Fetal circulation is a remarkable and complex system that supports the developing fetus by providing essential oxygen and nutrients while removing waste products.
Unlike the typical circulatory system in adults, fetal circulation includes specialized structures and pathways that adapt to the unique needs of a growing baby in the womb. This system ensures that the fetus thrives while relying entirely on the placenta for oxygenation and nutrient exchange.
Understanding how fetal circulation works provides insight into the intricacies of human development and the seamless transition that occurs at birth when the newborn’s circulatory system changes to support independent life.
What is Fetal Circulation?
Fetal circulation is a unique and specialized system that allows blood flow in a developing fetus, designed to bypass the non-functional lungs and rely on the placenta for oxygen and nutrient exchange. The oxygen-rich blood from the placenta is carried through the umbilical vein to the fetus, where it enters the liver and partially bypasses it via the ductus venosus to flow into the inferior vena cava.
This blood then enters the right atrium of the heart, where it predominantly passes through the foramen ovale—an opening that allows blood to move directly to the left atrium, bypassing the lungs. From the left atrium, the oxygenated blood flows into the left ventricle and is pumped out through the aorta to supply the brain and other vital organs.
Meanwhile, blood returning from the body and lower oxygen content enters the right atrium, passes into the right ventricle, and is pumped into the pulmonary artery. Most of this blood is shunted via the ductus arteriosus into the aorta, bypassing the lungs, which are not yet engaged in gas exchange.
The deoxygenated blood is then carried back to the placenta through the umbilical arteries, where it is reoxygenated. After birth, the structures like the foramen ovale and ductus arteriosus close as the newborn’s lungs take over the role of oxygenation, transitioning the circulation to the typical adult pattern.
Fetal Shunts
Fetal shunts are specialized structures in the fetal circulatory system that direct blood flow to bypass the non-functional lungs and liver, ensuring efficient oxygen delivery and nutrient distribution throughout the developing body.
These shunts are essential for adapting to the fetus’s unique environment and dependence on the placenta for oxygenation.
There are three main fetal shunts:
- Ductus Venosus: This shunt is located within the liver and allows a portion of oxygen-rich blood from the umbilical vein to bypass the liver and flow directly into the inferior vena cava. This facilitates quicker delivery of highly oxygenated blood to the heart and subsequently to vital organs such as the brain.
- Foramen Ovale: This is an opening between the right and left atria of the fetal heart. It allows blood to flow directly from the right atrium to the left atrium, bypassing the lungs, which do not function for gas exchange in utero. This shunt ensures that the most oxygenated blood is sent to the brain and upper body.
- Ductus Arteriosus: This vessel connects the pulmonary artery to the aorta, allowing most of the blood from the right ventricle to bypass the lungs and flow directly into the systemic circulation. This is important for maintaining circulation without involving the non-functional lungs during fetal development.
Note: These shunts close shortly after birth as the newborn’s lungs take over the oxygenation process, marking the transition to normal postnatal circulation.
Where Does Nutrient and Gas Exchange Occur in Fetal Circulation?
In fetal circulation, nutrient and gas exchange occur at the placenta. The placenta is a specialized organ that develops during pregnancy, serving as the interface between the mother and fetus. It facilitates the exchange of oxygen, carbon dioxide, nutrients, and waste products through a complex network of blood vessels and membranes.
Oxygen-rich blood and essential nutrients from the mother’s bloodstream diffuse across the placental barrier into the fetal blood. This oxygenated and nutrient-rich blood is then carried to the fetus via the umbilical vein.
Simultaneously, carbon dioxide and metabolic waste products from the fetal blood are transferred across the placental membrane to the maternal blood, where they are expelled through the mother’s excretory systems.
This efficient exchange ensures that the developing fetus receives the necessary oxygen and nutrients for growth while removing waste products, even though the fetal lungs and digestive system are not yet functional.
Fetal Circulation Practice Questions
1. What structures in fetal circulation play a role in shunting blood away from the lungs?
The ductus arteriosus and foramen ovale.
2. What structures in fetal circulation play a role in shunting blood away from the lungs and liver?
The ductus arteriosus, foramen ovale, and ductus venosus.
3. Which statement below accurately describes the role of the ductus arteriosus?
It allows blood to bypass the fetal lungs by connecting the pulmonary trunk to the aorta.
4. Is the umbilical cord made up of two umbilical veins and one umbilical artery?
No, the umbilical cord is made up of two umbilical arteries and one umbilical vein.
5. The right ventricle pumps what type of blood up through the pulmonary artery?
Deoxygenated blood.
