35 2 PHYSIOLOGICAL CHEMISTRY. 



uric acid is not a step in the ordinary protein metabolism, but 

 is a derivative of certain substances only, which break down 

 to a limited degree. The amount of uric acid which could 

 be formed in this way would not be very large at most. In 

 the metabolism of nitrogen, therefore, a number of normal 

 end products must be considered and these will be discussed 

 in the next few pages. It will be well to begin with the con- 

 sideration of the urine as a whole, as all these substances are 

 eliminated through that channel. 



THE GENERAL COMPOSITION OF URINE. 



The work of the kidneys in the discharge of the urine, or 

 more properly the separation of its constituents from the blood, 

 is usually spoken of as one of excretion. But something more 

 than simple elimination of worthless products is here con- 

 cerned; the work done by these organs is in part secretory, 

 as certain synthetic reactions are beyond question carried out 

 here. Years ago Bunge and Schmiedeberg demonstrated the 

 synthesis of hippuric acid from benzoic acid and glycocoll in 

 the kidney, and since then other changes have been brought to 

 light. Further than this, the peculiar mechanism of the kid- 

 ney accomplishes another very remarkable thing. The blood 

 circulating through the kidney contains valuable material to 

 be saved as well as worthless substances to be rejected. 

 Toward all these constituents the epithelial cells of the kidney 

 tubules exercise a sort of selective treatment. The proteins, 

 which are colloids, are retained by the blood, but the sugar, 

 which is a crystalloid, and very soluble, is retained also unless 

 its concentration passes a certain limit. The soluble salts are 

 in part passed through the kidneys and in part retained by the 

 blood, with the final result of maintaining a very nearly con- 

 stant osmotic pressure in that fluid. How this is done we 

 cannot say. It is indeed a problem of physiology and his- 

 tology rather than of chemistry. We know only this, that the 

 selective absorption and control of the blood concentration are 



