Final answer:
The foramen ovale in the fetal heart allows blood to flow from the right atrium directly to the left atrium, bypassing the nonfunctioning lungs.
Step-by-step explanation:
The structure in the fetal heart that allows blood to flow from the right atrium directly to the left atrium is the foramen ovale. It is an opening in the interatrial septum that allows oxygenated blood to bypass the nonfunctioning fetal lungs and flow directly to the left atrium. A valve associated with this opening prevents backflow of blood during the fetal period.