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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!
It is true if that's what you're asking.
It depends on the type of animal, but if, for example, you're talking about one lower on the energy pyramid (like a chicken, for example) then this is because of the energy distribution. When an animal consumes something else, it absorbs about 10 percent of the energy of that creature.
So, lets say a plant has 1000 units of energy. Then a bug comes along, and eats the plant, absorbing 100 units (10 percent) of that energy. Then, a chicken eats the bug, absorbing only 10 units of that energy. Finally, a human eats the chicken, getting only 1 unit of the original energy.
The reason that something like a bug has to eat less than a human, is because it gets more energy directly from the plant.