6. Which statement accurately describes how oxygen gets from the mother to the fetus?
Oxygen diffuses from the maternal blood through the placenta into the fetal blood.
7. What connects the atria of the fetal heart?
The foramen ovale.
8. Oxygenated blood from the placenta is delivered to the fetus through what?
The umbilical vein.
9. Which structure is responsible for bypassing the fetal liver and taking blood directly to the right atrium?
The ductus venosus.
10. A fetal circulatory structure is used to take blood from the pulmonary trunk and reroute it to the fetal aorta. What does this structure become after birth?
The ductus arteriosus becomes the ligamentum arteriosum.
11. Fetal blood is oxygenated by what?
The placenta.
12. Fetal circulation is uniquely designed to do what?
Prioritize oxygenated blood flow to the brain and heart while bypassing non-functional organs like the lungs.
13. The structure that diverts half of the oxygenated blood from the umbilical vein into the inferior vena cava is what?
The ductus venosus.
14. The structure that diverts oxygenated blood from the right atrium to the left atrium is what?
The foramen ovale.
15. The structure that diverts oxygenated blood from the pulmonary trunk to the fetal aorta is what?
The ductus arteriosus.
16. Following circulation, deoxygenated fetal blood returns to the placenta via the what?
The umbilical arteries.
17. What events happen during transitional circulation?
The lungs expand, the ductus arteriosus closes, the foramen ovale seals, and blood flow shifts to the lungs for oxygenation.
18. Which of the following is NOT true about postnatal development of the heart?
The ductus arteriosus remains open. (This is incorrect as it normally closes shortly after birth.)
19. The right ventricle pumps what type of blood up through the pulmonary artery?
Deoxygenated blood.
20. Where does nutrient and gas exchange occur in fetal circulation?
In the placenta.
21. What structure allows oxygen-rich blood to bypass the non-functional fetal lungs?
The ductus arteriosus.
22. What happens to the foramen ovale after birth?
It closes and forms the fossa ovalis.
23. How does blood bypass the fetal liver?
Through the ductus venosus, which directs blood from the umbilical vein to the inferior vena cava.
24. What is the function of the umbilical vein in fetal circulation?
To carry oxygenated blood from the placenta to the fetus.
25. What triggers the closure of the ductus arteriosus after birth?
Increased oxygen levels and reduced prostaglandin levels.
26. What adaptation ensures the fetus receives adequate oxygen to vital organs?
The strategic shunting of blood through the ductus venosus, foramen ovale, and ductus arteriosus.
27. How does blood flow change in the heart after birth?
The increased pressure in the left atrium causes the foramen ovale to close.
28. What occurs if the ductus arteriosus remains open after birth?
It can lead to patent ductus arteriosus (PDA), resulting in abnormal blood flow between the aorta and pulmonary artery.
29. How does the fetal right atrium receive oxygenated blood?
Via the inferior vena cava, which receives blood from the umbilical vein through the ductus venosus.
30. What is the purpose of the ductus venosus during fetal development?
To bypass the liver and direct oxygen-rich blood to the heart.
31. What change occurs in the fetal circulation when the umbilical cord is clamped?
Blood flow from the placenta stops, leading to the closure of the ductus venosus and umbilical vessels.
32. How does the fetal heart compensate for not using the lungs for oxygenation?
By redirecting blood flow through shunts such as the foramen ovale and ductus arteriosus.
33. What happens to the umbilical vein after birth?
It becomes the ligamentum teres in the liver.
34. What does the umbilical artery become after birth?
The medial umbilical ligament.
35. What causes the foramen ovale to close after birth?
The pressure in the left atrium becomes higher than in the right atrium as the lungs inflate and blood flow shifts.
36. How does oxygenated blood from the placenta enter the systemic circulation of the fetus?
Through the umbilical vein to the ductus venosus, then to the inferior vena cava, and into the right atrium.
37. What is the role of the placenta in fetal circulation?
To facilitate gas, nutrient, and waste exchange between the mother and fetus.
38. What feature helps ensure oxygen-rich blood reaches the fetal brain efficiently?
The shunting of blood through the foramen ovale, directing oxygenated blood from the right atrium to the left atrium.
39. What happens to the ductus venosus after birth?
It becomes the ligamentum venosum in the liver.
40. Why does blood bypass the fetal lungs in utero?
The lungs are not yet functioning for oxygen exchange, so blood is rerouted through the ductus arteriosus and foramen ovale.
41. What is the primary source of oxygen for the fetus?
The placenta.
