Fetal circulation is an essential, unique process that supports the growth and development of a fetus within the womb. It provides oxygen and nutrients to the developing baby, while also handling waste removal.
This process operates exclusively during pregnancy and undergoes significant changes post-birth as the newborn adapts to life outside the womb.
This article aims to explore the specifics of fetal circulation, including its function, structure, and postnatal transition.
What is Fetal Circulation?
Fetal circulation is a remarkable process that ensures the proper growth and development of a fetus in the womb.
It involves the circulatory system of a fetus, which is markedly different from postnatal (after birth) circulation.
The process is specifically designed to ensure oxygen, nutrients, and blood are efficiently supplied to the developing fetus while also removing waste products.
Key components of fetal circulation include:
- Umbilical cord
- Fetal shunts
This organ connects the fetus to the mother’s uterine wall and allows nutrient and gas exchange.
Oxygen and nutrients from the mother’s blood are transferred to the fetus, while carbon dioxide and waste materials are transferred from the fetus to the mother for disposal.
This connects the fetus to the placenta and contains two umbilical arteries and one umbilical vein.
The arteries carry deoxygenated blood and waste products from the fetus to the placenta, while the vein carries oxygenated blood and nutrients from the placenta to the fetus.
These are small passages that facilitate blood flow in the fetus. There are three main shunts in the fetal circulatory system:
- Ductus venosus: This shunt bypasses the liver and connects the umbilical vein to the inferior vena cava, allowing oxygen-rich blood from the placenta to flow directly to the heart.
- Foramen ovale: This is an opening between the two upper chambers of the heart (atria). It allows blood to flow from the right atrium to the left atrium, bypassing the lungs since oxygenation of the blood occurs through the placenta, not the fetal lungs.
- Ductus arteriosus: This connects the main pulmonary artery to the aorta, allowing blood to bypass the lungs and flow directly into the body’s main artery.
These fetal adaptations are critical for life inside the womb and usually close or regress soon after birth as the baby’s circulatory system adapts to independent life.
Process of Fetal Circulation
The process of fetal circulation is different from that of an adult because the fetus is not yet capable of breathing air and obtaining oxygen through the lungs.
Instead, oxygen and nutrients are supplied to the fetus by the placenta via the umbilical cord.
Oxygenated blood from the placenta enters the fetal body through the umbilical vein and flows into the hepatic circulatory system.
Approximately one-third of this blood goes to the lower trunk and extremities, while the other two-thirds flow through the ductus venosus and directly into the inferior vena cava.
This blood in the inferior vena cava is well-oxygenated, but it mixes with venous blood returning from the lower extremities entering the right atrium.
Around 50% of this mixed blood is shunted from the right atrium into the left atrium through the foramen ovale, which is a small opening in the interatrial septum.
The left atrial blood then flows into the left ventricle and eventually into the ascending aorta, supplying the brain, brachiocephalic trunk, and descending aorta.
Venous blood from the superior vena cava is directed downward into the right atrium, then into the right ventricle, and finally into the main pulmonary artery.
However, the relatively low PaO2 and the presence of prostaglandins in the fetal blood result in pulmonary vasoconstriction and increased pulmonary vascular resistance.
This leads to a higher pulmonary artery pressure than aortic blood pressure.
This causes approximately 90% of the blood entering the pulmonary artery to shunt through the ductus arteriosus, which is a muscular vessel connecting the trunk of the pulmonary artery to the aorta, into the systemic circulation.
Only the remaining 10% of blood actually flows into the lungs.
The mixed blood flowing through the ductus arteriosus is routed into the systemic circulation, supplying some of it to the gut, lower extremities, and placenta.
Where Does Nutrient and Gas Exchange Occur in Fetal Circulation?
The nutrient and gas exchange in fetal circulation occurs at the placenta, an organ that develops in the uterus during pregnancy. The placenta is a unique structure that connects the fetus to the mother’s uterine wall via the umbilical cord, which contains two arteries and one vein.
The umbilical vein carries oxygenated blood and nutrients from the mother’s bloodstream, via the placenta, to the fetus. Conversely, the umbilical arteries transport deoxygenated blood and waste products from the fetus back to the placenta.
In the placenta, the mother’s blood and the fetus’s blood come into close proximity (although they do not mix), enabling the exchange of nutrients, oxygen, and waste products by diffusion.
The mother’s blood supplies the fetus with the oxygen and nutrients it needs to grow and develop, while the fetal blood passes back carbon dioxide and other waste materials to be eliminated by the mother’s system.
This efficient exchange mechanism is a crucial aspect of development and survival, providing the fetus with the oxygen and nutrients necessary for growth and development until it can breathe air and take in oxygen through the lungs.
Note: During fetal circulation, 90% of blood bypasses the pulmonary circulation through the foramen ovale and ductus arteriosus (right-to-left shunting). Shunting that continues after birth is considered to be abnormal and may require medical intervention.
Fetal circulation demonstrates the intricacy and efficiency of nature’s design, ensuring the survival and growth of a fetus within the womb.
It is a unique and complex system that facilitates the exchange of nutrients and gases between the mother and the fetus via the placenta, all while bypassing certain routes used in postnatal life.
Once birth occurs, these specialized pathways undergo significant changes, adapting to a new physiological state.
Knowledge of fetal circulation is crucial, not just for understanding human development but also in diagnosing and treating prenatal conditions and managing the transition to life outside the womb.
By exploring the extraordinary processes of fetal circulation, we are able to appreciate the remarkable stages of human life even before birth.
John Landry, BS, RRT
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.