In the absence of antidiuretic hormone (ADH or vasopressin), the collecting duct is an impermeable tubule which conveys hypotonic fluid from the distal convoluted tubule to the renal pelvis.
ADH causes the CD to become permeable to water, allowing concentration of the urine as the fluid passes through the hypertonic renal medulla.
In the absence of ADH, the collecting duct is impermeable to water and the hypotonic fluid from the distal tubule passes out unaltered as urine.
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