What defines a vein in the circulatory system?

A vein is specifically defined as a blood vessel that returns blood to the heart. This is a crucial function within the circulatory system. Veins are responsible for collecting deoxygenated blood from the tissues of the body and guiding it back to the heart, where it can then be pumped to the lungs for oxygenation.

Veins have thinner walls compared to arteries and often contain valves that prevent the backflow of blood, ensuring it flows in the correct direction toward the heart. Understanding the role of veins is fundamental for comprehending how blood circulates throughout the body, especially in conjunction with the role of arteries, which carry oxygenated blood away from the heart.

The other options represent different components of the circulatory system or lymphatic system functions, but do not capture the defining characteristic of veins specifically. For instance, lymphatic vessels are involved in transporting lymphatic fluid, while capillaries serve as the site of gas exchange and nutrients between blood and tissues. This distinction is important for grasping the overall structure and function of the circulatory system.