Antidiuretic hormone (ADH), also known as vasopressin, is the hormone involved in maintaining water absorption in the kidneys. ADH is produced by the hypothalamus and released from the posterior pituitary gland. It acts on the kidneys to increase water reabsorption from the collecting ducts, reducing urine output and maintaining body water balance. When the body is dehydrated or blood volume is low, ADH secretion increases, leading to greater water reabsorption and decreased urine production. Conversely, when there is sufficient body water, ADH secretion decreases, resulting in increased urine output to eliminate excess water.