Which definition best describes a lumen in a tubular organ?

• A. An inner cavity or space surrounded by the organ walls.
• B. An outer protective layer.
• C. A nerve bundle inside.
• D. A type of connective tissue.

Answer: (A)

The lumen is the hollow interior of a tubular organ—the inner cavity that contents pass through. It is the space enclosed by the organ’s walls, so you can picture it as the central opening inside vessels and ducts. This interior space is what defines the lumen, while the outer protective layer, the nerve fibers inside the wall, and connective tissue are different parts of the structure. For example, in a blood vessel the lumen carries blood; in the intestine it contains digested material. So the best description is the inner cavity or space surrounded by the organ walls.