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HUMAN PHYSIOLOGY 



CHAP. 



tubes finally empty their excretion into an open space 

 near the concave surface of the kidney, called the pelvis. 

 The glands, tubules, and pelvis are lined with a mucous 

 membrane, thick in some parts and thin in others. 



The kidneys are very useful because they are the 

 principal organs for the removal of water, urea, and 



other waste matters. In warm 

 weather the skin probably 

 removes as much water as 

 the kidneys do. The skin 

 also excretes some salt and 

 urea, but most of these wastes 

 are removed through the kid- 

 neys. Urea is manufactured 

 from ammonia compounds in 

 the liver. It then enters the 

 blood and is distributed to all 

 parts of the body. When the 

 renal arteries carry it to the 

 glands and tubules of the kid- 

 neys, it is picked out of the 

 blood and poured into the pel- 

 vis. The solid urea and other 

 poisonous wastes are dissolved in a large amount of 

 water ; this is known as urine. All urine is carried from 

 the pelvis of the kidneys to the bladder by means of the 

 ureters, slender ducts about sixteen inches long. The 

 bladder serves as a reservoir for the urine, from which 

 it is periodically discharged from the body. About three 

 pints of urine are discharged from the body every day. 



FIG. 83. The kidneys and blad- 

 der (Cheever). 



