Anatomy

Fetal Circulation

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Fetal Circulation, Fetoplacental Circulation, Birth Transition from Fetal Circulation

  • Physiology
  • Fetus
  1. Fetal gas exchange occurs at the placenta (not the lung)
    1. Ductus venosus
      1. Carries oxygenated blood from the placenta
      2. Delivers to the inferior vena cava via the umbilical vein
        1. Mixes with deoxygenated blood returning from the lower extremity
    2. Pulmonary arteries
      1. Returns blood from the descending aorta to the placenta to be re-oxygenated
  2. Fetal right ventricular output
    1. Right to left shunt, bypassing the lungs (60% of flow)
      1. Increased pulmonary vascular resistance
      2. Ductus arteriosus
      3. Foramen ovale
    2. Pulmonary system (40% of flow)
      1. Alveoli are filled with amniotic fluid and result in minimal gas diffusion
  • Physiology
  • Birth Transition from Fetal Circulation
  1. Alveolar fluid is replaced with air on initial ventilations
    1. Critical first step that leads to reduced pulmonary vascular resistance
    2. Results in an increase in PaO2 with an associated decreased in pulmonary vascular resistance
  2. Negative intrathoracic pressure increases
    1. Initiated with uterine contractions and travel through the birth canal
    2. First few breaths add to the negative intrathoracic pressure
  3. Increased pulmonary perfusion
    1. Results from less resistance to flow through pulmonary system than through fetal shunts
    2. Increased systemic vascular resistance following delivery
    3. Flow reverses in the ductus arteriosus (now traveling aorta to pulmonary artery)
  4. Closure of shunts (last steps in transition)
    1. Ductus arteriosus closes within hours to days (unless Patent Ductus Arteriosus)
    2. Foramen ovale closes last
      1. Results from increased left atrial flow and pressure
  5. Myocardium changes
    1. Right ventricle wall thins
    2. Left ventricle wall hypertrophies
  • Resources
  • References
  1. Joseph and Webb (2015) Crit Dec Emerg Med 29(1): 10-8