The cardiovascular system develops early in the embryonic stage of development. It starts towards the end of the third week or at the beginning of the fourth week of fetal development. Once the cardiovascular system is fully established, blood circulation commences and the embryo can directly derive nutrients from its own blood supply.
What is fetal circulation?
As the fetus develops within the womb, fetal circulation is established during the early stages of development, allowing the growing fetus to receive the required oxygen and nutrients as well as dispose of waste products. This type of circulation refers to the circulatory system of a fetus which differs from postnatal circulation. Fetal circulation, unlike postnatal circulation, involves the umbilical cord and placental blood vessels which carry fetal blood between the fetus and the placenta. It is usually established in the fetal period of development and is designed to serve prenatal nutritional needs, as well as permit the switch to a neonatal circulatory pattern at birth. Good respiration in the neonate depends on normal circulatory changes occurring at birth (transitional circulation), which results in oxygenation of the blood in the lungs when fetal blood flow through the placenta ceases. Prenatally, the lungs do not provide gas exchange and the pulmonary vessels are vasoconstricted. Instead, the placenta acts as the gas exchange unit to oxygenate fetal blood. The three vascular structures most important in the transitional circulation are the ductus venosus, foramen ovale, and ductus arteriosus.
Fetal blood vessels and fetal circulation
Once the main arteries and veins as well as the heart are developed, usually after the 8th week of fetal development, deoxygenated blood is returned from the fetal systemic circulation to the placenta via two umbilical arteries, which branch off the fetal internal iliac arteries. Highly oxygenated, nutrient-rich blood flows from the placenta to the fetus via the umbilical vein. Approximately half of the blood in the umbilical vein bypasses the liver to flow into the ductus venosus, a fetal vessel connecting the umbilical vein to the inferior vena cava. The other half flows into the sinusoids of the liver and enters the inferior vena cava via the hepatic veins. Blood flow through the ductus venosus is regulated by a sphincter mechanism close to the umbilical vein. When the sphincter contracts, more blood is diverted to the portal vein and the hepatic sinusoids, and less to the ductus venosus. Although an anatomic sphincter in the ductus venosus has been described, its presence is not universally accepted. However, it is generally agreed that there is a physiologic sphincter that prevents overloading of the heart when venous flow in the umbilical vein is high, as seen during uterine contractions for example.
After a short course in the inferior vena cava, the blood enters the right atrium of the heart. Because the inferior vena cava also contains poorly oxygenated blood from the lower limbs, abdomen, and pelvis, the blood entering the right atrium flows in two streams, one highly oxygenated from the umbilical vein and the other poorly saturated in oxygen. At the junction of the inferior vena cava and right atrium, the Eustachian valve separates these two streams by directing the highly oxygenated blood from the right atrium to the left atrium through the foramen ovale. It is then ejected by the left ventricle into the ascending aorta to ensure a better perfusion of important fetal organs such as the myocardium and brain. The poorly oxygenated blood in the right atrium flows through the tricuspid valve to be ejected by the right ventricle , but around 90% of the flow in the pulmonary trunk is diverted into the descending aorta by the ductus arteriosus due to a high pulmonary vascular resistance. The other 10% flows into the fetal lungs to supply them with oxygen. The ductus arteriosus protects the lungs from circulatory overloading and allows the right ventricle to strengthen in preparation for functioning at full capacity at birth, when the transitional circulation establishes itself.