The collecting duct is situated at the very end of a nephron in the kidney. Multiple nephrons may connect to the same collecting duct. It is where the filtered solution passing through exits the nephron after the reabsorption of water and key substances such as salts and glucose. The collecting duct starts in the cortex of the kidney, however it moves through the medulla in order to empty urine (what is left of the filtrate) into the renal pelvis.
The filtrate that passes through the collecting duct is hypotonic to the medulla, but reabsorption of water at this stage may only occur if antidiuretic hormone (ADH) is present. ADH that has diffused into the medulla opens aquaporins in the collecting duct that allows the water to move out by osmosis, lowering the volume of urine that enters the bladder, but increasing its concentration.