Fetal Circulation

Fetal Circulation: Overview

• Fetal circulation occurs in two phases: before birth (in utero) and after birth (post-umbilical cord cutting).
• The circulatory structures used in fetal circulation transform into adult remnants after birth.

Fetal Circulation Before Birth

• The fetus obtains oxygen from the placenta while in utero, bypassing the lungs.
• Oxygen concentration within the alveoli of fetal lungs is virtually zero.
• Low oxygen levels in the alveoli trigger the pulmonary capillaries to constrict.
• Vasoconstriction shunts blood away from poorly oxygenated alveoli to better-oxygenated alveoli.
• In fetal circulation, all alveoli have low oxygen, causing widespread vasoconstriction.
• Hypoxic vasoconstriction increases pulmonary artery pressure, raising pressure on the right side of the heart.
• Right-side heart pressures are greater than left-side pressures in the fetus.
• Hypoxic vasoconstriction describes pulmonary vasoconstriction due to low oxygen.
• The placenta removes CO2 from and provides oxygen to the fetal blood.
• Blood travels from the placenta via the umbilical vein.
• The umbilical cord contains the umbilical vein.
• The left umbilical vein persists during vascular system development.
• Blood is shunted from the umbilical vein to the inferior vena cava via the ductus venosus.
• The ductus venosus connects the umbilical vein to the inferior vena cava.
• The umbilical vein carries oxygenated blood at approximately 85% saturation.
• Blood from the inferior vena cava enters the right atrium.
• Blood from the head and neck enters the right atrium via the superior vena cava.
• The foramen ovale is a hole between the right and left atria in the fetal heart.
• High right atrial pressure causes blood to flow through the foramen ovale into the left atrium.
• Roughly 70% of blood goes from the right atrium to the left atrium through the foramen ovale.
• About 30% of blood flows from the right atrium to the right ventricle.
• Blood in the left atrium flows into the left ventricle, then to the aorta.
• Blood in the right ventricle flows into the pulmonary trunk.
• The ductus arteriosus connects the pulmonary arteries to the aorta.
• High pulmonary artery pressure causes blood to flow from the pulmonary trunk through the ductus arteriosus to the aorta.
• The foramen ovale and ductus arteriosus prevent most blood from reaching the lungs.
• Blood flows down the descending aorta, bifurcating into the common iliac arteries.
• The common iliac arteries split into external and internal iliac arteries.
• Blood flows from the internal iliac arteries into the umbilical arteries.
• Two umbilical arteries carry deoxygenated blood from the fetus back to the placenta.
• Umbilical artery oxygen saturation is approximately 50%.
• In the placenta, gas exchange occurs at the chorionic villi.
• The umbilical arteries drop off CO2, and oxygen is picked up, returning via the umbilical veins.

Fetal Circulation After Birth

• The umbilical cord is cut, and the baby is born and takes its first breath.
• Oxygen floods into the alveoli, raising the oxygen concentration.
• Increased oxygen causes pulmonary vessels to dilate, decreasing pulmonary artery pressure.
• Decreased pulmonary artery pressure reduces pressure on the right side of the heart.
• Right-side heart pressures become less than left-sided heart pressures.
• Hypoxic vasoconstriction ceases under normal circumstances.
• The umbilical veins become insignificant and eventually transform.
• The foramen ovale closes due to pressure changes, becoming the fossa ovalis.
• Fibrous tissue forms, preventing blood flow between the atria.
• Blood flow through the pulmonary arteries increases.
• Blood returns to the left atrium via the pulmonary veins.
• Blood in the left atrium flows into the left ventricle and then into the aorta.
• The ductus arteriosus is sensitive to oxygen, prostaglandins, and bradykinins.
• High oxygen, low prostaglandins, and high bradykinins cause the ductus arteriosus to close.
• The ductus arteriosus separates pulmonary and aortic circulations.
• Blood reaches the right atrium from the inferior vena cava (lower limbs) and superior vena cava (upper extremities and head).
• Blood flows from the right atrium to the right ventricle, then to the pulmonary trunk and pulmonary arteries.
• Umbilical arteries degenerate into fibrotic tissue.

Fetal Structure Remnants

• Ductus venosus (connection between umbilical vein and inferior vena cava) becomes the ligamentum venosum.
• Foramen ovale (hole between right and left atria) closes and becomes the fossa ovalis.
  • Failure to close can cause a patent foramen ovale, potentially leading to a paradoxical embolus.
• Ductus arteriosus (connection between pulmonary trunk and aorta) closes and becomes the ligamentum arteriosum.
  • Failure to close results in patent ductus arteriosus (PDA), an acyanotic congenital heart defect.
• Umbilical vein (oxygenated blood from placenta) becomes the ligamentum teres.
  • The ligamentum teres connects on the liver.
  • In portal hypertension, it can recanalize and cause caput medusae.
• Umbilical arteries (deoxygenated blood to placenta) become the medial umbilical ligaments.
  • Portions of the umbilical artery can remain patent, supplying blood to the bladder as the superior vesicle artery.
• Allantois (in umbilical cord) becomes the urachus, then the median umbilical ligament.
• Examination of a cross-section of the umbilical cord shows the umbilical vein, two umbilical arteries, the allantois, the vitelline duct, vitelline arteries and veins, and Wharton's jelly.
• Wharton's jelly provides insulation.