Answer:
There are air filters to purify the air and water filters to clean the water. Well, the kidneys are like a filtering system, too. They contain nephrons, which filter waste from the blood in two stages. First, nephrons take waste as well as needed materials out of blood. Then, the materials the body needs (like water and glucose) are returned to the blood and wastes are eliminated from the body. This is one way the kidneys maintain homeostasis. The kidneys also regulate the amount of water in the body. The amount of water the kidneys return to the blood depends on the conditions inside and outside of the body. This is another way the excretory system maintains homeostasis. For example, you will make more urine on a cool day when you have drunk a lot of water than you will on a hot day when you've hardly had anything to drink.
(I answered a question similar to this some time ago so you might also find it there.) Hope It Helps!
