FETAL CIRCULATION: FUNCTION AND ANATOMICAL CHARACTERISTICS - ANATOMY AND PHYSIOLOGY - 2025

The fetal circulation is the way in which the blood is circulated through the circulatory system of the fetus in utero. Unlike in extrauterine life, oxygen is not obtained from the air through the lungs before birth. Instead, all the nutrients and oxygen come from the mother and reach the fetus through the placenta.

That is why in the fetal circulation there are right-left shunts or shunts that allow oxygenated blood from the placenta to be properly distributed.

Source: OpenStax College

Since the lungs do not function during pregnancy, the blood supply to them is minimal. Therefore, the minor circulation (pulmonary circulation) is practically abolished, and blood passes largely from the right side of the heart to the left.

This exchange is made through two major connections, present only during fetal life: the foramen ovale and the ductus arteriosus. Through these conduits, oxygenated blood passes almost entirely into the aorta to be distributed throughout the body.

In the case of venous blood, there is also a short circuit known as the ductus venosus, which derives part of the venous blood from the portal vein to the inferior vena cava without passing through the liver.

Circulation in extrauterine life

To understand the differences between fetal circulation and that of the baby once it is born (as well as that of children and adults), it is necessary to clearly understand how blood circulates during extrauterine life.

In this sense, it must be remembered that blood circulation has two major circuits: the major circulation (which carries oxygenated blood to all the tissues of the body) and the minor circulation (responsible for bringing deoxygenated blood to the lungs so that it becomes oxygenated again).

It is about two closed circuits, interconnected with each other through which blood flows without ceasing throughout life.

Greater circulation

The major circulation begins in the left ventricular outflow tract. From there, the blood passes through the aortic valve and passes to the aorta, from where it is directed to each of the corners of the body through the different branches of this artery.

Once the blood donates its oxygen and nutrients to the tissues in the arterial capillary bed, it becomes venous (deoxygenated) blood, so it enters the venous capillaries and from there to the main veins. All of them converge in the superior and inferior vena cava.

From the vena cavae, the blood reaches the right atrium, where the circuit of the greater circulation is completed.

Less circulation

In the right atrium there is deoxygenated blood that must be taken to the lungs to release carbon dioxide and be charged with oxygen. To do this, it is pumped from the right atrium to the right ventricle, and from there to the lungs through the pulmonary arteries.

Unlike the aorta, which carries oxygenated blood, the pulmonary arteries carry deoxygenated blood. This, upon reaching the peri-alveolar arterial capillaries, releases the carbon dioxide that it carries and is charged with oxygen.

Immediately afterwards the blood (now oxygenated) passes from the arterial capillary to the venous one; and from there, through a series of increasingly larger branches, it reaches the pulmonary veins.

The pulmonary veins flow into the left atrium, from where it is propelled to the left ventricle. This is the site where the circuit of the minor circulation formally ends and the major circulation begins once the ventricle contracts and ejects blood.

Anatomical features of the fetal circulation

During intrauterine life it is not possible for circulation to take place as previously explained. This is because the lungs are not working, and therefore cannot supply oxygen to the bloodstream.

In view of this situation, the fetus has accessory arteries and veins that connect it to the placenta and through it to the mother.

During the entire pregnancy, the placenta is in charge of oxygenate the blood and provide nutrients, the umbilical cord being the means of connection between the mother and the fetus. It is a structure that exits the abdomen of the fetus through what will later become the navel.

In the umbilical cord there are three vascular structures: two umbilical arteries and an umbilical vein.

As in the minor circulation, the umbilical arteries carry non-oxygenated blood from the fetus to the placenta; and the umbilical vein brings oxygen- and nutrient-rich blood back from the placenta to the fetus.

Once inside the body of the fetus, this oxygenated blood must be distributed throughout the body efficiently. However, for this to occur, the circulatory system of the unborn baby has a series of particular anatomical characteristics that allow blood to circulate towards the capillary beds, where it is most needed.

These anatomical features are:

- The oval hole.

- The ductus arteriosus.

- The ductus venosus.

Anatomy and Physiology of the Umbilical Arteries

The umbilical arteries are present only during intrauterine life. They are the first branch of the internal or hypogastric iliac artery, and are directed attached to the abdominal wall to the point of emergence of the abdomen, where after birth will be the navel.

There are two umbilical arteries, each of the arteries coming from one of the iliac arteries: right and left.

The umbilical arteries carry partially deoxygenated blood from the fetus to the placenta. There the blood releases carbon dioxide and takes in oxygen to return to the body of the fetus through the umbilical vein.

