hours - ~80% closed; by 96 hours - 100% closed in full-term neonates
• Anatomic closure: 1-4 months after birth (fibrosis of lumen)
• Adult remnant: Ligamentum arteriosum

Umbilical Vessels

Neonatal Circulation Diagram

Ventricular Wall Changes

Summary Table

• The Developing Human: Clinically Oriented Embryology (Moore et al.), pp. 869-876
• Guyton and Hall Textbook of Medical Physiology, pp. (Changes in Fetal Circulation at Birth)
• Ganong's Review of Medical Physiology, 26th ed.