Fetal Circulation

Abstract

According to embryology rules, during transformation into a 4-chambered structure, the single heart tube starts to beat at 23 days of growth and development and till 30 days blood circulates through the embryo. At about 22 days, the heart tube initiates to bend ventrally, toward the right (looping). In order to separate the systemic and pulmonary circulations, the original single-chamber heart begins to be partitioned. Also in the fetal circulation, unlike in the adult circulation, the placenta provides gas and metabolite exchange. In addition, in a fetal circulation, the right and left ventricles are in a parallel circuit, as opposed to the series circuit in a neonatal or adult circulation. The ductus venosus, foramen ovale, and ductus arteriosus are three structures in the cardiovascular system that are unique to the fetus and are important for maintaining this parallel circulation. At birth, the elimination of the low-resistance placental circulation and the expansion of the lungs lead to closure of the ductus venosus, foramen ovale, and ductus arteriosus. During the first few weeks of life, pulmonary vascular resistance drops from high fetal level to the low adult level and the wall thickness of the right ventricle decreases slowly. Anomalies can result from deficiencies in any of these basic developmental patterns.