Neurons in the hypothalamus produce ADH, which the posterior pituitary releases in response to a decreasing water concentration in blood or a decrease in blood volume. When ADH reaches the kidney, it increases the permeability of the epithelial linings of the distal convoluted tubule and collecting duct to water, and water moves rapidly out of these segments by osmosis (water is reabsorbed). Consequently, urine volume falls, and urine concentrates soluble wastes and other substances in minimal water. Concentrated urine minimizes loss of body fluids when dehydration is likely.
If body fluids become too dilute, ADH secretion decreases. In the absence of ADH, the epithelial linings of the distal segment and collecting duct become less permeable to water, less water is reabsorbed, and urine is more dilute, excreting the excess water.