42. How is deoxygenated blood carried from the fetus to the placenta?
Through the paired umbilical arteries.
43. What happens to the fetal circulation when the lungs begin to function after birth?
The pressure in the lungs decreases, promoting increased blood flow through the pulmonary arteries and closure of the ductus arteriosus.
44. Why is the foramen ovale crucial during fetal life?
It allows blood to bypass the non-functional fetal lungs by moving from the right atrium to the left atrium.
45. What structural change marks the transition from fetal to postnatal circulation?
The closure of the foramen ovale and ductus arteriosus, leading to separated pulmonary and systemic circulations.
46. What is the function of the umbilical arteries in fetal circulation?
To carry deoxygenated blood and waste products from the fetus to the placenta.
47. What helps prevent backflow through the foramen ovale in utero?
A flap-like valve on the left side of the septum between the atria.
48. How does oxygenated blood bypass the right ventricle in fetal circulation?
Through the foramen ovale, which directs blood from the right atrium to the left atrium.
49. What role does prostaglandin play in fetal circulation?
It helps keep the ductus arteriosus open during fetal life.
50. What triggers the ductus venosus to close after birth?
The loss of blood flow from the umbilical vein after the umbilical cord is clamped.
51. How does fetal hemoglobin differ from adult hemoglobin?
Fetal hemoglobin has a higher affinity for oxygen, allowing better oxygen uptake from the maternal blood.
52. Why does the right ventricle have a high workload in fetal circulation?
Because it pumps blood to both the pulmonary artery and through the ductus arteriosus to the aorta.
53. What happens to the foramen ovale if it does not close after birth?
It can result in a condition known as a patent foramen ovale (PFO), which may lead to abnormal blood flow between the atria.
54. How does the fetal liver receive blood during gestation?
Partially via the umbilical vein, but most of it bypasses the liver through the ductus venosus.
55. What ensures that oxygenated blood reaches the brain and heart in the fetus?
The strategic shunting through the foramen ovale and ductus arteriosus.
56. What happens to the oxygen saturation of blood in the umbilical vein?
It is the highest oxygen saturation in the fetal circulation system.
57. Why is the fetal circulation considered parallel?
Because both the right and left sides of the heart pump blood simultaneously to systemic and placental circulations.
58. What function do the umbilical arteries serve after birth?
They become the medial umbilical ligaments in the abdominal wall.
59. What is the difference in pressure between the fetal right and left atria before birth?
The right atrium has higher pressure than the left atrium, facilitating blood flow through the foramen ovale.
60. Why is the ductus arteriosus essential in fetal circulation?
It diverts blood from the pulmonary trunk directly to the aorta, bypassing the non-functional lungs.
61. What is the role of the umbilical vein in fetal circulation?
It carries oxygenated blood from the placenta to the fetus.
62. Why does the fetus not rely on its lungs for oxygenation?
Because oxygen is supplied through the placenta, not the lungs.
63. What circulatory change happens immediately after birth?
The lungs expand, causing a drop in pulmonary pressure and increased blood flow through the pulmonary arteries.
64. What fetal structure bypasses the liver, directing blood to the inferior vena cava?
The ductus venosus.
65. How does blood flow from the placenta reach the fetal heart?
Via the umbilical vein, which connects to the inferior vena cava.
66. What causes the foramen ovale to close at birth?
Increased left atrial pressure as blood flow from the lungs returns to the left atrium.
67. What is the ductus arteriosus’ function in fetal circulation?
It connects the pulmonary artery to the aorta, allowing blood to bypass the non-functional fetal lungs.
68. What happens to the ductus arteriosus after birth?
It constricts and eventually becomes the ligamentum arteriosum.
69. How does the fetal circulatory system prioritize blood flow?
By directing the most oxygen-rich blood to the brain and heart through shunting mechanisms.
70. What is the oxygen saturation of blood in the umbilical arteries compared to the umbilical vein?
It is lower in the umbilical arteries, as they carry deoxygenated blood back to the placenta.
71. Why is the pressure higher on the right side of the fetal heart?
Because the fetal lungs are collapsed, increasing resistance in the pulmonary arteries.
72. What causes the ductus venosus to close after birth?
The cessation of blood flow from the umbilical vein once the umbilical cord is cut.
73. What condition occurs if the ductus arteriosus remains open after birth?
Patent ductus arteriosus (PDA), which can lead to abnormal blood flow between the aorta and pulmonary artery.
74. Why is fetal circulation considered a parallel system?
Because both ventricles pump blood into the systemic circulation simultaneously.
75. What change in blood pressure occurs when the newborn starts to breathe?
The pressure in the pulmonary circulation drops significantly.
