In a blood vessel such as an artery, the lumen is the interior space through which contents move.

• A. Artery
• B. Nerve
• C. Bone
• D. Muscle

Answer: (A)

The lumen is the hollow interior of a tube through which contents flow. In the circulatory system, the lumen is the central passage where blood moves. An artery is a hollow tube built to carry blood, with its lumen forming the space inside through which blood travels. The wall layers around it—the endothelium lining the inner surface, a thick muscular layer, and supporting outer layers—enclose that space and help regulate blood flow and pressure. Other structures like nerves, bones, or muscles aren’t primarily hollow channels for circulating fluids in the same sense, so they don’t fit the idea of a lumen as the interior passage for moving contents.