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 occurs via the uterine arteries (a. uterina). 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 most crucial function is realized — gas exchange and the transfer of nutrients into the fetal circulatory system. After exchange takes place, blood drains into the endometrial veins and then collects into the extensive uterine venous plexus (plexus uterina).
Pathway from the placenta to the heart: oxygenation and mixing
Inside the capillaries of the chorionic villi, the fetal blood is enriched with oxygen, acquiring an arterial character. Next, it collects into a 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 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:
- Branch to the portal vein (v. portae): heads toward the liver, providing this organ with the purest arterial blood compared to any other tissues.
- Branch to the inferior vena cava (v. cava inferior): performs a direct blood shunt.
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 to the right atrium through the inferior vena cava.
Intracardiac hemodynamics and stream separation
The architectonics of the fetal heart are arranged in such a way as to preferentially supply oxygen to the brain. In the right atrium (atrium dextrum), two completely different streams meet, the direction of which depends on their source.
- Stream from the inferior vena cava: brings mixed, but relatively oxygen-rich blood. A special endocardial fold (the valve of the inferior vena cava) directs this volume through the foramen ovale in the interatrial septum directly into the left atrium. Next, the route runs through the left ventricle into the ascending aorta. This redistribution guarantees that the head and upper extremities receive blood of the best available quality.
- 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 pushed out into the pulmonary trunk (truncus pulmonalis), heading into the lesser circulation, which functions minimally.
Blood supply to the trunk 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 through the ductus arteriosus (botallduct) (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 rate of development of the pelvis and legs compared to the cephalic end of the body.
In the tissues of the legs and trunk, the blood becomes definitively venous and drains into the inferior vena cava. To complete the cycle, paired umbilical arteries (aa. umbilicales) branch off from the iliac arteries (a. iliacae). They pass as part of the umbilical cord and carry subvenous (mixed) blood back to the placenta. There, the arteries break down into capillaries of the chorionic villi, exchange with the maternal blood of the lacunae occurs, and the cycle resumes.
Briefly: answers to topical questions
Into which 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) — supplies the fetal liver with the purest arterial blood.
- Branch to the inferior vena cava (v. cava inferior) — provides a direct blood shunt (via the ductus venosus), bypassing the liver.
What are the specific vascular shunts (ducts) in the fetal circulatory system called?
Three specific shunts function in the fetal circulatory system.
- Ductus venosus (Arantius' duct) (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 (Botallo's duct) (ductus arteriosus / ductus arteriosus Botalli) — reroutes blood from the pulmonary trunk directly into the descending aorta.
When and how does the obliteration (closure) of fetal shunts occur after birth?
The closure of fetal shunts occurs at various times after birth.
- Ductus arteriosus (Botallo's duct): 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 time 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, it remains in reduced dimensions throughout life.
- Ductus venosus (Arantius' duct): closes within a few minutes after umbilical cord clamping due to the contraction of smooth muscle elements. Sources indicate different timelines for subsequent obliteration: by the end of the 2nd week, it transforms into the round ligament of the liver, while another source states that it obliterates after 2 months. In a separate source, the ductus venosus is noted as transforming into the ligamentum venosum.
The rule of numbers will help you remember the composition of the umbilical cord: ONE vein (carries the purest, arterial blood to the fetus) and TWO arteries (return subvenous/mixed blood to the placenta).
Class and exam questions
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 type of blood is found in the fetus's inferior vena cava?
Mixed blood flows in the fetal inferior vena cava. It is formed from purely venous blood from the lower body, subarterial blood from the hepatic veins, and pure arterial blood coming directly from the placenta.
Why do the lower extremities of the fetus develop more slowly than the head and upper limbs?
This is due to the oxygenation gradient. The ascending aorta (supplying the head and arms) receives more oxygen-saturated blood from the inferior vena cava. The descending aorta (supplying the legs and pelvis) receives subvenous blood after shunting from the pulmonary trunk, which contains less oxygen.
What else to study on this topic
- Structure and functions of chorionic villi
- Histology of placental lacunae
- Topography of the uterine venous plexus
- Anatomy of the Eustachian valve of the inferior vena cava
- Formation of the umbilical ring
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