Fetal Circulation and Postnatal Changes

Fetal Lung Development and Circulation

Fetal Lungs

  • Fetal lungs are collapsed, meaning they do not participate in gas exchange.
  • The fetus is surrounded by amniotic fluid within the amniotic sac inside the mother's uterus; therefore, the fetus doesn't breathe with air but with fluid.
  • Because the lungs are collapsed, the peripheral resistance in the pulmonary circuit is much higher than in the systemic circuit.
  • Only a small amount of blood passes through the fetal lungs.

Placenta

  • The placenta is a temporary organ attached to the uterine wall.
  • It facilitates gas exchange between the fetal blood and the mother's blood.
  • The placenta is connected to the fetus via the umbilical cord.

Umbilical Cord Vessels

  • The umbilical cord contains three vessels:
    • One umbilical vein (red): Carries oxygenated blood from the placenta to the fetus.
    • Two umbilical arteries (blue): Carry deoxygenated blood away from the fetus to the placenta.

Fetal Blood Circulation

  • Oxygenated blood travels via the umbilical vein through the umbilicus into the fetus's body.
  • Immediately upon entering the body, the umbilical vein becomes the ductus venosus, which bypasses the liver.
  • The ductus venosus empties oxygenated blood into the inferior vena cava.
  • The inferior vena cava carries mixed blood (oxygenated and deoxygenated) to the right atrium.
  • Fetal circulation uses two structures in the heart to bypass the lungs because the lungs are collapsed, and pulmonary pressure is high.

Foramen Ovale

  • Foramen Ovale: In the interatrial septum, the foramen ovale (oval window) allows blood to flow from the right atrium directly into the left atrium, bypassing the pulmonary circulation.

Ductus Arteriosus

  • Ductus Arteriosus: Connects the pulmonary trunk to the aortic arch, allowing blood from the pulmonary trunk to bypass the lungs and enter the systemic circulation.
  • Deoxygenated blood is carried away from the fetal body via two umbilical arteries through the umbilicus to the placenta for gas exchange.

Postnatal Changes in Circulation

  • When a baby is born, the first breath inflates the lungs.
  • Lung inflation causes pulmonary pressure to drop, reversing the pressure gradient between the systemic and pulmonary circuits.
  • The umbilical cord is clamped and then cut.

Obliteration of Fetal Vessels

  • Cutting the umbilical cord causes the blood vessels within to empty, collapse, and obliterate.
  • Umbilical Vein: Becomes the ligamentum teres hepatis (round ligament of the liver) inside the baby.
  • Umbilical Arteries: Become the medial umbilical folds on the posterior surface of the lower anterior abdominal wall.
  • Foramen Ovale: Closes and becomes the fossa ovale (a shallow depression in the interatrial septum, visible from the right atrium).
  • Ductus Arteriosus: Obliterates and becomes the ligamentum arteriosum, connecting the pulmonary trunk and the aortic arch.

Anatomical References

  • Page 37, 38 and 57 may contain relevant information. Consult those pages for more reference.