Lumen

A lumen is the interior space enclosed by a tube, sac, or organelle membrane. It is distinct from the surrounding tissue or the membrane itself.

Biological membrane: A thin boundary of lipid molecules and proteins that separates a cell or compartment from its surroundings. A membrane encloses a lumen and controls exchange with the space around it.
Intestinal lumen: The central passage through the intestine, where digested material encounters the absorptive surface. It is a large, continuously connected lumen central to digestion and nutrient uptake.
Selective permeability: The property of a barrier that allows some substances to cross more readily than others. Membrane permeability governs which molecules enter or leave an enclosed lumen.
Lumen (unit): The SI unit of luminous flux, equal to the visible light emitted per unit time, weighted by human visual sensitivity. This optical unit shares the name but has no connection to an enclosed biological space.
Epithelial tissue: A tissue layer that covers body surfaces and lines cavities and hollow organs. Epithelial cells often form the boundary between a lumen and surrounding tissue.
Vascular lumen: The interior passage of a blood or lymphatic vessel through which fluid flows. Its diameter sets the space available for blood or lymph transport.
Membrane transport: The movement of substances across biological membranes by passive or energy-dependent processes. Transport across the enclosing membrane changes luminal contents.
Vacuole: A membrane-bound cellular compartment used for storage, degradation, or maintaining pressure. A vacuole is a bounded structure; its interior, not the structure, is the lumen.
Extracellular space: The space outside cells, including fluid-filled regions between them. It lies beyond the epithelial or cellular boundary rather than inside the enclosed passage.
Airway lumen: The interior passage of an airway through which air moves. Airway narrowing reduces the lumen available for ventilation.

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