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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.

An endocrine gland releases hormones into the surrounding internal fluid for delivery to target tissues, commonly through the blood. Unlike an exocrine gland, it does not use a duct to discharge its hormonal secretion onto an epithelial surface or into an anatomical lumen.^[ref]

The thyroid and pituitary are examples. The pancreas shows why classification can be functional: its endocrine cells release hormones such as insulin, while other pancreatic cells send digestive secretion through ducts. Hormone delivery alone does not specify the response. A target cell's receptors and intracellular signaling determine whether and how it reacts, and feedback can regulate further secretion.

Insufficient blood supply to intestinal tissue relative to its needs. Impaired oxygen delivery can disrupt the mucosal barrier and, when severe, cause necrosis; the mechanism is vascular rather than simply blockage of the bowel anatomical lumen.

A hernia trapping only part of the bowel wall; severe vascular compromise may occur without complete obstruction of the anatomical lumen.