The metabolism that goes on in all our body cells results in the formation of waste products. The chief of these, carbon dioxide, is taken care of by means which were described in the last chapter. Besides this gaseous substance there are produced a number of compounds which are soluble in water and which are poured out from the cells into the tissue spaces surrounding them and which pass thence into the blood stream. The special places in the body wherein these substances leave the blood stream make up the apparatus for the removal of waste. This apparatus must be able to take out of the blood stream all the waste products of cell metabolism except carbon dioxide, also any materials which may have been absorbed into the blood stream from the digestive tract, but not used by the tissues, and finally all the accumulations of water. The discharge of water makes up a topic by itself which will be considered after a word has been said about the apparatus by which the removal of wastes is carried on. Sharing in this function we have first the kidneys, second the liver, and third the sweat glands. The kidneys are the most important of the organs for removing waste. They are a pair of bean-shaped bodies lying in the small of the back. Each [Illustration: DIAGRAM OF KIDNEY (From Martin’s “Human Body”)] one is made up of thousands of tiny tubes; each of these tubes starts as a little sac in which is a knot of blood vessels which is really a tiny filter, filtering out from the blood water and the inorganic salts that are dissolved in it.