Which of the following structures is NOT part of the systemic circuit?

The pulmonary veins are not part of the systemic circuit; instead, they are part of the pulmonary circuit, which is responsible for carrying oxygenated blood from the lungs to the heart. The pulmonary circuit includes all the structures involved in transporting blood between the heart and the lungs, where carbon dioxide is exchanged for oxygen.

In contrast, the systemic circuit involves the aorta, which distributes oxygen-rich blood from the left ventricle to the body, the left atrium, which receives oxygenated blood from the pulmonary veins before it moves into the left ventricle, and the venae cavae, which bring deoxygenated blood from the body back to the heart. Each of these structures plays a crucial role in the systemic circulation that delivers oxygen and nutrients to tissues throughout the body.