Collecting duct
= Collecting duct
A <collecting duct> receives tubular fluid from several <nephrons> and participates in the final adjustment of <urine>. <Collecting ducts> merge as they pass through the <renal medulla>, conducting the remaining fluid toward the renal outlet. Although continuous with the <renal tubules>, the shared <collecting duct system> is not technically part of an individual <nephron>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 25.4, Collecting Ducts]{ref}
Its <epithelium> can change its <membrane permeability> to <water>. <Vasopressin> promotes the presence of <aquaporin-2> in the <apical membrane> facing the <tubular lumen>. When an appropriate <osmotic pressure> difference favours outward movement, <water> crosses toward the surrounding <interstitial fluid> and can return to the <blood>. The channel changes access to an <osmosis> pathway; it does not itself pump <water> against that driving difference.
A 1993 study by <K. Fushimi> and collaborators identified a collecting-tubule water-channel <Complementary DNA>, then tested the encoded channel by <gene expression> in <Xenopus> <oocytes>. Expression increased osmotic water permeability. Localization to the apical region of collecting-tubule <biological cells> connected that functional result to its site in the <kidney>.<Cloning and expression of apical membrane water channel of rat kidney collecting tubule by K. Fushimi and others>[Abstract: collecting-tubule localization and osmotic permeability in expressing oocytes]{ref}
The regulated movement was tested more directly in 1995. Investigators measured osmotic water permeability in isolated perfused rat <collecting ducts>, then used <immunoelectron microscopy> to examine channel distribution in the same tubules before, during and after <vasopressin> exposure. Increased permeability accompanied more channel labelling at the <apical membrane> relative to intracellular vesicles. Withdrawing <vasopressin> reversed both changes.<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: same-tubule perfusion, immunolocalization and withdrawal comparison]{ref}
This links a physiological change to <membrane trafficking>: <exocytosis> can add channel-bearing membrane to the surface, while <endocytosis> can retrieve it. The amount of channel at the correct surface, as well as its presence somewhere inside the <epithelial cell>, determines the available water pathway.