What is the hollow interior of a blood vessel called?

The hollow interior of a blood vessel is referred to as the lumen. This space is crucial for the transportation of blood within the circulatory system. The lumen allows for the flow of blood to reach various tissues and organs, facilitating the delivery of oxygen and nutrients while removing waste products. The structure of the lumen can vary depending on the type of blood vessel; for example, arteries have a narrower lumen compared to veins, which can hold more blood at lower pressure.

In contrast, the vessel wall consists of multiple layers surrounding the lumen, providing structural support and elasticity. Capillaries, although they are blood vessels, specifically refer to the smallest vessels that allow for exchange between blood and tissues. Endothelium is the thin layer of cells lining the lumen of blood vessels, playing a critical role in regulating blood flow and the passage of substances; however, it does not define the hollow space itself. The concept of the lumen is fundamental in understanding blood vessel anatomy and physiology.