204 OUR BODIES AND HOW WE LIVE 



276. The Waste Matters described. Besides the water 

 which comes from the food we eat, we drink a great deal 

 of it as plain water. We need to keep the tissues continu- 

 ally moist to help to dissolve the food, and also to flush 

 them and cleanse them of their useless matters and impu- 

 rities. As we wash the surface of the body to keep it 

 clean, so nature is ever bathing our tissues to wash away 

 their impurities. 



The red blood corpuscles, as we know, are the tiny boats 

 which carry the oxygen breathed in by the lungs along the 

 blood stream to every tissue. The tissues contain carbon, 

 which in some mysterious way unites with the oxygen, 

 forming carbon dioxide. 



The tissues, especially the muscles, yield nitrogen. A 

 compound of nitrogen, called urea, is filtered out of the 

 body, through the kidneys. It is a peculiar substance, 

 something like ammonia only more complex. 



277. The Chief Organs of Excretion. The process by 

 which the body gets rid of its waste material is called 

 excretion, meaning separation from, or sifting out. 



The chief organs of excretion, or the three main channels 

 by which the waste products leave the body, are the skin, 

 the lungs, and the kidneys. 



The functions of these three organs are closely allied. 

 They differ very much in appearance, but are built on the 

 same general principle. The blood, as it passes through 

 the numberless capillaries in these organs, is purified by a 

 sifting process. The waste matters are, as it were, sifted 

 from the blood, and finally removed from the body. 



The structure of the skin has already been described. 

 One function of the skin, as we have seen, is to rid the 

 body of water and other matters in the form of sweat. 



