Fetal Circulation in Utero – Pathway, Shunts (Foramen Ovale, Ductus Arteriosus, Ductus Venosus) & Placental Role

Fetal circulation is a unique circulatory system that operates only before birth. Unlike postnatal circulation, where the lungs oxygenate blood, the fetus relies entirely on the placenta for oxygen and nutrient exchange. Because the fetal lungs are filled with fluid and not functional for gas exchange, the circulation system uses special shunts to bypass the lungs and liver, ensuring that oxygen-rich blood reaches vital organs such as the heart and brain.

Understanding fetal circulation is crucial for medical students, obstetricians, pediatricians, and nurses, as it explains congenital heart defects, perinatal complications, and the dramatic changes that occur at birth.

Key Differences Between Fetal and Postnatal Circulation

1. Placenta replaces the lungs as the site of oxygen and carbon dioxide exchange.

2. Special shunts are present in the fetus to redirect blood flow.

3. Fetal blood contains higher hemoglobin levels and fetal hemoglobin (HbF), which has a greater affinity for oxygen.

4. Pulmonary circulation is high resistance due to fluid-filled lungs, while systemic circulation is low resistance thanks to the placenta.

Role of the Placenta

The placenta acts as the lifeline between mother and baby, performing the roles of:

• Gas exchange: Oxygen moves from maternal to fetal blood, while carbon dioxide moves in the opposite direction.
• Nutrient transfer: Glucose, amino acids, and fatty acids pass from mother to fetus.
• Waste removal: Urea and other waste products move from fetal blood to maternal circulation.
• Endocrine functions: Secretes hormones such as hCG, progesterone, and estrogen.

Blood vessels involved in placental exchange:

• Umbilical vein: Carries oxygenated blood from placenta to fetus.
• Umbilical arteries (2): Carry deoxygenated blood and waste products from fetus to placenta.

Pathway of Fetal Circulation

1. Oxygen-rich blood from the placenta enters the fetus through the umbilical vein.

2. Most of this blood bypasses the liver through the ductus venosus and enters the inferior vena cava.

3. Blood enters the right atrium of the heart, where it is mixed with deoxygenated blood from the superior vena cava.

4. Due to pressure differences, blood is shunted through the foramen ovale from the right atrium to the left atrium, bypassing the lungs.

5. Blood that does enter the right ventricle is pumped into the pulmonary artery, but most bypasses the lungs via the ductus arteriosus, entering the aorta.

6. The mixed blood in the aorta is distributed to fetal tissues.

7. Deoxygenated blood returns to the placenta through the umbilical arteries to be re-oxygenated.

The Three Major Shunts of Fetal Circulation

1. Ductus Venosus

• Connects the umbilical vein to the inferior vena cava, bypassing the liver.
• Function: Ensures that highly oxygenated blood from the placenta reaches the heart and brain quickly.

2. Foramen Ovale

• An opening between the right and left atria.
• Function: Shunts blood from the right atrium directly to the left atrium, bypassing the non-functioning lungs.

3. Ductus Arteriosus

• Connects the pulmonary artery to the aorta.
• Function: Diverts blood away from the fluid-filled lungs and into systemic circulation.

Oxygen Saturation in Fetal Circulation

Oxygen saturation differs at various points:

• Umbilical vein (from placenta): ~80% oxygenated
• Inferior vena cava (after mixing): ~67%
• Right atrium: Mixed
• Left atrium and ventricle: Higher oxygen concentration (direct from placenta via foramen ovale)
• Umbilical arteries (return to placenta): ~58%

This system ensures that the most oxygenated blood is delivered to the heart and brain, the most vital organs for fetal survival.

Transition to Postnatal Circulation

At birth, a newborn undergoes dramatic circulatory changes when the umbilical cord is clamped and the first breath is taken:

1. Lungs expand, decreasing pulmonary resistance.

2. Foramen ovale closes due to increased left atrial pressure, eventually becoming the fossa ovalis.

3. Ductus arteriosus constricts due to increased oxygen and decreased prostaglandins, forming the ligamentum arteriosum.

4. Ductus venosus closes, becoming the ligamentum venosum in the liver.

5. Umbilical vessels collapse — the umbilical vein becomes the ligamentum teres, and the umbilical arteries become the medial umbilical ligaments.

These changes establish normal postnatal circulation, where the lungs now oxygenate blood.

Clinical Significance of Fetal Circulation

Understanding fetal circulation is essential in recognizing and managing congenital and perinatal conditions:

Patent Foramen Ovale (PFO): Failure of foramen ovale closure, common but usually asymptomatic.

Patent Ductus Arteriosus (PDA): Persistence of ductus arteriosus, leading to left-to-right shunt and heart failure if untreated.

Persistent Pulmonary Hypertension of the Newborn (PPHN): Failure of pulmonary resistance to fall, leading to cyanosis.

Congenital Heart Defects: Many anomalies alter fetal and neonatal circulation patterns.

Table: Key Fetal Shunts and Their Fate

Shunt	Location	Function (Fetus)	Postnatal Remnant
Ductus Venosus	Umbilical vein → IVC	Bypasses liver	Ligamentum venosum
Foramen Ovale	Between right & left atria	Bypasses lungs	Fossa ovalis
Ductus Arteriosus	Pulmonary artery → Aorta	Bypasses lungs	Ligamentum arteriosum
Umbilical Vein	Placenta → Fetus	Carries oxygenated blood	Ligamentum teres
Umbilical Arteries	Fetus → Placenta	Return deoxygenated blood	Medial umbilical ligaments

Conclusion

Fetal circulation is an elegant and highly efficient system that ensures optimal oxygen delivery to vital organs in the womb. With the placenta functioning as the lungs and liver, specialized shunts play a key role in bypassing nonfunctional fetal organs. After birth, these shunts close, and the newborn transitions into the postnatal circulatory system.

A solid understanding of this physiology not only helps in medical education but is also vital in diagnosing and treating congenital cardiac conditions in neonates.

FAQs on Fetal Circulation

Q1. Why does fetal blood bypass the lungs?

Because the fetal lungs are filled with fluid and non-functional for gas exchange, blood is diverted through shunts.

Q2. Which fetal shunt bypasses the liver?

The ductus venosus shunts blood from the umbilical vein to the inferior vena cava.

Q3. What happens if the ductus arteriosus does not close after birth?

It results in Patent Ductus Arteriosus (PDA), which can cause heart failure if untreated.

Q4. What is the oxygen saturation difference between umbilical vein and arteries?

The umbilical vein carries oxygen-rich blood (~80%), while umbilical arteries carry deoxygenated blood (~58%).

Q5. When does the foramen ovale close?

Functionally at birth due to pressure changes, and anatomically within the first year of life.