Fetal Circulation by L. McCabe | OPENPediatrics

Summary

This lecture by Lisa McCabe, a Clinical Nurse Specialist at Children's Hospital Boston, explains fetal circulation and the transition to postnatal circulation. Key structures involved in fetal circulation include the placenta, umbilical vein, ductus venosus, foramen ovale, ductus arteriosus, and umbilical arteries. Oxygenated blood from the placenta flows through these structures to nourish the fetus. After birth, the circulation changes as the lungs expand, the placenta is removed, and certain fetal structures close. The foramen ovale and ductus arteriosus normally close, transitioning the baby to postnatal circulation.

Takeaways

• 👶 Fetal circulation includes key structures: the placenta, umbilical vein, ductus venosus, foramen ovale, ductus arteriosus, and umbilical arteries.
• 🌬 Blood rich in oxygen and nutrients flows from the placenta through the umbilical vein to the ductus venosus, then into the heart's right atrium.
• 💖 The foramen ovale allows blood to bypass the lungs by flowing from the right atrium to the left atrium, then into the left ventricle, which pumps it to the body.
• 🫁 Due to high resistance in the fluid-filled fetal lungs, most blood from the right ventricle flows through the ductus arteriosus into the aorta, bypassing the lungs.
• 🩸 Only 8% of blood from the right ventricle reaches the lungs, which are not yet needed for gas exchange, as the placenta handles oxygenation.
• 🪡 After birth, the umbilical cord is clamped, and the placenta is removed from circulation, starting the transition to postnatal circulation.
• 🏃‍♂️ The increase in systemic vascular resistance and decrease in pulmonary pressure after birth allows blood to flow into the lungs for oxygenation.
• 🚫 The foramen ovale closes as left atrial pressure rises, and in most cases, it permanently seals within three months.
• 🔒 The ductus arteriosus and ductus venosus close shortly after birth, becoming the ligamentum arteriosum and ligamentum venosum, respectively.
• 📊 While the foramen ovale closes in most people, studies show 15-25% of adults have a patent foramen ovale (PFO) that never fully closes.

Q & A

What are the main structures associated with fetal circulation?

The main structures associated with fetal circulation include the placenta, the umbilical vein, the ductus venosus, the foramen ovale, the ductus arteriosus, and the umbilical arteries.

What is the function of the umbilical vein in fetal circulation?

The umbilical vein carries blood rich in oxygen and nutrients from the placenta to the ductus venosus, which then directs the blood to the fetal heart.

How does blood flow between the right atrium and the left atrium in fetal circulation?

In fetal circulation, blood flows from the right atrium to the left atrium through the foramen ovale, allowing oxygenated blood to bypass the lungs and reach the left ventricle and aorta.

Why does only a small amount of blood flow to the fetal lungs?

Because the fetal lungs are fluid-filled and have high vascular resistance, only about 8% of the right ventricular output flows to the lungs, with most of the blood bypassing the lungs via the ductus arteriosus to the aorta.

What happens to the placenta after birth and how does this affect circulation?

After birth, the placenta is removed from the systemic circulation, causing systemic vascular resistance to rise and initiating the transition to postnatal circulation.

How does the baby’s first breath influence the pulmonary circulation?

With each breath, the baby’s alveoli expand and the surrounding blood vessels dilate in response to oxygen, which lowers pulmonary vascular resistance.

What is the role of the foramen ovale after birth?

After birth, the foramen ovale functionally closes as left atrial pressure increases, preventing blood flow between the atria. In most cases, it permanently closes within three months.

What happens if the foramen ovale doesn’t close after birth?

If the foramen ovale remains open, a condition called patent foramen ovale (PFO) occurs. While the PFO usually has a small shunt, autopsy studies show that 15-25% of adults may have a PFO that never closed.

When does the ductus arteriosus typically close after birth?

The ductus arteriosus begins to close shortly after birth and usually closes completely within 4-10 days.

What is the fate of the umbilical vessels after birth?

After birth, the umbilical vein and arteries are infiltrated with fibrin and eventually become ligaments within about a week.