Lumen (anatomy)

A lumen is the internal space of a hollow organ or vessel. In a blood vessel, the lumen contains the flowing blood and is bounded by endothelium. In the small intestine, the lumen contains material being digested and is bounded by an epithelium. The space, its contents and its surrounding wall are distinct anatomical objects.^[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; its opposite side faces underlying tissue. This orientation explains why transport into the lumen and transport into the surrounding interstitial fluid are physiologically different directions, even when the same substance moves across the same layer of biological cells.^[ref]

A tubular lumen is the continuous internal passage of a tube-shaped anatomical structure. In a renal tubule, its lining epithelium separates the tubular fluid from surrounding interstitial fluid. The apical membrane faces the tubular lumen, so movement along the tube and movement across its wall follow different paths.^[ref]

A straight cylinder gives a useful scale estimate. If the internal radius is $10\mu \mathrm{m}$ and the length is $1\mathrm{mm}$, the luminal volume isThis is an ideal geometric example; real renal tubules change shape and diameter along their length.

Experiments on isolated perfused collecting ducts exploit the same compartment distinction. Fluid is supplied through the tubular lumen, while a separate surrounding solution contacts the outside of the epithelium. Changing the conditions permits measurement of water crossing the wall. In a 1995 study, the increased membrane permeability during vasopressin exposure accompanied movement of aquaporin-2 toward the apical membrane.^[ref]