Answer:
The mechanism of by which kidney forms dilute or concentrated urine is based on the amount of ADH( Antidiuretic hormone) present in the blood. Collecting duct have receptors for the binding of ADH hormone.
Formation of concentrated Urine:
When the ADH hormone binds to the receptors present on the cells of collecting duct it open the water channels present on it. Once the water channel is opened the duct becomes permeable to water and reabsorption of water takes place in the body making urine concentrated.
Formation of diluted urine:
When ADH is not secreted in the blood, the collecting duct remains impermeable to water molecules which results in production of diluted urine. This happens when blood contains sufficient water.