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 that carries 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 from the internal iliac arteries. They run alongside 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-functional or developing organs, blood bypasses them via specialized pathways (shunts):
- Ductus venosus (Ductus venosus). Located in the region of the liver, it directly connects the umbilical vein to the inferior vena cava (V. cava inferior). A significant portion of arterial blood bypasses the hepatic capillary bed, although some still enters the hepatic 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 blood to bypass the pulmonary circuit. 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) to the aortic arch (Arcus aortae). Blood entering the right ventricle is pumped into the pulmonary trunk, but rather than going to the lungs (Pulmo), it is shunted into the descending aorta.
From the aorta, blood is distributed through the systemic circulation, 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 fetal pathways lose their function. Their obliteration begins—a gradual involution 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 fibroses into the ligamentum arteriosum (lig. arteriosum).
- Umbilical arteries (distal parts) form fibrous cords (chordae aa. umbilicales) on the inner surface of the anterior abdominal wall.
- Foramen ovale closes, leaving the fossa ovalis (fossa ovalis) in the heart.
Vascular Characteristics of the Newborn
Newborns possess the same major vessels as adults, but they differ in topography and caliber:
- Aorta and pulmonary trunk: The aortic arch is positioned higher, and the caliber of the pulmonary trunk is larger than that of the aorta.
- Carotid arteries: The common carotid artery is slightly tortuous, and its bifurcation is high—at the level of the 2nd cervical vertebra (gradually descending later).
- Renal vessels: Lie obliquely due to the low anatomical position of the kidneys.
- Veins: Grow more intensively than arteries and are straighter, but their valve apparatus is insufficiently developed.
Development of the Vascular and Lymphatic Systems
Blood vessels develop during the 3rd week from mesenchymal cells (mesoblast). Initially, blood islands form both extra- and intra-embryonically. 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 during the 6th–7th weeks as lymph sacs along major veins. Lymph nodes form by the end of the 3rd month (first in the jugular and inguinal regions). In the newborn, there are more regional nodes, lymphocyte germinal centers are poorly developed, and the walls of lymphatic vessels are very thin.