76. What fetal circulatory structure ensures blood bypasses the right ventricle?
The foramen ovale.
77. How does the placenta function in fetal circulation?
As the site of oxygen and nutrient exchange between maternal and fetal blood.
78. What occurs to the foramen ovale if there is a defect in closure?
It remains open as a patent foramen ovale (PFO), which can cause potential issues with blood mixing.
79. How does oxygenated blood reach the left side of the heart in the fetus?
It passes through the foramen ovale from the right atrium to the left atrium.
80. What structural adaptation allows the fetal circulation to direct more oxygenated blood to the brain?
The foramen ovale and ductus arteriosus ensure oxygen-rich blood reaches essential organs.
81. What is the role of Wharton’s jelly in the umbilical cord?
It provides structural support and protection to prevent compression of the umbilical vessels.
82. What is the difference between fetal hemoglobin (HbF) and adult hemoglobin (HbA)?
Fetal hemoglobin has a higher affinity for oxygen, facilitating oxygen transfer from maternal to fetal blood.
83. How does blood bypass the fetal liver to reach the heart more directly?
Through the ductus venosus, which channels blood from the umbilical vein to the inferior vena cava.
84. What triggers the closure of the foramen ovale after birth?
The increase in left atrial pressure as the newborn’s lungs expand and blood flow shifts.
85. Why is the right atrium pressure higher than the left atrium in fetal circulation?
Because of increased resistance in the pulmonary arteries due to non-functional lungs.
86. What change occurs to the umbilical arteries after birth?
They constrict and become part of the medial umbilical ligaments.
87. What is the primary source of oxygen for the fetus?
Oxygen is supplied through the maternal blood via the placenta.
88. How does the ductus arteriosus respond to increased oxygen levels after birth?
It contracts and begins to close, eventually becoming the ligamentum arteriosum.
89. What effect does clamping the umbilical cord have on fetal circulation?
It stops blood flow from the placenta, initiating the closure of fetal shunts and transitioning to neonatal circulation.
90. What maintains the patency of the ductus arteriosus during fetal life?
Prostaglandins and low oxygen levels help keep it open.
91. How does the fetal circulatory system prioritize oxygen delivery to vital organs?
Through the shunting of oxygen-rich blood via the foramen ovale and ductus arteriosus.
92. What prevents blood from returning to the right atrium from the left atrium after birth?
The foramen ovale closes as pressure in the left atrium exceeds that in the right.
93. What is the purpose of fetal shunts?
To bypass non-functional organs such as the lungs and liver, directing blood to essential areas.
94. What is the fate of the umbilical vein after birth?
It becomes the ligamentum teres, or round ligament of the liver.
95. What is the oxygen saturation level of blood in the fetal aorta compared to the umbilical vein?
The oxygen saturation in the aorta is lower due to the mixing of oxygenated and deoxygenated blood.
96. What happens to the fetal circulatory system when the newborn takes its first breath?
Pulmonary pressure decreases, promoting increased blood flow to the lungs and reducing reliance on fetal shunts.
97. Why does blood flow preferentially through the foramen ovale in utero?
Because of the higher pressure in the right atrium compared to the left atrium.
98. What is the function of the placenta in fetal circulation?
It acts as the organ for gas exchange, nutrient delivery, and waste removal for the fetus.
99. Why is it important for the ductus arteriosus to close postnatally?
To prevent oxygenated blood from mixing with deoxygenated blood and ensure proper circulation through the lungs.
100. What role do pulmonary vessels play in fetal circulation before birth?
They have high resistance, limiting blood flow through the lungs and facilitating shunting through the ductus arteriosus.
Final Thoughts
Fetal circulation plays a vital role in the development of a healthy baby, showcasing the body’s ability to create specialized mechanisms to sustain life even before birth.
From the efficient design of the placental exchange to the temporary shunts that guide blood flow, each component works in harmony to ensure that the fetus receives the oxygen and nutrients it needs.
As birth approaches, the body prepares for the shift to independent breathing and circulation, demonstrating the incredible adaptability and precision of human physiology. Understanding these processes highlights not only the complexity of fetal development but also the marvel of life itself.
Written by:
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.
References
- Remien K, Majmundar SH. Physiology, Fetal Circulation. [Updated 2023 Apr 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
- Marty M, Kerndt CC, Lui F. Embryology, Fetal Circulation. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
- Friedman AH, Fahey JT. The transition from fetal to neonatal circulation: normal responses and implications for infants with heart disease. Semin Perinatol. 1993.