# Stents in Congenital Heart Disease: State of the Art and Future Scenarios
## 3 Congenital Heart Disease
### Characteristic Features of Fetal Circulation  
The fetal circulation is markedly different from the adult circulation. Indeed, the cardiovascular system undergoes several changes during the whole fetal life and the first days after birth leading to the final adult configuration and structure. Despite these differences, the primary function of the cardiovascular system is always the same: providing oxygen and nutrients from the external environment to the tissues and removing carbon dioxide and waste products. In physiological conditions, once development is completed, the heart is composed of four main chambers, namely the right atrium and the right ventricle and the left atrium and the left ventricle, which perfuse a series system constituted by the pulmonary and systemic circulation. The oxygen-poor blood coming from body tissues and organs returns to the right atrium through the superior and inferior vena cava. The blood enters the right ventricle and then, through the pulmonary artery, it is pumped into the lungs where oxygenation occurs. The oxygenated blood returns to the left atrium through the pulmonary veins and then reaches the left ventricle, where it is pumped towards the various body districts through the aorta and its branches. Adult circulation can thus be schematized as a closed circuit, with four components in series (Figure 2a) in which the blood flow pumped by the right ventricle into the pulmonary circulation should equal the blood flow pumped by the left ventricle into the systemic circulation, defining the cardiac output. The primary difference between adult and fetal circulation is that in the fetus gas exchange takes place in the placenta (the organ that connects the maternal and fetal circulatory systems) rather than in the lungs. The placenta substitutes the respiratory, the gastrointestinal, and renal systems during the whole fetal life, assuring the exchange of nutrients and waste products and allowing for blood oxygenation. The deoxygenated  
Figure 2: Comparison between (**a**) adult circulation, schematized as a series system, and (**b**) fetal circulation, constituting an intricate parallel circuit. Blue arrows define the path of poorly oxygenated blood, while red arrows stand for oxygenated blood. The magenta color indicates intermediate oxygenation. The dashed arrows in the fetal system indicate the additional pathways the fetal shunts allow. DV = ductus venous, FO = foramen ovale, RA = right atrium, LA = left atrium, RV = right ventricle, LV = left ventricle, SystUB = systemic upper body, SystLB = systemic lower body. blood comes to the placenta through the umbilical arteries, which originate from the iliac arteries, just beyond the aortic bifurcation. Oxygen-rich blood is returned to the fetus through the umbilical vein, and approximately 50-60% of this blood flow directly reaches the inferior vena cava through the ductus venosus, a fetal shunt that allows part of the placental blood flow to bypass the liver [35]. Oxygen-rich blood in the inferior vena cava enters the right atrium and preferentially streams into the left atrium through the foramen ovalae (a hole in the atrial septum present only during gestation) without mixing with the oxygen-poor blood coming from the lower and upper limbs which instead enters the right ventricle. Thanks to the foramen ovalae, the most oxygenated blood directly reaches the left heart, is pumped into the ascending aorta, and then is delivered to the coronary circulation and the brain. The remaining part of desaturated blood in the right atrium is delivered to the right ventricle and is pumped into the pulmonary artery. As the lungs are not working in fetal life, only a small portion of blood enters the pulmonary circulation due to the associated high pulmonary vascular resistance [35]. Indeed, most of the pulmonary artery blood flow (90%) bypasses the lungs flowing into the aorta through the ductus arteriosus (also indicated as Botallo's duct).