Which of the following is an example of a lumen?

• A. The space within a blood vessel.
• B. The wall of a blood vessel.
• C. Blood inside the vessel.
• D. The lining of the vessel.

Answer: (A)

A lumen is the hollow interior space of a tubular structure, such as a blood vessel. In a blood vessel, the lumen is the central channel through which blood flows. The wall of a blood vessel is the surrounding structure that gives shape and regulates diameter. Blood inside the vessel is the fluid occupying the lumen, not the space itself. The lining of the vessel refers to the inner surface, the endothelium, which coats the lumen rather than being the space. So the space within a blood vessel best fits the definition of a lumen, the open passage that allows blood to move.