Fetal Circulation: Development and Changes at Birth

Fetal circulation works differently than adult circulation.The organ that grows and implants in the mother's uterus during pregnancy is the placenta, which is connected to the fetus by the umbilical cord. 

The mother's placenta, which is connected to the umbilical cord by blood vessels, provides the fetus with all of the nutrients, oxygen, and life support that it needs. The umbilical cord and placenta return waste materials and carbon dioxide from the developing fetus to the mother's circulation so she can expel them.

Read this blog further to get a quick overview of this important topic for anatomy and ace your NEET PG exam preparation.

Fetal Circulation

Inside the umbilical cord in the beginning there are 2 umbilical arteries and 2 umbilical veins. Later on the right one is regressed and only the left one persists so the Umbilical cord contains 1 vein & 2 arteries. 

Umbilical vein carries oxygenated blood (SpO2 - 80%). Single umbilical artery can be seen in 5-10 births out of 1000 births and is associated with increased risk of renal anomaly. It is most commonly seen in twin pregnancy and is associated with Trisomy 18. Delayed fall of umbilical cord is seen in Leukocyte adhesion defect which is a form of immunodeficiency. 

Three shunts, or tiny channels, are used by the fetal circulatory system to direct blood that needs to be oxygenated. These shunts are meant to bypass organs like the liver and lungs, which are still forming while the fetus is still in the womb. 

The foramen ovale, which transports blood from the right atrium of the heart to the left atrium, and the ductus arteriosus are the two shunts that bypass the lungs and  transports blood from the pulmonary artery to the aorta.

The fetus receives nutrients and oxygen from the mother's blood through the placenta. As it passes through the liver and the umbilical cord, the improved blood splits into three branches. The blood then continues to the inferior vena cava, the main vein of the heart. 

The ductus venosus, a shunt that also moves highly oxygenated blood from the liver to the inferior vena cava and subsequently to the right atrium of the heart, is responsible for carrying the vast majority of this blood. Some of this blood is delivered straight to the liver to give it the nutrients and oxygen it needs.

The right atrium, a chamber on the upper right side of the heart, receives blood. The majority of the blood that enters the right atrium travels through the foramen ovale and into the left atrium. The main artery that emerges from the heart, the aorta, is then reached after blood enters the left ventricle.

Blood is delivered to the brain, arms, and heart muscle itself from the aorta. The blood then circulates there before returning through the superior vena cava to the right atrium of the heart. There is very little of this blood that is less oxygenated that mixes with the oxygenated blood and enters the right ventricle rather than returning through the foramen ovale.

This blood, which has less oxygen, is pumped into the aorta by the right ventricle. The blood only goes to the lungs in modest amounts. The ductus arteriosus passes the  blood to the descending aorta. After traveling through the umbilical arteries, this blood is then received by the placenta. While waste products and carbon dioxide from the placenta are discharged into the mother's circulatory system, oxygen and nutrients from the mother's blood are released into the blood of the fetus.

The baby's umbilical cord is clamped at birth, cutting off the mother's supply of oxygen and nutrition. The lungs start to grow with the first breaths of life. The alveoli in the lungs are drained of fluid as the lungs expand. 

The baby's pulmonary pressures noticeably decrease and their blood pressure increases, which reduces the need for the ductus arteriosus to shunt blood so the shunt is closed. These changes cause the pressure in the left atrium of the heart to increase while it decreases in the right atrium.

Changes in Circulation at or After Birth

• Lungs become the source of O₂ instead of placenta.
• Pulmonary vascular resistance decreases and Blood flow to lung increases 
• Systemic vascular resistance increases 
• Foramen ovale closes
• Ductus venosus closes and forms ligamentum venosus, Functional closure is 10-96 hrs after birth and anatomical closure occurs 2-3 weeks after birth.
• Ductus arteriosus closes and forms ligamentum arteriosus. Functional closure occurs  immediately after birth at 10-15 hrs due to smooth muscle contraction and Anatomical closure occurs at day 15 to day 21 or up to 3 months of life , due to proliferation of cells of intima of ductus arteriosus.

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