Placental circulation
The connection between the fetus and the maternal organism is provided by the placenta (Placenta). Blood flow is organized to efficiently deliver nutrients and remove metabolic waste:
- Umbilical vein (V. umbilicalis). This is the only unpaired vessel carrying arterial (oxygen-rich) blood from the placenta to the fetal body. It passes through the umbilical ring and heads toward the liver (Hepar).
- Umbilical arteries (Aa. umbilicales). These are paired vessels (right and left) branching off from the iliac arteries. They run along the sides of the urinary bladder (Vesica urinaria) and return venous (oxygen-poor) blood back to the placenta for gas exchange.
Three main fetal shunts
To prevent oxygen loss in non-functioning or actively developing organs, blood bypasses them via specialized pathways (shunts):
- Ductus venosus (Ductus venosus). Located in the liver region. It directly connects the umbilical vein to the inferior vena cava (V. cava inferior). A significant portion of arterial blood bypasses the capillary network of the liver, although some still enters the portal vein (V. portae hepatis).
- Foramen ovale (Foramen ovale). An opening in the interatrial septum. Mixed blood from the inferior vena cava enters the right atrium (Atrium dextrum) and is immediately shunted into the left atrium (Atrium sinistrum). This allows the pulmonary circuit to be bypassed. Blood from the head and upper limbs arrives via the superior vena cava (V. cava superior).
- Ductus arteriosus (Botallo's duct) (Ductus arteriosus). Connects the pulmonary trunk (Truncus pulmonalis) with the aortic arch (Arcus aortae). Blood entering the right ventricle is pumped into the pulmonary trunk, but instead of going to the lungs (Pulmo), it is shunted into the descending aorta.
From the aorta, blood is distributed through the systemic circuit, nourishing the head, upper limbs, and internal organs (e.g., the left kidney via the A. renalis sinistra).
Postnatal transformation
After birth, the newborn takes its first breath, the lungs expand, and the fetal communications lose their function. Their obliteration begins — a gradual closure and transformation into ligaments:
- Umbilical vein transforms into the round ligament of the liver (lig. teres hepatis).
- Ductus venosus becomes the ligamentum venosum (lig. venosum).
- Ductus arteriosus obliterates into the ligamentum arteriosum (lig. arteriosum).
- Umbilical arteries (distal parts) form cords (chordae aa. umbilicales) on the internal surface of the anterior abdominal wall.
- Foramen ovale closes, leaving the fossa ovalis (fossa ovalis) in the heart.
Features of newborn blood vessels
Newborns have the same major vessels as adults, but they differ in topography and caliber:
- Aorta and pulmonary trunk: The aortic arch is located higher, and the caliber of the pulmonary trunk is larger than that of the aorta.
- Carotid arteries: The common carotid artery is slightly curved, and its bifurcation is high — at the level of the 2nd cervical vertebra (it gradually descends).
- Renal vessels: Lie obliquely due to the low position of the kidneys.
- Veins: Grow faster than arteries and are straighter, but their valvular apparatus is insufficiently developed.
Development of the vascular and lymphatic systems
Blood vessels develop on the 3rd week from mesenchyme (mesoblast) cells. Initially, blood islands form outside and inside the embryo's body. The inner layer of the vessel (endothelium) forms directly from mesenchymal cells, while the middle and outer layers form from the surrounding mesenchyme.
The lymphatic system also develops from mesenchyme on the 6th–7th week as sacs along the major veins. Lymph nodes form by the end of the 3rd month (first in the jugular and inguinal regions). In newborns, there are more regional nodes, lymphocyte germinal centers are poorly developed, and the walls of lymphatic vessels are very thin.
Briefly: answers to topical questions
What arteries give rise to the umbilical arteries?
The umbilical arteries arise from the iliac arteries. Specifically, they are visceral branches of the anterior trunk of the internal iliac artery.
- Umbilical artery (a. umbilicalis) is a paired vessel that originates in the pelvic cavity and runs along the sides of the urinary bladder to the umbilical ring to return venous blood to the placenta.
What branches arise from the aortic arch in the fetus?
Three large trunks arise from the fetal aortic arch, providing blood supply to the head and upper limbs.
- Brachiocephalic trunk (Truncus brachiocephalicus) arises on the right, dividing into the right common carotid and right subclavian arteries.
- Left common carotid artery (a. carotis communis sinistra) arises directly from the aortic arch.
- Left subclavian artery (a. subclavia sinistra) runs upward and to the left toward the cervical region.
What ligaments do the distal parts of the umbilical arteries turn into after birth?
The distal parts of the umbilical arteries obliterate after birth, forming umbilical artery cords running along the internal surface of the superior/anterior abdominal wall. The proximal segments remain patent and give rise to the superior vesical arteries.
Which fetal organs receive the most oxygenated blood?
The most oxygenated arterial blood in the fetal body is received by the liver.
Blood comes from the placenta via the unpaired umbilical vein (v. umbilicalis), which in the abdominal cavity gives off a branch to the portal vein (v. portae). It is this branch that supplies the liver with the purest arterial blood compared to other organs. The rest of the fetal organs are nourished by mixed (arteriovenous) blood, as mixing of blood streams occurs in the inferior vena cava and the heart.
Three shunts become three ligaments: ductus venosus — ligamentum venosum, ductus arteriosus — ligamentum arteriosum, umbilical vein — round ligament of the liver.
Class and exam questions
What type of blood flows through the fetal umbilical vein?
The umbilical vein (V. umbilicalis) carries arterial, maximally oxygen-saturated blood from the placenta to the fetal body.
Why is the ductus arteriosus (Botallo's duct) needed?
It shunts blood from the pulmonary trunk directly into the aorta. This allows blood to bypass the fetal lungs, which do not perform gas exchange before birth.
At what level is the carotid bifurcation located in infants?
In newborns, the bifurcation of the common carotid artery is located high — at the level of the 2nd cervical vertebra. With age, the bifurcation descends.
What else to study on this topic
- Topography and branches of the fetal aortic arch
- Histogenesis of the vascular wall from mesenchyme
- Anatomical features of newborn veins
- Formation and topography of fetal regional lymph nodes
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