Fetal Circulation

The fetus obtains oxygen and removes carbon dioxide through the placenta, not through the lungs. The fetal lungs are fluid-filled and have high pulmonary vascular resistance (PVR), so they receive blood for growth rather than for gas exchange. This circuit is parallel, not the postnatal series circuit. The right and left ventricles pump at the same time into the systemic circulation, and shunts allow blood to bypass organs that are not yet performing their postnatal functions. Approximately 45% of combined ventricular output perfuses the placenta, while only about 8% to 15% reaches the pulmonary circulation. Blood returning from the placenta travels through the umbilical vein. Some enters the liver; the remainder is directed through the ductus venosus to the inferior vena cava (IVC). The most oxygen-rich stream is preferentially directed toward the right atrium and upper-body circulation. That arrangement gives the brain and myocardium relatively better-oxygenated blood than the lower body, although fetal blood is still mixed compared with postnatal arterial blood.