CHAPTER XIV 

 WASTE AND KXCKKTION 



WE saw that the essential characteristics of living tissues are 

 that they are constantly undergoing oxidation, and constantly 

 building up their substance anew. We have seen how the blood 

 obtains and supplies them with oxygen, and that the blood derives 

 all the other substances they require from the food in the ali- 

 mentary canal. We have seen also how the crude substances in 

 the food are acted on and prepared by the various digestive juices, 

 anil that some of the substances are further modified as they 

 are being absorbed by the epithelial cells of the intestine. 

 Certain substances in the blood coming from the alimentary 

 canal, especially the carbohydrates, arc further acted on by the 

 liver. The blood is also influenced during its circulation 

 through other organs such as the spleen, so that it is brought 

 into and kept in a condition tit for the proper nourishment of 

 the tissues. 



The oxidation which the tissues are constantly undergoing 

 is a process of waste. The living substance takes up oxygen, 

 and the oxygen unites with the elements or groups of elements 

 of which the living substance is composed, and is given off 

 again in union with them in the waste products. The chief 

 of these waste products arc carbonic acid, water, and urea. 

 These substances which are made in the tissues pass from 

 the tissues into the blood. Just as the tissues derive all 

 their nutrient matter and oxygen from the blood, so they 

 return to the blood their waste products. These substances 

 being injurious to the tissues have to be got rid of; they 

 must be removed from the blood and be discharged from the 

 body. They have to be excreted as it is called. Kx< rction 



