= Tubular lumen
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.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Sections4.2 and25.4: epithelial polarity and renal-tubule organization]{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> is
$$
V=\pi r^2L=\pi(10\,\mu\mathrm{m})^2(1000\,\mu\mathrm{m})\approx0.314\,\mathrm{nL}.
$$
This 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>.<Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane by S. Nielsen and others>[Abstract: isolated perfused tubules and same-tubule immunolocalization]{ref}
Back to article page
