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Updated: Aug 14 2021

Fetal Circulation and Erythropoiesis

  • Overview
    • Umbilical vein
      • returns 80% saturated blood from the placenta to the fetus
    • Umbilical arteries
      • carry mainly deoxygenated blood back to the placenta for oxygenation
    • Erythropoeisis
      • begins at week 3
  • Circulatory Shunts
    • Ductus venosus
      • oxygenated blood returning from the placenta via the umbilical vein is shunted away from the liver via the ductus venosus to the IVC
      • becomes ligamentum venosum upon closure at birth
    • Foramen ovale
      • due to the increased pulmonary resistance, oxygenated blood entering the right atrium is diverted from the lungs through the foramen ovale into the left atrium to deliver oxygenated blood to the body
      • closes after birth due to increased left atrial pressure and becomes fossa ovalis
    • Ductus arteriosus
      • connects pulmonary artery to aortic arch
      • deoxygenated blood that enters the right ventricle to be pumped through the pulmonary arteries is mostly diverted from the lungs to the body into the aortic arch via the ductus arteriosus
      • becomes ligamentum arteriosum upon closure at birth
  • Infant's first breath
    • First breath
      • pulmonary resistance ↓ causing ↓ in right atrial pressure
      • ↑ venous return to left atria causes ↑ pressure resulting in the foramen ovale to close
      • ↑ in O2 tension leads to ↓ in prostaglandin production resulting in closure of ductus arteriosus
        • indomethacin, an NSAID, closes ductus arteriosus
        • with closure of the ductus arteriosus, the right ventricular contribution to systemic cardiac output decreases as the left ventricle takes over
        • prostaglandins keeps open the ductus arteriosus
          • e.g., alprostadil is an PGE1
  • Fetal Erythropoiesis
    • Hemoglobin
      • composed of 2 sets of 2 subunits
        • α2γ2 - fetal form
        • α2β2 - adult form
        • γ subunit has greater affinity than β subunit to allow for oxygen transfer across the placenta
      • hemoglobin production
        • week 3-8: yolk sac
        • week 6-30: liver
        • week 9-28: spleen
        • week 28: bone marrow
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