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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