During fetal circulation, blood bypasses the lungs.

Question

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Because the fetal lungs are not functioning, this blood mostly bypasses the lungs. Most of the blood entering the right atrium by the inferior vena cava is shunted directly into the left atrium through the foramen ovale. There is a small valve on the left side of the atrial septum called the septum primum. This valve keeps blood from going back into the right atrium. The remaining fetal blood that has entered the right atrium contains a large amount of oxygen-poor blood from the superior vena cava and travels to the right ventricle and into the pulmonary trunk. The pulmonary blood vessels have a high resistance to blood flow because of the collapsed state of the lungs; thus they only allow a small amount of blood to enter the pulmonary circulation. This small amount is enough to nourish the pulmonary tissue.

During Fetal Circulation, Blood Bypasses the Lungs