CHAPTER XXIX 

 EXCRETION 



As living matter is constantly being torn down and built 

 up, the removal of waste is as important as supplying 

 food or oxygen. The process of getting rid of waste 

 materials is known as excretion. It is performed by sev- 

 eral organs such as the kidneys, the liver and the skin, 

 each of which carries on its own peculiar kind of excretory 

 activity. Every cell gives off waste into the blood just 

 as every cell respires and assimilates food. A part of 

 this waste is CO 2 and is gotten rid of mainly through the 

 lungs, while other waste materials are solid and escape 

 from the body only by the medium of water, in which 

 they become dissolved. As the lungs are specialized to 

 get rid of the gaseous waste, so other organs are peculiarly 

 adapted to get rid of other forms of waste which are given 

 off into the blood by the cells of the body. Chief among 

 these organs of excretion are the kidneys, two reddish 

 organs one on either side of the spinal column just below 

 and behind the stomach. Each kidney receives a large 

 artery from the aorta (the renal artery) and gives off a 

 large vein (the renal vein) that joins the inferior vena 

 cava. Passing from each kidney is a duct called the 

 ureter; the two ureters pass downward to connect with the 

 bladder which is a thin-walled, elastic sac which serves as 

 a reservoir for the storage of the fluid secreted by the 

 kidneys. 



The kidney is a gland consisting mainly of numerous 

 uriniferous tubules. Each of these structures begins in a 



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