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collecting duct Any of the ducts in the mammalian kidney that drains into the renal pelvis, which leads to the ureter. They are the main sites of water reabsorption from the glomerular filtrate, which drains into the ducts from the distal convoluted tubules of the nephrons. The cells of the collecting ducts are relatively impermeable to water. However, the influence of antidiuretic hormone increases the permeability of the collecting ducts, allowing the reabsorption of water and controlling the final urine concentration according to the body's state of hydration.