= Lumen
{disambiguate=anatomy}
{wiki}
A <lumen (anatomy)> is the internal space of a hollow <organ> or vessel. In a <blood vessel>, the <lumen (anatomy)> contains the flowing <blood> and is bounded by <endothelium>. In the <small intestine>, the <lumen (anatomy)> contains material being digested and is bounded by an <epithelium>. The space, its contents and its surrounding wall are distinct anatomical objects.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section4.2: epithelial lining of hollow organs, apical surfaces and endothelium]{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 (anatomy)>; its opposite side faces underlying <tissue>. This orientation explains why transport into the <lumen (anatomy)> and transport into the surrounding <interstitial fluid> are physiologically different directions, even when the same substance moves across the same layer of <biological cells>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section4.2: epithelial polarity and basal attachment]{ref}
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