= Anatomical lumen
An <anatomical lumen> is the internal space of a hollow <organ> or tubular structure. <Blood> occupies the lumen of a <blood vessel>, while intestinal contents occupy the lumen of the gut. The word identifies a space, not its wall, contents or diameter.
For a tube lined by <epithelium>, the apical <cell> surface faces the lumen and the basolateral surface faces the underlying <tissue>. This orientation matters for <membrane transport>: a substance can cross one side of a <cell> and leave through the other. In a <nephron>, for example, moving a solute from tubular lumen back toward the <blood> is reabsorption.
The gut illustrates why geometrical location and physiological access differ. Material can be inside the outline of the body yet still be in a lumen separated from internal <tissues> by an epithelial barrier. Absorption requires crossing that barrier. This anatomical meaning is separate from the photometric unit called the lumen.
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