What structure allows blood to bypass the lungs in fetal circulation?

The structure that allows blood to bypass the lungs in fetal circulation is the foramen ovale. In a fetus, the lungs are not yet functioning to oxygenate blood, so a significant portion of blood needs to be diverted away from them. The foramen ovale is an opening between the right and left atria of the heart that enables this bypass.

When blood enters the right atrium, it can flow directly into the left atrium through the foramen ovale instead of moving into the right ventricle and then into the pulmonary arteries, which lead to the lungs. This mechanism not only conserves energy by avoiding the non-functional lungs but also directs oxygen-rich blood from the placenta to systemic circulation, where it is needed for fetal development.

Although other structures like the ductus arteriosus also play a crucial role in directing blood flow by connecting the pulmonary artery to the aorta, they do so primarily for shunting blood after it has already moved towards the lungs. The foramen ovale is essential for initially bypassing the lungs and is critical for maintaining proper fetal circulation.