ABSTRACT
In adults, blood circulates sequentially through the systemic and pulmonary vasculature and there is essentially no mixture of blood oxygenated in the lung and that in the systemic circulation. Fetal cardiac hemodynamics differs from that seen postnatally. During fetal life, blood is oxygenated in the placenta and returns to the fetal body via the umbilical vein. Studies in chronically instrumented fetal lambs have shown that in physiological conditions, about 55 % of umbilical vein blood bypasses the hepatic circulation, entering the inferior vena cava directly via the ductus venosus. 1 From the inferior vena cava this highly oxygenated blood preferentially streams through the foramen ovale into the left atrium, left ventricle, and descending aorta. 2 On the other hand, poorly oxygenated blood from the hepatic and superior vena cava circulations enters the right atrium and is almost completely directed through the tricuspid valve into the right ventricle and pulmonary artery. 2 Because fetal blood is not oxygenated by the lungs, an additional shunt (i.e. the ductus arteriosus) operates to bypass the pulmonary circulation, preferentially directing the right ventricle output to the descending aorta. As a consequence, both ventricles eject into the systemic circulation in parallel. The output of the left ventricle is directed through the ascending aorta to the upper body organs, thus making the most highly oxygenated blood available to the heart and brain. The right ventricle ejects through the patent ductus arteriosus and the descending aorta to the lower body and placenta.
