Answer:
Hormones are involved in the osmoregulation of kidneys by regulating the levels of water.
Such as if the water level is excess in the body fluids, the hormone will help the kidneys to filter as much water as they can from the glomerular capillaries. So, As a result dilute urine is formed.
Now, If the water level is very low in the body fluids, the hormone will help the kidneys to absorb most water and filter less water from the glomerular capillaries. As a result the urine formed is concentrated.
In Chapter 3, "Coordination and Control" You will come to know that the hormone is ADH (Anti-diuretic hormone) called vasopressin.