Fetal Circulation

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One of the neonatology/pediatric points the CREOG exam will test on is flow of blood through the fetal circulation. It can be quite confusing, but it’s worth remembering. We’ll take you on the journey of a red blood cell in today’s episode.

The important foundational bit of knowledge for this is the nomenclature of arteries and veins. Arteries carry blood away from the heart (Arteries Away), whereas veins carry blood towards the heart. Arteries and veins do not denote oxygenation status, particularly in the fetal circulation!

(C) Children’s Hospital of Philadelphia

Let’s start at the umbilical vein, which is carrying oxygenated blood from the placenta towards the fetal heart. Remember there is a single large umbilical vein with normal umbilical cords.

  • The umbilical vein enters at the umbilicus, and moves superiorly towards the liver, where it ultimately needs to meet the inferior vena cava. However, the umbilical vein naturally empties into the portal hepatic vein. 

    • This is where we encounter our first fetal shunt, the ductus venosus.

      • This allows oxygenated blood from the umbilical vein to connect to the inferior vena cava, bypassing the portal vein and the liver. 

      • The ductus venosus closes functionally in term infants within minutes of birth, and full closure naturally occurs within one week of birth. In preterm infants this may take longer. The remnant structure is known as the ligamentum venosum.

  • From the IVC, we can get blood into the right atrium of the heart. Now blood will move to the right ventricle naturally in adult circulation. In fetal circulation, though, the lungs have yet to open. The pulmonary circulation is of very high resistance. Rather than take the long, high resistance trip around the lungs, we encounter our second shunt, the foramen ovale between the right and left atria. 

    • The relatively high pressure in the right atrium allows for blood to move across this shunt into the left atrium. 

    • With the first neonatal breaths, the lungs open and the resistance to the pulmonary circulation drastically drops. This allows for the foramen ovale to close, as the septum secundum (some tissue in the right atrium where the foramen ovale is located), is effectively a one way valve from right to left; when flow starts to go left-to-right, this valve closes.

      • In up to 25% of adults, this one-way valve closure is not completely effective, leading to the patent foramen ovale.

  • Now blood is in the left heart, where it can move from left atria, to left ventricle, to aorta, and now supply the fetal brain and other tissues.

  • However, some blood may still move to the right ventricle in spite of the pressure gradient, and try to move through the pulmonary circulation. 

    • To exit the pulmonary circulation more quickly and supply oxygenated blood to the lower extremities, we encounter our third shunt, the ductus arteriosus. This connects the pulmonary artery to the descending aorta. After birth, this closes and becomes the ligamentum arteriosum. 

    • In some individuals, the ductus arteriosus remains open, leading to a patent ductus arteriosus. Because of the change in pressure after birth, now oxygenated blood is leaving the aorta and overloading the pulmonary artery. This can lead to pulmonary hypertension and right heart failure. 

      • In PDA and rarely PFO, but more commonly with ventricular septums, the pulmonary hypertension becomes so great as to change the pressure differential again (i.e., pulmonary or right heart circulation pressure is greater than left heart or systemic circulation). This changes the shunt to send deoxygenated blood from the right heart into the systemic circulation, and is known as Eisenmeger syndrome. 

  • Now that we’ve gotten all the blood to the left heart, it moves through the arteries to supply organs and tissues, and will end up in the veins. 

    • Coming from superiorly, blood will end up in the superior vena cava, and end up back in the right atrium. From here, it’s the same cycle all over again -- some will go through the foramen ovale, some will go to the right ventricle and pass through the ductus arteriosus. 

    • If blood went inferiorly (i.e., went through the descending aorta/ductus arteriosus), the umbilical arteries will carry blood back towards the placenta for re-oxygenation and deposition of CO2 and waste products. 

      • The umbilical arteries originate off the internal iliac arteries bilaterally. After birth, they become obliterated and are known as the medial umbilical ligaments. These can be seen during laparoscopic surgery and are good markers for the position of the superior vesical arteries. More on that in a future episode on pelvic anatomy!

Though we have the anatomic picture above, some folks may find a schematic helpful. Run through this a few times before your exam and you’ll be golden!

(C) Epomedicine