218 ADVANCED PHYSIOLOGY 



sorb only a certain amount of these wastes, but if they can 

 be constantly removed from it, the blood may go on absorb- 

 ing them continually. A part of these materials, we have 

 already learned, is excreted through the lungs, since there 

 carbon dioxid gas and water are being constantly eliminated, 

 respiration thus being in part an excretion. The wastes ex- 

 creted through the skin will be considered in a special chapter. 

 The portion secreted through the kidneys will be noticed here. 



UREA: ITS SOURCE AND EXCRETION 



Proteid food contains nitrogen; therefore, among the body 

 wastes there must be some which hold this element. The 

 main one is urea, which is essentially broken down proteid 

 material, with the chemical formula (N 2 H 4 ) CO. Although 

 muscles are largely proteid this does not necessarily mean 

 that muscular work will result in a large amount of urea 

 formation, for the energy of muscular contraction comes 

 chiefly from the oxidation of starches, sugars and fats; since 

 these contain no nitrogen their oxidation, of course, produces 

 no urea. A considerable quantity of urea does come from 

 the muscles, but this happens whether they are working or 

 not; for the very act of living results in the breaking down of 

 proteid material, and consequently in nitrogenous waste. 



The waste from muscles does not leave them as urea, but 

 in a slightly different form. It is probably carried by the blood 

 to the liver, where it is changed into urea. Thus we learn of 

 another function of the liver, viz. in converting this waste 

 from the form in which it leaves the muscles into a form ca- 

 pable of excretion. The proof of this fact is that an animal 

 from which the liver has been removed, or in which it is 

 diseased, gives off very little urea, though an unusual amount 

 of ammonia, which also contains nitrogen, is excreted in 

 such a case. From the liver the urea passes into the blood 

 again, to be carried to the kidneys whose function it is tQ 

 remove urea from the blood. 



