The waste products of metabolism and all unutilized food, along with the products of oxidation, are eliminated from the system in several ways. Water escapes through the lungs, skin and kidneys. Aside from the obvious fluid perspiration, a steady evaporation takes place from the skin (insensible perspiration). The carbon dioxide produced by oxidation escapes by the lungs, and nitrogenous products of protein metabolism by the urine. The feces are made up chiefly of the residues of food, but also contain biliary matters and other products of the body. When there is excessive putrefaction of the fecal matter, some of the soluble material is absorbed and eliminated in the urine. Generally speaking, putrefaction (of nitrogenous food) and fermentation (of carbohydrates) hold each other in check. It is known that fasting and starving people pass some feces, thus showing that the bowel is a true excretory organ.