Fetal Circulation – Definition, Working, Importance, FAQs

The umbilical vein, carrying the oxygenated blood (80% saturated) from the placenta, enters the fetus at the umbilicus and runs along the free margin of the falciform ligament of the liver. In the liver,  it gives off branches to the left lobe of the liver and receives deoxygenated blood from the portal vein. The greater portion of the oxygenated blood, mixed with some portal Venus blood, short circuits the liver through the ductus venosus to enter the inferior Vena cava (IVC) and then to the right atrium of the heart. The Oxygen content of this mixed blood is thus reduced. Although both the ductus venosus and hepatic portal/fetal trunk blood enter the right atrium through the ivc, there is little mixing. The terminal part of the IVC receives blood from the right hepatic vein. 

In the right atrium, most of the well-oxygenated {75%} ductus venosus blood is preferentially directed into the foramen ovale by the valve of the IVC and Crista dividens and passes into the left atrium. Here it is mixed with a small amount of venous blood returning from the lungs through the pulmonary veins. This left atrial blood is passed on through the pulmonary veins. This left atrial blood is passed on through the mitral opening into the left ventricle. A remaining lesser amount of blood {25%}, after reaching the right atrium via the superior and inferior Vena cava (carrying the venous blood from the cephalic and caudal parts of the respectively) passes through the tricuspid opening into the right ventricle.  During ventricular systole, the ventricular blood is pumped into the ascending arch of the aorta and distributed by their branches to the heart, head, neck, brain, and arms. The right ventricular blood with low oxygen content is discharged into the pulmonary trunk. Since the resistance in the pulmonary arteries during fetal life is very high, the ductus arteriosus into the descending aorta bypasses the lungs, where it mixes with the blood from the proximal aorta. About 70% of the cardiac output (60% from the right and 10% from the left ventricle) is carried by the ductus arteriosus to the descending aorta. About 40% of the combined output goes to the placenta through the umbilical arteries. The deoxygenated blood leaves the body by way of two umbilical arteries to reach the placenta, where it is oxygenated and gets ready for recirculating. The mean cardiac output is comparatively high in the fetus and is estimated to be 350mL/kg/min.

How does the Fetal Circulation  System work?

The fetal circulation system is distinctly different from adult circulation. This intricate system allows the fetus to receive oxygenated blood and nutrients from the placenta. It is composed of the blood vessels in the placenta and the umbilical cord, which contains two umbilical arteries and one umbilical vein. Fetal circulation bypasses the lungs via a shunt known as the ductus arteriosus; the liver is also bypassed via the ductus venosus and blood can travel from the right atrium to the left atrium via the foramen ovale. Normal fetal heart rate is between 110 and 160 beats per minute. When compared to adults, fetuses have decreased ventricular filling and reduced contractility. Fetal circulation undergoes a rapid transition after birth to accommodate extra-uterine life.  Human understanding of fetal circulation originated from fetal sheep, but ultrasound and magnetic resonance imaging (MRI) during the fetal period now provide detailed information. There are distinct differences in fetal circulation that if not appropriately formed can lead to childhood or adult diseases.

The difference in Hb OF adult and fetus

Two Sub-units Make One Big Difference

Both fetal and adult hemoglobin is composed of four subunits: both have two identical alpha subunits, but in fetal hemoglobin, the two beta subunits found in adult hemoglobin are replaced with two gamma subunits.

Physiology of  Fetal Circulation

Fetal circulation bypasses the lungs via a shunt known as the ductus arteriosus; the liver is also bypassed via the ductus venosus and blood can travel from the right atrium to the left atrium via the foramen ovale. Normal fetal heart rate is between 110 and 160 beats per minute.

Circulation before Birth

Fetal circulation bypasses the lungs via a shunt known as the ductus arteriosus; the liver is also bypassed via the ductus venosus and blood can travel from the right atrium to the left atrium via the foramen ovale. Normal fetal heart rate is between 110 and 160 beats per minute.

Circulation after Birth

In postnatal circulation, when the baby takes its first breath, pulmonary resistance decreases, and blood flow through the placenta ceases. Blood commences flowing through the lungs, and the pressure on the left side becomes higher than on the right. As a result, the shunts mentioned above close.

