Fetal Circulation Notes

Fetal Circulation

Placental Exchange

  • The placenta is the site of nutrient, gas, and waste exchange between mother and fetus.
  • Maternal and fetal blood do not mix due to potential blood type incompatibility.
  • Diffusion is the primary method for transferring water, glucose, amino acids, and inorganic salts.
  • Diffusion requires a concentration gradient; the partial pressure of oxygen is higher in maternal blood than in fetal blood.

Fetal Hemoglobin (HbF)

  • Fetal blood cells contain HbF, which has a greater affinity for oxygen compared to adult hemoglobin (HbA).
  • This facilitates the transfer and retention of oxygen in the fetal circulatory system.
  • Waste materials and carbon dioxide move from fetal to maternal circulation.

Placental Barrier and Immunity

  • The placenta provides a barrier and immunity to the fetus.
  • The fetus is immunologically naive and can be accidentally exposed to pathogens in utero.
  • Antibodies cross the placental membrane, providing protection.

Placenta as an Endocrine Organ

  • The placenta produces:
    • Progesterone
    • Estrogen
    • Human Chorionic Gonadotropin (HCG)
  • These hormones are essential for maintaining pregnancy.

Umbilical Vessels

  • Umbilical arteries carry blood away from the fetus to the placenta (deoxygenated).
  • Umbilical vein carries blood towards the fetus from the placenta (oxygenated).
  • Oxygenation occurs at the placenta, not the fetal lungs.

Differences Between Fetal and Adult Circulation

  • Fetal lungs and liver have limited function before birth.
  • Gas exchange occurs at the placenta.
  • The mother's liver handles detoxification and metabolism.
  • Nutrient and waste exchange occurs at the placenta.
  • The fetal lungs and liver are underdeveloped and sensitive to high blood pressure after birth.

Fetal Shunts

  • Fetal shunts direct blood away from the lungs and liver during development.
Foramen Ovale
  • A one-way valve that connects the right atrium to the left atrium.
  • Allows blood from the inferior vena cava to flow from the right atrium to the left atrium, bypassing the right ventricle, and then into the aorta and systemic circulation directly.
  • In the fetus, pressure is higher on the right side of the heart than the left, pushing blood through the foramen ovale.
  • After birth, the pressure differential reverses, causing the foramen ovale to close.
Ductus Arteriosus
  • Shunts leftover blood from the pulmonary artery to the aorta.
  • Driven by the pressure differential between the right and left sides of the heart.
Ductus Venosus
  • Shunts blood returning from the placenta via the umbilical vein directly into the inferior vena cava, bypassing the liver.
  • The liver still receives some blood supply from smaller hepatic arteries through systemic circulation.