Fetal and postnatal (adult) circulation: in fetuses, placenta is the organ of gas exchange. The umbilical vein carries oxygenated blood from the placenta has the highest fetal oxygen saturation of approximately 80%. Oxygen saturation in various blood vessels is shown in crimson circles. Oxygenated blood is streamed through the ductus venosus and along the left margin of the inferior vena cava. In the heart, oxygenated blood gets shunted across the foramen ovale (FO) to the left heart to supply the cerebral and coronary circulation. Poorly oxygenated blood from the upper half of the body goes to the right side of the heart, the main pulmonary artery and gets shunted through the ductus arteriosus (DA) to the descending aorta. Blood in the descending aorta has a saturation of around 55 %, and this blood flows back to placenta for oxygenation through umbilical arteries. In spite of relative hypoxia, adequate oxygen delivery to the fetal tissues is maintained by (a) high cardiac output, (b) high hemoglobin level in the term fetus and (c) the presence of fetal hemoglobin (HbF) with high oxygen affinity. The saturation gradient across the placenta (85 – 55 = 30%) is similar to the saturation gradient across the lungs in adults (98 – 70% = 28%) but is achieved at a much lower partial pressure of oxygen (PO2 ~ 32 mmHg in the umbilical vein) in the fetus compared to the adult (PaO2 ~ 95 mmHg) limiting the fetal risk for oxygen toxicity. (copyright Satyan Lakshminrusimha).