The fetal circulation

different from the adult circulation. In the fetus, gas exchange does not occur in the lungs but in the placenta. The placenta must therefore receive deoxygenated blood from the fetal systemic organs and return its oxygen rich venous drainage to the fetal systemic arterial circulation. In addition, the fetal cardiovascular system is designed in such a way that the most highly oxygenated blood is delivered to the myocardium and brain. These circulatory adaptations are achieved in the fetus by both the preferential streaming of oxygenated blood and the presence of intracardiac and extracardiac shunts. Thus, the fetal circulation can be defined as a ‘shunt-dependent’ circulation. In the fetus, deoxygenated blood arrives at the placenta via the umbilical arteries and is returned to the fetus in the umbilical vein. The partial pressure of oxygen (PO2) in the umbilical vein is around 4.7 kPa and fetal blood is 80–90% saturated. Between 50–60% of this placental venous flow bypasses the hepatic circulation via the ductus venosus (DV) to enter the inferior vena cava (IVC). In the IVC, the better oxygenated blood flow from the DV tends to stream separately from the extremely desaturated systemic venous blood, which is returning from the lower portions of the body with an SVO2 of around 25–40%. At the junctionof the IVCand the right atrium (RA) is a tissue flap known as the Eustachian valve. This flap tends to direct the more highly oxygenated blood, streaming along the dorsal aspect of the IVC, across the foramen ovale (FO) and into the left atrium (LA). In the LA, the oxygen saturation of fetal blood is 65%. This better oxygenated blood enters the left ventricle (LV) and is ejected into the ascending aorta. The majority of the LV blood is delivered to the brain and coronary circulation thus ensuring that blood with the highest possible oxygen concentration is delivered to these vital structures. Desaturated blood (SVO 2 25–40%), from the superior vena cava (SVC) and the coronary sinus, in addition to the IVC’s anteriorly streamed flow (comprised mainly of venous return from lower body and hepatic circulation), is directed across the tricuspid valve and into the right ventricle (RV). This blood is then ejected into the pulmonary artery (PA). Because of the high pulmonary vascular resistance (PVR) only about 12% of the RV output enters the pulmonary circulation, the remaining 88% crossing the ductus arteriosus (DA) into the descending aorta. The lower half of the body is thus supplied with relatively desaturated blood (PO2 2.7 kPa).