It is important to note that it is partially deoxygenated blood, since it is the same type of blood that is circulating throughout the body of the fetus. However, when compared to the blood that comes through the umbilical vein, the oxygen content is lower.

After birth, the umbilical arteries are obliterated giving rise to the medial umbilical ligaments in the anterior abdominal wall.

Anatomy and Physiology of the Umbilical Vein

The umbilical vein forms in the placenta, and from there it runs into the umbilical cord until it reaches the abdomen of the fetus. Once there, it passes through what will later be the sickle-cell ligament of the liver to divide into two small portions.

One of them is the terminal portion of the umbilical artery, which joins the portal vein. From there, fresh blood rich in oxygen and nutrients reaches the liver. Between 60 and 70% of the umbilical vein flow is channeled through this branch.

The second branch, about 2 cm long, is known as the ductus venosus.

Once the fetus is born, the umbilical vein becomes obliterated to become the round ligament of the liver, while the ductus venosus gives rise to the venous ligament of the liver.

Anatomy and physiology of the ductus venosus

The ductus venosus is a vein present only during intrauterine life. Its objective is to function as a by-pass so that between 30 and 40% of the oxygenated blood goes to the inferior vena cava without first passing through the liver.

This is because the metabolic rate of the liver during intrauterine life is not as high as in extrauterine life. In addition, it ensures that a part of the blood reaches the heart with a high oxygen concentration.

Otherwise, the liver would trap most of the oxygen molecules, leaving less available to the rest of the body.

Beyond the ductus venosus, blood from the liver reaches the inferior vena cava through the suprahepatic veins and from there it reaches the right atrium. Due to the difference in density of the blood in the ductus venosus and the suprahepatic veins, they do not mix, reaching the right atrium in parallel flows.

Within a few minutes after birth, the ductus venosus closes due to pressure changes in the circulatory circuits, completely obliterating between 3 and 7 days later. Its remains give rise to the venous ligament of the liver.

Anatomy and physiology of the foramen ovale

Under normal conditions, blood would flow from the right atrium to the lungs. However, in intrauterine life this is not necessary, since the lungs do not carry out any gas exchange.

In view of this, most of the blood in the right atrium passes directly to the left atrium through the foramen ovale. Only a small fraction reaches the right ventricle and the pulmonary arteries, providing the minimum necessary flow to the lungs so that they can develop.

The foramen ovale is a communication in the interatrial septum that allows the passage of blood from the right side of the heart to the left, without having to go through the minor circulation circuit.

This ensures that oxygenated blood is directed to the vascular bed, where it is most needed, reserving only a minimal supply of partially oxygenated blood for the lungs. At this stage of development, these organs have very low metabolic requirements.

The foramen ovale closes spontaneously shortly after birth, due to increased pressure in the pulmonary circuit once the fetus is born and begins to breathe.

When this does not occur, a congenital heart condition known as "persistent foramen ovale" or "atrial septal defect" develops, which in most cases requires surgical correction.

Anatomy and physiology of the ductus arteriosus

As previously mentioned, most of the blood that reaches the right atrium passes directly to the left atrium. However, a portion of this still reaches the right ventricle and from there it passes to the pulmonary arteries.

However, despite the foramen ovale, the volume of blood that reaches the pulmonary artery is still greater than that required by the lungs. Therefore, there is a communication that shunts the flow from the pulmonary artery to the aorta.

This communication is known as the ductus arteriosus, and it allows the excess blood that reached the minor circulation to divert to the aorta and the major circulation, leaving only a minimal amount available for the lungs.

As with all other temporal structures in the fetal circulation, the ductus arteriosus closes shortly after birth, giving rise to the ligamentum arteriosus. When this does not happen, it is usually necessary to carry out some type of corrective procedure in order to avoid future heart complications.

References

1. Kiserud, T., & Acharya, G. (2004). The fetal circulation. Prenatal diagnosis, 24 (13), 1049-1059.
2. Kiserud, T. (2005, December). Physiology of the fetal circulation. In Seminars in Fetal and Neonatal Medicine (Vol. 10, No. 6, pp. 493-503). WB Saunders.
3. Haworth, SG, & Reid, L. (1976). Persistent fetal circulation: newly recognized structural features. The Journal of pediatrics, 88 (4), 614-620.
4. Hecher, K., Campbell, S., Doyle, P., Harrington, K., & Nicolaides, K. (1995). Assessment of fetal compromise by Doppler ultrasound investigation of the fetal circulation: arterial, intracardiac, and venous blood flow velocity studies. Circulation, 91 (1), 129-138.
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