CHAPTER VII 

 EXCRETION 



Excretions are substances formed in the course of metabolism 

 of cells. Such substances are in the nature of end products of 

 chemical changes in the tissues and are useless as further sources 

 of energy to the animal body. Not only are they useless but 

 actually detrimental if allowed to remain in the body. A secre- 

 tion differs from an excretion in that the secretion is of some 

 definite use in metabolism, whereas the excretion is not. Gastric 

 juice is a secretion of the glands in the stomach; urine is an 

 excretion of the kidneys. Obviously undigested food, and 

 inorganic salts and nitrogenous products of putrefaction absorbed 

 from the alimentary canal, are not excretions. In man the major 

 part of the solid excretions in solution, and certain substances 

 occurring largely as by-products of digestion, absorbed from 

 the alimentary canal by the blood, are excreted by the kidneys. 

 On the other hand, the bile pigments, which are derived from 

 broken-down erythrocytes and which are true excretory products, 

 leave the body by way of the alimentary canal. Carbon dioxide, 

 a gaseous excretory product, is eliminated by the respiratory 

 organs — gills, lungs, or the surface of the body. In man some 

 of the constituents of perspiration are excretory in character. 

 Excretory substances are formed by and in the living tissues, 

 from which they are collected by the blood and eliminated by 

 various organs. A number of organs take part in excretion, but 

 the kidney is unique in that the absorption of excretory products 

 from the blood and the subsequent excretion of these products 

 are its sole functions. The discussion of excretion therefore 

 centers about the structure and function of the kidney. 



Protonephridia. — One of the important functions of the blood 

 is the collection or absorption of products of metabolism from the 

 cells of the body and their transport to the excretory organ, 

 which removes them from the blood stream. In some forms 

 lacking a blood-circulating system, excretory products collect in 



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