How does antidiuretic hormone (ADH) regulate water reabsorption in the collecting ducts?

ADH makes the principal cells of the collecting ducts more permeable to water by triggering insertion of aquaporin water channels into their apical membranes. The number of aquaporins inserted, and therefore the amount of water reabsorbed by osmosis, is determined by how much ADH is present. This ADH-dependent movement is called facultative water reabsorption; when ADH levels are low, the collecting ducts remain relatively impermeable to water. ADH also promotes urea reabsorption in the deep medullary collecting duct, which strengthens the medullary osmotic gradient and supports concentrated urine production.

ADH is released in large amounts when a person is dehydrated. At the collecting duct, it acts on principal cells to insert aquaporins into their apical membranes. In the absence of ADH, these aquaporins are virtually absent, so the collecting duct is not very water permeable. With ADH present, water is reabsorbed by osmosis through the aquaporins, and the amount of ADH controls how many aquaporins are inserted and thus how much water is reclaimed. This ADH-dependent process is called facultative water reabsorption. ADH also increases urea transport out of the medullary collecting duct, which enhances urea recycling and strengthens the medullary osmotic gradient. When ADH is at maximal levels, up to 99% of the water in the filtrate can be reabsorbed, and urine output can fall to about half a liter per day of highly concentrated urine. Conversely, when the body is overhydrated, ADH secretion decreases and the collecting ducts become relatively impermeable to water, allowing more dilute urine to be produced.