Foetal circulation

Master answer

Foetal Circulation

• Placenta
  • Supplied by paired umbilical artery and drains to single umbilical vein
  • Capillary networks in parallel à high flow, low resistance system, decreases overall systemic vascular resistance.
  • Oxygenated blood (SpO₂ 80% PO₂ 35mmHg) from placenta returns to foetus via umbilical vein
  • 60% of blood from umbilical vein bypasses liver via ductus venosus into IVC
    • Blood in IVC from systemic circulation is hypoxaemic (SpO₂ 30%)
• Heart
  • Blood from IVC enters right atrium and oxygenated blood, directed via the Eustachian valve, preferentially flows across foramen ovale to left ventricle à supplies ascending aorta (SpO₂ 65%, PaO₂ 25mmHg)
    • This allows most oxygenated blood to supply the brain and coronary arteries
  • Blood from SVC enters right atrium and continues to right ventricle and pulmonary arteries → flows into aorta via ductus arteriosus and predominantly supplies descending aorta (SpO₂ 60%, PaO₂ 20mmHg) and organs
• Lungs
  • High pulmonary vascular resistance due to;
    • Unexpanded lungs
    • Hypoxic pulmonary vasoconstriction
  • Therefore blood in pulmonary artery preferentially perfuses systemic circulation via ductus arteriosus
  • Only approx 12% of pulmonary arterial blood enters the pulmonary circulation

Changes at birth

• Placenta
  • Vasoconstriction occurs due to longitudinal stretch of umbelical vessels and increase in PaO₂
  • Augemented by clamping of vessels
  • Loss of placenta increases systemic vascular resistance
  • Increased left heart pressures
• Lungs
  • After first breath…
    • Hypoxic pulmonary vasoconstriction is relieved,
    • Lung expands
      • Recruitment of collapsed parallel capillary networks
      • Dilation of existing vasculature
    • Pulmonary blood flow increases 10-fold and pulmonary vascular resistance decreases
  • Combined with decreased venous return (from placenta), decreases right heart pressures
• Heart
  • Foramen ovale
    • Equilibration of right and left heart pressures closes flap of foramen ovale (occurs within minutes to hours)
    • Pulmonary arterial blood flow further increases
  • Ductus arteriosus
    • The ductus arteriosus shunt become bi-directional
    • Closure
      • Due vasospasm caused by increased PO₂
      • Also, loss of PGE₂ produced by placenta causes loss of vasodilation
      • Occurs within 96 hours

Sakurai 2016

Answer by Sakurai