= Collecting duct system
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The <collecting duct system> is the converging network of <collecting ducts> that receives fluid from many <nephrons>, modifies its composition, and conducts the remaining <urine> toward the outlet of the <kidney>. <Collecting ducts> run from the <renal cortex> through the <renal medulla>, joining into larger passages that empty at a <renal papilla> into a <renal calyx>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Sections25.3–25.4: renal pyramids/papillae and collecting ducts]{ref}
The network arrangement matters: a single downstream <collecting duct> can receive output from several upstream <nephrons>. Consequently the shared <collecting duct system> is not a separate terminal tube assigned exclusively to each <nephron>. Flow is pooled while further exchange across the lining <epithelium> adjusts the final <urine>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section25.4, Collecting Ducts]{ref}
A shared outlet can still regulate its water loss. In isolated rat <collecting ducts>, <vasopressin> increased water permeability while <aquaporin-2> labelling shifted toward the <apical membrane>. Withdrawal reversed both changes. This supplies a molecular mechanism for regulating the network's epithelial water pathway; a surrounding <osmotic pressure> difference supplies the driving force for <osmosis>.<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: perfusion measurements, immunoelectron microscopy and withdrawal comparison]{ref}
For example, removing <water> from the pooled tubular fluid increases its remaining <solute> concentration even without adding more <solute>. Delivery along the network and exchange across its walls jointly determine what leaves the <kidney>.
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