(Ninja Nerds) Embryology | Development of Fetal Circulation (Shorter Video)

Fetal Circulation Overview

• Fetal circulation involves understanding blood flow before birth (in utero) and after birth (post-umbilical cord cutting).
• Fetal structures transform into adult remnants.

Fetal Circulation Before Birth

• A baby in utero gets oxygen from the placenta, not by breathing air; alveolar oxygen concentration is near zero.
• Low oxygen in alveoli causes pulmonary capillaries to constrict pulmonary arteries, diverting blood from that area.

Hypoxic Vasoconstriction

• Low oxygen levels induce hypoxic vasoconstriction in the pulmonary circulation, specifically the pulmonary arteries.
• Vasoconstriction shunts blood away from poorly oxygenated alveoli to better-oxygenated ones, but fetal lungs have overall low oxygen.
• Intense vasoconstriction in the fetal pulmonary circulation increases pulmonary artery pressure.
• High pulmonary artery pressure increases pressure in the right ventricle and right atrium as they work harder to pump blood.
• Right-side heart pressure is greater than left-side heart pressure in the fetus due to hypoxic vasoconstriction.

Placenta and Umbilical Cord

• The placenta removes CO2 and provides oxygen to the fetal blood.
• Oxygenated blood runs from the placenta via the umbilical vein.
• The umbilical cord houses the umbilical vein.
• The left umbilical vein is the one that persists.
• The umbilical vein connects to sinusoids within the liver.
• The umbilical vein also connects to the inferior vena cava via the ductus venosus.
• The ductus venosus is a shunt that moves blood from the umbilical vein to the inferior vena cava.
• Blood in the umbilical vein is highly oxygenated (around 85% saturation), unlike most veins.
• The hepatic portion of the inferior vena cava comes from the right vitelline vein during development.

Blood Flow Through the Heart

• Blood from the inferior vena cava enters the right atrium.
• The superior vena cava brings blood from the head and neck to the right atrium.
• The foramen ovale is a hole between the right and left atria in the fetal heart.
• The septum primum, septum secundum and ostium secundum form the foramen ovale.
• Blood preferentially flows from the high-pressure right atrium to the low-pressure left atrium through the foramen ovale.
• Some blood (around 30%) flows from the right atrium to the right ventricle.

Pulmonary Trunk and Ductus Arteriosus

• Blood from the right ventricle flows into the pulmonary trunk.
• The ductus arteriosus connects the pulmonary arteries to the aorta.
• Due to high pulmonary artery pressure, blood is shunted from the pulmonary trunk to the aorta via the ductus arteriosus.
• The foramen ovale and ductus arteriosus prevent much blood from reaching the lungs.

Aorta and Umbilical Arteries

• Blood flows through the aorta, ascending aorta, aortic arch, and descending aorta.
• The descending aorta bifurcates into common iliac arteries.
• The common iliac arteries split into external and internal iliac arteries.
• The internal iliac arteries give rise to the umbilical arteries.
• Umbilical arteries carry deoxygenated blood with low oxygen saturation (around 50%) from the baby to the placenta.
• Blood moves via the umbilical arteries to the chorionic arteries, then to the chorionic villi in the placenta.
• Gas exchange occurs in the placenta where CO2 is dropped off and oxygen is picked up, then blood returns via umbilical veins to start the circulation again.

Fetal Circulation After Birth

• After birth, the umbilical cord is cut, and the baby begins breathing air.
• Oxygen floods the alveoli, increasing oxygen concentration.
• Increased alveolar oxygen causes pulmonary vessels to dilate, reducing vasoconstriction.
• Pulmonary artery pressure decreases, reducing pressure on the right side of the heart.
• Right-side heart pressure becomes less than left-side heart pressure.

Foramen Ovale Closure

• After birth, blood cannot flow backward through the foramen ovale due to the interatrial septum structure.
• The foramen ovale should close after birth, turning into the fossa ovalis (fibrotic tissue).

Blood Flow Post-Birth

• As the pulmonary vessels open, blood flow through the pulmonary arteries increases.
• Pulmonary veins carry blood to the left atrium.
• Blood can no longer flow between the atria.
• Blood flows from the left atrium to the left ventricle and then to the aorta.

Ductus Arteriosus Sensitivity

• The ductus arteriosus closes due to sensitivity to oxygen, prostaglandins, and bradykinins.
• High oxygen, low prostaglandins, and high bradykinin levels cause the ductus arteriosus to close.
• After closing, no blood is shunted between the pulmonary trunk and the aorta.
• Blood circulates separately through the aortic and pulmonary circulations.

Blood Flow from the Aorta

• Blood continues to flow through the aorta, descending aorta, and common iliac arteries.
• The umbilical arteries degenerate into fibrotic tissue.
• The umbilical veins degrade into fibrotic tissue.
• The ductus arteriosus closes and becomes fibrotic tissue.

Fetal Structure Adult Remnants

• Understanding fetal circulation requires knowing the adult remnants of key fetal structures.

Ductus Venosus

• The ductus venosus (connection between umbilical vein and inferior vena cava) becomes the ligamentum venosum.

Foramen Ovale

• The foramen ovale (hole between right and left atria) becomes the fossa ovalis after closing.
• Failure of the foramen ovale to close results in a patent foramen ovale, which can lead to a paradoxical embolus and stroke.

Ductus Arteriosus

• The ductus arteriosus (connecting pulmonary trunk and aorta) becomes the ligamentum arteriosum.
• Failure of the ductus arteriosus to close results in a patent ductus arteriosus, a type of acyanotic congenital heart defect.

Umbilical Vein

• The umbilical vein (carrying oxygenated blood from placenta) becomes the ligamentum teres, which connects to the liver.
• The ligamentum teres can recanalize in cases of portal hypertension.

Umbilical Arteries

• The umbilical arteries (two of them) become the medial umbilical ligaments.
• Some portions of the umbilical artery can remain patent and provide blood flow from the internal iliac to the bladder, and this is called superior vesicle artery.

Allantois

• The allantois becomes the urachus, which then becomes the median umbilical ligament.

Umbilical Cord Cross-Section

• A cross-section of the umbilical cord reveals the umbilical vein, two umbilical arteries, the allantois, vitalin duct, vitalin arteries, vitalin veins and Wharton’s jelly.
• Wharton’s jelly provides insulation to the umbilical cord structures.