372 EXCRETION 



urea 2.2%, other nitrogenous wastes 0.17%, mineral salts 

 about 1.6%. 



The liver as an excretory organ. — We learned (see p. 

 144) that through the bile the liver has the power to 

 remove various poisons and other wastes. In this respect 

 it is also an excretory organ. It is now believed that the 

 urea or nitrogenous waste is not formed in the kidneys, 

 but is present in the blood which flows to the kidneys, 

 these simply secreting it from that blood. It has been 

 further proved that urea is formed, in part at least, in the 

 liver and then sent in the blood to the Icidneys for removal. 

 Just how the wastes of the cells are made into urea, and 

 where it takes place, is still uncertain, but it is already 

 demonstrated that the liver performs an important part 

 in this operation. 



Hygiene of the kidneys. — The skin excretes very little 

 urea and much water. The kidne3^s, on the other hand, 

 excrete much urea. When much water is given off by the 

 skin, as in summer, less is given off by the kidneys ; while 

 in winter, when perspiration is less, the amount of water 

 disposed of by the kidneys is correspondingly increased. 

 The fact that more blood passes to the skin in summer 

 also has an effect in this direction. Further, the perspira- 

 tion and blood supply to the skin is governed mainly by 

 the external temperature, while the amount of water 

 excreted by the kidneys is governed partly by the amount 

 of water swallowed. Now a decrease below a certain 

 limit in the amount of water excreted by the kidneys 

 tends to prevent the proper washing away of the urea, 

 while, if not washed away, the accumulation of this urea 

 colors the urine and tends to clog the action of the 

 kidneys. 



