Final answer:
Antidiuretic hormone (ADH) regulates water balance in the kidneys by controlling the amount of water reabsorbed into the bloodstream.
Step-by-step explanation:
The hormone that controls water regulation in the kidney is antidiuretic hormone (ADH), also known as vasopressin. ADH is crucial for preserving the body's water balance by increasing the permeability of the kidney tubules to water, thereby allowing more water to be reabsorbed back into the bloodstream and decreasing urine output. This hormone is synthesized in the hypothalamus and stored and released by the posterior pituitary gland. When the body needs to conserve water due to dehydration or increased blood plasma concentration, ADH is released, prompting the kidneys to reclaim more water.