Fetal Circulation Study Notes

Introduction to Fetal Circulation

  • Speaker Information: Lisa McCabe, Clinical Nurse Specialist at Children's Hospital Boston, Cardiovascular Program.

  • Topic Overview: Detailed discussion on fetal circulation and the transition to postnatal circulation post-birth.

Key Structures of Fetal Circulation

  • Placenta: The organ that provides oxygen and nutrients to the fetus.

  • Umbilical Vein: Carries oxygen-rich blood from the placenta to the fetus.

  • Ductus Venosus: A vessel that allows blood to bypass the fetal liver.

  • Foramen Ovale: An opening between the right atrium and left atrium of the heart.

  • Ductus Arteriosus: A vessel connecting the pulmonary artery to the aorta, allowing blood to bypass the lungs.

  • Umbilical Arteries: Carry deoxygenated blood from the fetus back to the placenta.

Blood Flow Through Fetal Circulation

  • Oxygen-Rich Blood Pathway:

    • Blood rich in oxygen and nutrients flows from the placenta through the umbilical vein.

    • It enters the ductus venosus, allowing it to bypass the liver, and goes into the inferior vena cava.

    • This blood mixes with blood from the upper body (via the superior vena cava) and lower body (via the inferior vena cava) as it enters the right atrium.

    • Right Atrium Distribution:

    • Some blood flows to the right ventricle.

    • Some blood is shunted through the foramen ovale to the left atrium.

    • The oxygenated blood in the left atrium then enters the left ventricle and is pumped into the aorta and distributed to the rest of the body.

    • Clarification Point: This flow permits oxygenated blood from the placenta to reach the coronary arteries and cerebral circulation, which are critical for the developing fetus due to their high metabolic activity.

  • Right Ventricle Function:

    • Blood in the right ventricle is pumped into the pulmonary artery.

    • The lungs are fluid-filled in the fetus, causing high vascular resistance in the pulmonary circulation.

    • Due to this high resistance, blood is more likely to flow into the ductus arteriosus rather than the lungs.

    • Outflow from the pulmonary artery via the ductus arteriosus enters the aorta, thus bypassing the high-resistance pulmonary circulation.

  • Atrial Pressure Dynamics:

    • Higher pressure in the right atrium (due to significant blood flow from the superior and inferior vena cava) than in the left atrium promotes blood flow through the foramen ovale.

    • Only 8% of right ventricular output reaches the lungs, supplying nutrients necessary for developing lung tissues.

    • Majority of blood travels through the ductus arteriosus into the aorta and is circulated throughout the body.

Transition to Postnatal Circulation

  • After birth, when the baby takes the first breath:

    • The umbilical cord is clamped, and the placenta is detached from circulation.

    • Transition Initiation: The immediate removal of the placenta results in a rise in systemic vascular resistance.

    • Each breath in the newborn increases oxygen in the lungs:

    • Alveoli expand and pulmonary vessels dilate, leading to a decrease in pulmonary pressures.

    • Pulmonary vascular resistance decreases to normal within 6-8 weeks, following birth.

  • Changes in Heart Function:

    • In postnatal circulation, blood flow does not occur through the foramen ovale or the ductus arteriosus:

    • Increase in left atrial pressure after birth causes the septum primum to push against the septum secundum, effectively closing the foramen ovale.

    • Closure may become permanent within three months as fibrin deposits bind the septal walls.

    • Patent Foramen Ovale (PFO):

    • While the foramen ovale may close functionally by three months, studies indicate that 15-25% of adults may retain a patent foramen ovale that does not close.

  • Ductus Arteriosus Closure:

    • Closure of the ductus arteriosus begins shortly after the infant starts breathing and typically completes within 4-10 days after birth.

    • The ductus venosus remains open initially, allowing for central venous access through the umbilical vein and naturally closes within 3-7 days after birth due to fibrin infiltration, resulting in the formation of the ligamentum venosum.

    • The umbilical vein and umbilical arteries also undergo similar changes, becoming ligaments within a week following birth.

Conclusion

  • Final Remarks: This lecture provided a comprehensive understanding of fetal circulation and its transition to postnatal circulation, highlighting the key anatomical structures and physiological changes that occur post-birth. Further inquiries and improvements on the content are welcomed.