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Fetal Circulation

Circulatio fetalis

For medical students3 min readUpdated 2026-10-10

Fetal circulation is a unique hemodynamic system that differs cardinally from adult circulation. The placenta acts as the primary organ for gas exchange and metabolism, completely altering classical circulatory anatomy. The key feature of the prenatal period is the presence of specific intracardiac and vascular shunts that redirect the most oxygenated blood to the brain, ensuring its priority development.

Umbilical veinCarries the purest arterial blood from the placenta to the fetal tissues.
Liver supplyThe portal vein receives the most oxygen-saturated blood.
Foramen ovaleDirects blood from the inferior vena cava directly into the left atrium.
Blood shuntingThe ductus arteriosus transfers volume from the pulmonary trunk to the aorta.

Maternal Circulation in the Placenta

The placenta is the central organ ensuring the viability of the developing organism. From the maternal organism, arterial blood influx is carried out via the uterine arteries (aa. uterinae). Small branches of these vessels penetrate through the muscular layer of the uterus (myometrium) and open directly into the placental lacunae.

In the lacunae, maternal blood flows freely and bathes the chorionic villi. It is at this stage that the crucial function is realized — gas exchange and the transfer of nutrients into the fetal circulatory system. After exchange takes place, blood flows out into the endometrial veins and then collects into the extensive uterine venous plexus (plexus uterinus).

Path from Placenta to Heart: Oxygenation and Mixing

Inside the capillaries of the chorionic villi, fetal blood is enriched with oxygen, acquiring an arterial character. Next, it collects into the large unpaired umbilical vein (v. umbilicalis). It is important to note an anatomical nuance: this vessel is called a vein solely because it carries blood away from the capillary bed toward the heart, despite its arterial composition.

Entering the abdominal cavity through the umbilical ring, the umbilical vein divides into two main branches:

As a result, a complex composition is formed in the inferior vena cava. It receives venous blood from the lower half of the body, "subarterial" (slightly less oxygen-saturated) blood from the hepatic veins, and pure arterial blood from the placenta via the direct shunt. The outcome is that mixed blood flows into the right atrium through the inferior vena cava.

Intracardiac Hemodynamics and Stream Division

The architectonics of the fetal heart are arranged in such a way as to predominantly supply oxygen to the brain. In the right atrium (atrium dextrum), two completely different streams meet, the direction of which depends on their source.

  1. Stream from the inferior vena cava: brings mixed, but relatively oxygen-rich blood. A special endocardial fold (valvula of the inferior vena cava) directs this volume through the foramen ovale in the interatrial septum directly into the left atrium. Further, the route runs through the left ventricle into the ascending aorta. This redistribution guarantees that the head and upper limbs receive blood of the best available quality.
  2. Stream from the superior vena cava (v. cava superior): collects venous, oxygen-depleted blood from the head and arms and empties into the right atrium. The valve does not impede this movement, so the stream rushes into the right ventricle (ventriculus dexter) and is expelled into the pulmonary trunk (truncus pulmonalis), heading into the pulmonary circuit, which functions minimally.

Trunk Blood Supply and Return to the Placenta

Since the fetal lungs do not yet participate in gas exchange, venous blood from the pulmonary trunk must be redirected. Shunting is carried out via the ductus arteriosus (ductus arteriosus) directly into the descending aorta.

From the descending aorta, blood is distributed to the arteries of the lower half of the body (including the iliac arteries and lower limb arteries). Subvenous blood, which is significantly poorer in oxygen compared to the blood going to the brain, enters these regions. The physiological consequence of this oxygenation gradient is a slower developmental rate of the pelvis and legs compared to the cranial end of the body.

In the tissues of the legs and trunk, the blood becomes finally venous and drains into the inferior vena cava. To close the cycle, paired umbilical arteries (aa. umbilicales) branch off from the iliac arteries (a. iliacae). They run as part of the umbilical cord and carry subvenous (mixed) blood back to the placenta. There, the arteries break up into capillaries of the chorionic villi, exchange occurs with the maternal blood of the lacunae, and the cycle resumes.

Mnemonic

Remember the composition of the umbilical cord using the digit rule: ONE vein (carries the purest arterial blood to the fetus) and TWO arteries (return subvenous/mixed blood to the placenta).

Frequently asked questions

Into what branches does the umbilical vein divide after entering the fetal abdominal cavity?

After passing through the umbilical ring into the abdominal cavity, the umbilical vein divides into two branches.

  • Branch to the portal vein (v. portae) — provides the fetal liver with the purest arterial blood.
  • Branch to the inferior vena cava (v. cava inferior) — performs direct blood shunting (via the ductus venosus), bypassing the liver.
What are the specific vascular shunts (ducts) in the fetal circulatory system?

Three specific shunts function in the fetal circulatory system.

  • Ductus venosus (ductus venosus) — directs blood from the umbilical vein into the inferior vena cava, bypassing the liver.
  • Foramen ovale (foramen ovale) — an interatrial communication through which blood is directed from the right atrium to the left.
  • Ductus arteriosus (ductus arteriosus / ductus arteriosus Botalli) — redirects blood from the pulmonary trunk directly into the descending aorta.
When and how does the obliteration (closure) of fetal shunts occur after birth?

Closure of fetal shunts occurs at various times after birth.

  • Ductus arteriosus: spasms under the influence of oxygen; complete closure occurs within 24–48 hours of life.
  • Foramen ovale: functionally closes by the 3rd month of life. The timing of anatomical closure is indicated in sources as 5–7 months or 1 year. In 50% of children, a small opening persists up to 5 years of age, and in 25% of individuals — in reduced sizes throughout life.
  • Ductus venosus: within a few minutes after clamping the umbilical cord, it closes due to the contraction of smooth muscle elements. Sources indicate different terms for subsequent obliteration: by the end of the 2nd week it turns into the round ligament of the liver, while another source indicates it scars 2 months later. In a separate source, the ductus venosus is indicated as transforming into the ligamentum venosum.
Why is the umbilical vein called a vein if it carries arterial blood?

In anatomy, the type of vessel is determined not by the blood composition, but by the direction of blood flow. The umbilical vein carries blood out of the placental capillary bed and directs it toward the fetal heart, so it is logically classified as a vein.

What kind of blood is found in the fetal inferior vena cava?

Mixed blood flows in the fetal inferior vena cava. It is formed from pure venous blood of the lower body, subarterial blood from the hepatic veins, and pure arterial blood coming directly from the placenta.

Why do the fetal lower extremities develop slower than the head and arms?

This is due to the oxygenation gradient. The ascending aorta (to the head and arms) receives more oxygen-saturated blood from the inferior vena cava. The descending aorta (to the legs and pelvis) receives subvenous blood after shunting from the pulmonary trunk, which contains less oxygen.

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