CHAPTER XL 

 THE EXCRETION OF WASTE FROM THE BODY 



241. Need for Excretion. So long as the body is alive it 

 needs a steady supply of food, partly to build up new tissues 

 and to repair worn ones, and partly to supply the energy by 

 means of which the tissues are able to work. Ultimately, 

 through wear or oxidation, the materials of the body break 

 down into substances that are not only useless but harmful, 

 and these wastes are thrown out by the organs of excretion. 

 When carbohydrates and fats are oxidized in the body, they 

 return to the elements from which they were made carbon 

 dioxide and water. Most of this carbon dioxide and part 

 of the water are given off through the lungs. These organs, 

 in addition to their function of supplying oxygen to the 

 body, are thus seen to be true organs of excretion. A small 

 amount of carbon dioxide and a much greater amount of 

 water are excreted by the skin, and this organ also excretes 

 some salt and urea, the latter a waste from the nitrogenous 

 part of the food. The liver is seldom thought of as an organ 

 of excretion, though the bile, useful as it is in digestion, con- 

 tains much waste matter which, poured into the small intes- 

 tine, passes out of the body with the refuse from the food. 

 The liver also builds up certain substances from the waste 

 matters in the blood which are returned to the blood to be 

 later excreted by the kidneys. These latter are the principal 

 organs for the elimination of protein wastes from the body and 

 also share with the skin the duty of disposing of the excess 

 water absorbed. 



280 



