 Lumen (anatomy)

A lumen is the internal space of a hollow organ or vessel. In a blood vessel, the lumen contains the flowing blood and is bounded by endothelium. In the small intestine, the lumen contains material being digested and is bounded by an epithelium. The space, its contents and its surrounding wall are distinct anatomical objects.^[ref]

A transverse section makes the distinction visible. Model a circular tube with inner radius $r$ and outer radius $R$, where $R>r$. The luminal cross-sectional area is $\pi r^2$, whereas the wall occupies $\pi (R^2-r^2)$. If $R=2r$, the wall's cross-sectional area is three times the luminal area. Measuring the outside of a tube therefore does not directly measure its internal opening.

For a lining epithelium, the apical membrane faces the lumen; its opposite side faces underlying tissue. This orientation explains why transport into the lumen and transport into the surrounding interstitial fluid are physiologically different directions, even when the same substance moves across the same layer of biological cells.^[ref]