Features of placental circulation
Placental circulation is established by approximately the 3rd–4th month of intrauterine development and functions until birth. Its fundamental differences from postnatal blood flow are as follows:
- Low-intensity blood flow in the lungs, since gas exchange occurs via the placenta and the fetal lungs do not breathe.
- Presence of three additional fetal shunts: the venous duct (ductus venosus, Arantius' duct), the arterial duct (ductus arteriosus, Botallo's duct), and the oval opening (foramen ovale).
- Circulation of mixed blood with varying degrees of oxygenation through the systemic circulation.
Blood pathway from the placenta to the fetus
Arterial blood, enriched with oxygen and nutrients in the placenta, flows out via the umbilical vein (vena umbilicalis), which runs within the umbilical cord (funiculus umbilicalis). Through the umbilical ring (anulus umbilicalis), the vessel enters the fetal abdominal cavity and heads toward the liver, where it divides into two pathways:
- Part of the blood passes through the ductus venosus (ductus venosus) directly into the inferior vena cava (vena cava inferior) — this is where the first mixing with deoxygenated blood from the lower body occurs.
- Another part passes through the liver parenchyma and enters the inferior vena cava via the hepatic veins — the second mixing.
Circulation in the fetal heart
Mixed blood with a relatively high oxygen content flows from the inferior vena cava into the right atrium. Through the foramen ovale (foramen ovale) of the interatrial septum, the greater part of it is directed into the left atrium, then into the left ventricle and the aorta. Due to this pathway, the upper half of the body (head, neck, upper extremities) receives the most oxygenated blood.
Venous blood from the upper half of the body enters the right atrium via the superior vena cava, where it partially mixes, passes into the right ventricle, and further into the pulmonary trunk. The greater part of it, bypassing the pulmonary circuit, is shunted through the ductus arteriosus (ductus arteriosus) into the aorta distal to the origin of the left subclavian artery. This unloads the pulmonary circuit and delivers less oxygenated blood to the lower half of the body.
From the descending aorta, blood reaches the fetal organs via its branches, and venous blood returns to the placenta through two umbilical arteries (arteriae umbilicales) running within the umbilical cord.
Blood distribution by oxygenation degree
- Arterial blood: umbilical vein, ductus venosus.
- Mixed blood with high oxygen content: inferior vena cava, left atrium, left ventricle, aorta proximal to the ductus arteriosus.
- Purely venous blood: superior vena cava, coronary sinus of the heart.
- Mixed blood with the lowest oxygen content: right atrium, right ventricle, pulmonary trunk, aorta distal to the ductus arteriosus.
Transformation of structures after birth
After clamping of the umbilical cord and the onset of pulmonary respiration, the fetal shunts atrophy and transform into connective tissue structures:
- Umbilical vein → ligamentum teres hepatis (ligamentum teres hepatis).
- Ductus venosus → ligamentum venosum (ligamentum venosum).
- Ductus arteriosus → ligamentum arteriosum (ligamentum arteriosum).
- Umbilical arteries → medial umbilical ligaments (ligamenta umbilicalia medialia), forming the basis of the medial folds of the anterior abdominal wall.
- Foramen ovale → fossa ovalis (fossa ovalis); the valve of the inferior vena cava (valvula venae cavae inferioris) forms a small fold.
Timing of obliteration: ductus arteriosus — 8–10 days; umbilical arteries — 2–3 days; umbilical vein — about a week; foramen ovale — during the first year of life, though in 1/3 of cases it remains patent for life.
Congenital defects associated with fetal shunts
Patent ductus arteriosus. If the ductus arteriosus fails to obliterate after birth, the direction of blood flow reverses: arterial blood is shunted from the aorta into the pulmonary artery. Initially, the shunt is intermittent (during crying or feeding), and later becomes continuous, leading to pulmonary hypervolemia and volume overload of the lungs.
Atrial septal defect. If communication between the atria persists, the blood flow also changes direction: arterial blood flows from the left atrium into the right, then into the right ventricle and pulmonary trunk. Pulmonary hypervolemia develops with corresponding clinical manifestations discussed in the pediatrics course.
Key points
- Placental circulation functions from the 3rd–4th month of intrauterine development until birth.
- Three fetal communications: ductus venosus, ductus arteriosus, foramen ovale.
- The umbilical vein carries arterial blood to the fetus, and two umbilical arteries carry venous blood to the placenta.
- The ductus arteriosus empties into the aorta distal to the origin of the left subclavian artery, ensuring better oxygenation of the upper body.
- After birth, fetal shunts obliterate and transform into ligaments; the foramen ovale becomes the fossa ovalis.
- Closure timing: umbilical arteries — 2–3 days, ductus arteriosus — 8–10 days, umbilical vein — about a week, foramen ovale — up to 1 year.
- Failure of ducts and openings to close leads to congenital heart defects with reverse blood shunting and pulmonary hypervolemia.