Final answer:
The foramen ovale and ductus arteriosus are the two primary structures in the fetus that reroute blood away from the nonfunctional lungs.
Step-by-step explanation:
The two structures in the developing fetus that allow most of the blood to bypass the lungs are the foramen ovale and the ductus arteriosus. The foramen ovale is a shunt in the interatrial septum allowing blood to flow directly from the right atrium to the left atrium, thus avoiding the pulmonary trunk and the nonfunctional fetal lungs. On the other hand, the ductus arteriosus is a temporary vessel connecting the pulmonary artery to the aorta, ensuring that blood mainly bypasses the lungs and is circulated into the systemic circulation. This network of shunts is essential because the fetal lungs are filled with amniotic fluid and not yet functional, with gas exchange occurring via the placenta.