Describe the fetal circulation

Describe the fetal circulation.

The fetal circulation (Figure 64.1) is a parallel circuit in contrast to a series circuit in the adult. In the fetus, gas exchange occurs at the placenta and not the lung. Blood leaving the placenta enters the fetus via the umbilical vein. This relatively well oxygenated blood (pO₂ 30–35 mmHg) enters the inferior vena cava, predominantly bypassing the liver via the ductus venosus. Most of this blood upon entering the right atrium is preferentially shunted across the patent foramen ovale to the left side of the heart and out the ascending aorta to the cerebral and coronary circulation. Thus, the brain and heart receive most of the relatively well oxygenated blood. Blood returning from the cerebral circu-lation via the superior vena cava, which is considerably less oxygenated (pO₂ 12–14 mmHg), enters the right side of the heart. This blood is preferentially directed to the right ventricle and exits through the pulmonary artery. Because of the high pulmonary vascular resistance (PVR) that exists in utero, only 10% of this blood enters the pulmonary circulation to provide nutrients for lung growth. The remaining blood is shunted across the ductus arteriosus because of the relatively low systemic vascular resistance (SVR) secondary to the presence of the placenta. Blood enters the descending aorta and supplies the lower fetal body, returning to the placenta via the umbilical arteries.