What vessels are responsible for carrying de-oxygenated blood?

Veins are the vessels responsible for carrying de-oxygenated blood back to the heart. In the circulatory system, blood takes on two primary forms: oxygenated and de-oxygenated. Oxygenated blood, which is rich in oxygen, is carried away from the heart by arteries. In contrast, after the blood has delivered oxygen to the tissues and collected carbon dioxide, it becomes de-oxygenated. This de-oxygenated blood is then transported back to the heart through veins.

Veins are designed with thinner walls and larger lumens than arteries, facilitating the flow of blood back to the heart, often against the force of gravity, aided by valves that help prevent backflow. They play a crucial role in the circulatory system by ensuring that the heart is consistently replenished with blood that will be re-oxygenated in the lungs.

Understanding the role of veins also contextualizes the functions of the other choices. For example, arteries carry oxygenated blood away from the heart, capillaries are the sites of exchange between oxygen and carbon dioxide, and the aorta is the largest artery in the body, which primarily carries oxygenated blood. Thus, veins are specifically associated with transporting de-oxygenated blood.