CHAPTER 11 



THE URINARY SYSTEM 



The importance of the circulatory system for maintaining homeo- 

 stasis or a constant internal environment has already been emphasized, 

 but its work would be impossible without the aid of the organs of ex- 

 cretion. Only in stable surroundings can the many complex processes 

 characteristic of the organism take place. The blood maintains this 

 homeostasis by bringing in foods and oxygen, carrying away waste 

 products, and distributing the secretions of the different endocrine 

 glands. 



The metabolism of foods for the production of energy and the re- 

 pair of tissues is accompanied by the accumulation of waste materials. 

 These are removed from the sites where produced by the blood. How- 

 ever, if there were no manner in which they could be excreted from 

 the organism, they would accumulate and cause great harm. Thus 

 the importance of homeostasis is further emphasized and the role as- 

 sumed by the organs of excretion becomes more important. 



In mammals there are several different systems which aid in the 

 excretion of waste materials. Carbon dioxide is lost from the lungs ; 

 some salts and small amounts of urea are eliminated through the skin ; 

 the large intestine is able to excrete waste heavy metals. Most im- 

 portant, however, is the work done by the kidneys. By means of their 

 many tiny tubules, the entire blood supply of the body is constantly 

 filtered. Excess materials are removed, and homeostasis is maintained. 

 So vital is this work, that life itself is impossible once their functioning 

 stops for even a short time. 



All animals from the single-celled protozoans to the multi-cellular 

 mammal have the same problem of disposing of their metabolic wastes. 

 For the small aquatic forms it is simply a matter of the waste products 

 diffusing out through the cell membrane. With increasing body size, 

 the problem of this disposal becomes more complex. This has been 



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