Difference between Fetal and Adult

Criteria	Adult Circulation	Fetal Circulation
Artery	Carries oxygenated blood away from the heart	Carries non-oxygenated blood away from the heart
veins	Carries non-oxygenated blood toward the heart	Carries oxygenated blood back to the heart
Exchange of gases	It takes place in the lungs	It takes place in the Placenta
Pressure	Increase pressure on the left side of the heart	Increase pressure on the right side of the heart

Adult circulation sequence 

1. Nonoxygenated blood enters the right atrium via the inferior and superior vena cava.
2. Increased level of blood in the right atrium causes the tricuspid valve to open and drain the blood to the right ventricle.
3. The pressure of blood in the right ventricle causes the pulmonic valve to open and non-oxygenated blood is directed to the pulmonary artery, then to the lungs.
4. The exchange of gases occurs in the lungs. Highly oxygenated blood is returned to the heart via the pulmonary vein to the left atrium.
5. From the left atrium, the pressure of the oxygenated blood causes the mitral valve to open and drain the oxygenated blood to the left ventricle.
6. The left ventricle then pumps the oxygenated blood that opens the aortic valve. Blood is then directed to the ascending and descending aorta to be distributed in the systemic circulation.

Fetal Circulation Sequence

1. The exchange of gases occurs in the placenta. Oxygenated blood is carried by the umbilical vein toward the fetal heart.
2. The ductus venosus directs part of the blood flow from the umbilical vein away from the fetal liver (filtration of the blood by the liver is unnecessary during the fetal life) and directly to the inferior vena cava.
3. Blood from the ductus venosus enters the inferior vena cava. Increase levels of oxygenated blood flows into the right atrium.
4. In adults, the increased pressure of the right atrium causes the tricuspid valve to open, thus, draining the blood into the right ventricle. However, in fetal circulation, most of the blood in the right atrium is directed by the foramen ovale (opening between the two atria) to the left atrium.
5. The blood then flows from the left atrium to the left ventricle, going to the aorta. The majority of the blood in the ascending aorta goes to the brain, heart, head and upper body.
6. The portion of the blood that drained into the right ventricle passes to the pulmonary artery.
7. As blood enters the pulmonary artery (carries blood to the lungs), an opening called ductus arteriosus connects the pulmonary artery and the descending aorta. Hence, most of the blood will bypass the non-functioning fetal lungs and will be distributed to the different parts of the body. A small portion of the oxygenated blood that enters the lungs remains there for fetal lung maturity.
8. The umbilical arteries then carry the non-oxygenated blood away from the heart to the placenta for oxygenation.

Conceptual Questions 

Question 1: What main structures are involved in fetal circulation?

Answer:

The fetal circulation system is distinctly different from adult circulation. This intricate system allows the fetus to receive oxygenated blood and nutrients from the placenta. It is composed of the blood vessels in the placenta and the umbilical cord, Which contains two umbilical arteries and one umbilical vein.

Question 2: What are some features of fetal circulation?

Answer:

Most of the blood that leaves the right ventricle in the fetus bypasses the lungs through the second of the two extra fetal connections known as the ductus arteriosus. The ductus arteriosus sends the oxygen poor blood to the organs in the lower half of the fetal body.

Question 3: What happens to fetal circulation after birth?

Answer:

The umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of air, the lungs start to expand, and the ductus arteriosus and the foramen ovale both close. The baby’s circulation and blood flow through the heart now function like an adult’s.

Question 4: What are the three fetal shunts in fetal circulation?

Answer:

Therefore, the current anatomical nomenclature of the fetal cardiac shunts is historically inappropriate. The three cardiac shunts are:

1. Foramen Ovale
2. Ductus Arterious
3. Ductus Venosus.

Question 5: How are wastes removed in fetal circulation?

Answer:

Through the blood vessels in the umbilical cord, the fetus all needs nutrition and oxygen. The fetus gets life support from the mother through the placenta. Waste products and carbon dioxide from the fetus are sent back through the umbilical cord and placenta to the mother’s circulation to be removed.

Question 6: Why is fetal circulation a low-pressure system? 

Answer:

Parental Circulation- Role of the foramen ovale-Fetal circulation has unique characteristics, as gas exchange occurs in the placenta instead of the fetal lungs. In the fetus, the placenta has the lowest vascular resistance and receives 40% of the fetal cardiac output, which results in low systemic pressure.

Question 7: What are the last organs to develop in a fetus?

Answer:

Most babies move to a head-down position in the uterus toward the end, with the head on the mother’s pubic bone. The lungs are the major organ to finish developing.