Normal Circulation in a Fetus

In the fetus, blood entering the right side of the heart has already been oxygenated via the placenta. Because the lungs are not ventilated, only a small amount of blood needs to go through the pulmonary artery. Most of the blood from the right side of the heart bypasses the lungs through the

Normally, these 2 structures close shortly after birth.

Red arrows represent the most highly oxygenated fetal blood (oxygen saturation ≥ 65%). Blue arrows represent the least highly oxygenated blood (oxygen saturation ≤ 45%). Purple arrows represent intermediate oxygen saturation (oxygen saturation 50–60%). Note that oxygen saturation throughout is significantly lower than in postnatal life.