The purpose of the excretory system

During the metabolism of proteins, fats and carbohydrates break down into the most basic components in its composition. Carbon, hydrogen and oxygen from their molecules with absorbed by oxygen in the air to form as a last slag carbon dioxide and water. Nitrogen is released when the protein, liver collects from the entire body and then produces in the blood in the form of urea. The last together with uric acid is the end product of nitrogen metabolism in the body.
These are the waste products of metabolism that must be removed from the body. Carbon dioxide, which during the day, even at rest is formed by more than 600 g is excreted through the lungs. Water, which should be displayed as much as you enter, i.e. 2-2,5 liters, also partially (350 g) is removed through the lungs. The rest of the water with dissolved waste nitrogen metabolism, as well as many other toxins excreted by the kidneys and sweat glands of the skin. These organs are the organs of excretion in the proper sense of the word. Because through the skin is removed during the day, 0.5 liters of water (in hot weather is much more), and kidney 1.5 liters, with the main mass of urea and other end products of metabolism is removed through the kidneys, they are the main, most important excretory organ, without which life is impossible.
The reader may ask: why the intestine is not named among the bodies allocation? The thing is that the bowel emits only remnants of food, and not products of metabolism of the body cells. Only some of matter in the colon move from the blood in the bowel lumen and are excreted in the feces. These substances little, but because the intestine although it may be listed in this sense to the excretory organs, has little value.