Anatomy and Physiology II Practice Test

Which structure in the fetus shunts blood from the right atrium to the left atrium, allowing most of the blood to bypass the fetal lungs?

1. Aorta

2. Ductus arteriosus

3. Foramen ovale

4. Foramen rotundum

The correct answer is: Foramen ovale

The foramen ovale is a crucial structure in the fetal circulatory system that allows blood to bypass the non-functioning lungs. This oval-shaped opening is located between the right and left atria of the heart. In fetal development, since the lungs are not yet employed for gas exchange, oxygen-rich blood from the placenta enters the right atrium and mostly flows directly into the left atrium through the foramen ovale. This mechanism ensures that oxygenated blood is efficiently distributed to the growing tissues and organs of the fetus while minimizing the amount of blood that would otherwise need to flow to the undeveloped lungs. Once the baby is born and takes its first breath, the foramen ovale typically closes, transforming into a fibrous structure known as the fossa ovalis. This closure is crucial for establishing normal postnatal circulation, where the right atrium now receives deoxygenated blood from the body to send it to the lungs for oxygenation. The other structures mentioned do not perform the same function: the aorta is primarily involved in carrying oxygenated blood to the body from the left ventricle, the ductus arteriosus connects the pulmonary artery to the aorta in the fetus but is responsible for shunting blood away from