The fetal cardiovascular system operates differently than a newborn’s. Inside the womb, a baby’s lungs, digestive system, and liver are not fully active. The placenta provides oxygen and nutrients and removes waste. This unique arrangement requires specialized, temporary circulatory pathways to ensure the growing baby receives what it needs until birth.
Key Fetal Cardiovascular Structures
The fetal cardiovascular system features several specialized structures designed to efficiently direct blood away from the non-functional lungs and liver.
The foramen ovale is an opening located between the right and left atria of the heart. This allows oxygenated blood from the inferior vena cava to largely bypass the pulmonary circulation by flowing directly from the right atrium into the left atrium, and then into the left ventricle and aorta to supply the brain and heart with oxygen-rich blood.
The ductus arteriosus connects the pulmonary artery directly to the aorta. This shunt diverts most of the blood pumped by the right ventricle away from the high-resistance pulmonary circulation and into the systemic circulation, allowing it to bypass the fluid-filled fetal lungs.
The ductus venosus is a specialized shunt that bypasses the liver. It connects the umbilical vein, which carries oxygenated blood from the placenta, directly to the inferior vena cava, minimizing blood flow through the fetal liver.
The umbilical cord contains two umbilical arteries and one umbilical vein, unique to fetal circulation. The two umbilical arteries carry deoxygenated blood and waste products from the fetus back to the placenta for removal. Conversely, the single umbilical vein transports oxygenated, nutrient-rich blood from the placenta to the developing fetus. These vessels are the direct link between the maternal and fetal circulations, facilitating all necessary exchanges.
Transformations to Postnatal Structures
Upon birth, the specialized fetal cardiovascular structures undergo significant transformations, either closing off or becoming non-functional ligaments.
The foramen ovale, the opening between the atria, closes to become the fossa ovalis. This closure occurs as pressure in the left atrium surpasses that in the right atrium, pressing the septum primum against the septum secundum, eventually leading to fibrous fusion. The fossa ovalis remains as a shallow depression in the interatrial septum, serving as a remnant of this fetal bypass.
The ductus arteriosus, which shunted blood from the pulmonary artery to the aorta, transforms into the ligamentum arteriosum after birth. This process begins with functional constriction shortly after birth, followed by anatomical closure through fibrosis. The ligamentum arteriosum is a fibrous cord connecting the aorta and the pulmonary artery, signifying its former role in bypassing the lungs.
The ductus venosus, which bypassed the fetal liver, becomes the ligamentum venosum. Its closure starts within minutes of birth, with complete obliteration occurring over time, forming a fibrous band. This ligament is a fibrous remnant located in the liver, indicating the pathway that once allowed highly oxygenated blood to bypass the hepatic sinusoids.
The umbilical arteries, responsible for carrying deoxygenated blood to the placenta, largely obliterate after birth to become the medial umbilical ligaments. These fibrous cords are found on the anterior abdominal wall.
The umbilical vein, which transported oxygenated blood from the placenta, transforms into the round ligament of the liver, also known as the ligamentum teres hepatis. This fibrous cord extends from the umbilicus to the liver’s transverse fissure.
The Crucial Transition at Birth
The shift from fetal to postnatal circulation is a rapid and complex physiological event, primarily triggered by the baby’s first breath and the clamping of the umbilical cord. When a newborn takes its first breath, the lungs inflate and clear of fluid. This aeration leads to a dramatic decrease in pulmonary vascular resistance, allowing blood to flow freely into the lungs for oxygenation. The influx of oxygen into the pulmonary arterioles contributes to their dilation, further reducing resistance.
Concurrently, the clamping of the umbilical cord severs the connection to the placenta, which was a low-resistance vascular bed. This cessation of placental blood flow causes an immediate and significant increase in systemic vascular resistance throughout the baby’s body. The combined effects of decreased pulmonary resistance and increased systemic resistance lead to a reversal of pressure gradients within the heart and major vessels.
The increased blood flow to the lungs and the subsequent increase in pulmonary venous return raise the pressure in the left atrium. This elevated left atrial pressure, now higher than the right atrial pressure, forces the septum primum flap of the foramen ovale against the septum secundum, functionally closing the opening. Similarly, the increased arterial oxygen tension and the altered blood flow dynamics cause the ductus arteriosus to constrict and functionally close. The changes in pressure and blood flow also lead to the collapse and eventual obliteration of the ductus venosus and the umbilical vessels, directing all blood entering the liver through the hepatic sinusoids.