What structure carries blood back to the heart after it has circulated through the body?

The structure that carries blood back to the heart after it has circulated through the body is the vein. Veins are part of the cardiovascular system and are responsible for returning deoxygenated blood from the body back to the heart. They have thin walls and often contain valves that help prevent the backflow of blood as it moves toward the heart, especially in areas where blood must travel against gravity.

In contrast, arteries carry oxygenated blood away from the heart to the tissues of the body. Capillaries are small blood vessels that facilitate the exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and tissues. Arterioles are smaller branches of arteries that lead to capillaries and are involved in regulating blood flow and pressure but do not serve the primary function of returning blood to the heart. Thus, veins are specifically structured and designated for the purpose of returning blood to the heart, making them the correct answer.