178 BIOLOGICAL CHEMISTRY 



The composition of the blood must either be the result of an 

 equilibrium or work must be done in maintaining a uniform 

 composition. It must be clearly understood that we have 

 here to deal with the difference in composition between blood 

 and urine. For instance, in the human being the blood 

 plasma contains 0*03 per cent, of urea and the urine approxi- 

 mately 2-0 per cent. In certain fish, however, the osmotic 

 pressure of the blood is maintained by urea and the blood 

 contains 2-61 per cent, urea, whilst the urine contains less.* 

 These differences must depend upon the activity of the kidneys 

 in removing urea. 



Two processes are known to occur in the kidney, one a fil- 

 tration and the other a process requiring the transformation of 

 energy. The former occurs through the flat epithelial cells 

 of the Malpighian corpuscle. The energy for this process is 

 derived from the blood pressure and is thus due to the activity 

 of the heart muscle. The urine thus formed is almost the same 

 composition as the blood plasma, but without the proteins. 

 It is of equal osmotic concentration (isosmotic) with the blood 

 and its production is not accompanied by increased oxida- 

 tion in the kidneys, f If the cells allow some substances to 

 filter through more easily than others there will be a partial 

 separation of the constituents. 



The second or selective function is believed to be a function 

 of the cubical cells lining the kidney tubules. The com- 

 position of this secretion differs markedly from the com- 

 position of the blood plasma and the energy for its separation 

 is obtained by oxidation in the kidney cells, f 



I shall not attempt any explanation of the phenomenon as 

 there is no satisfactory one yet. We can look upon it like 

 secretion in being a pumping action of the cell which requires a 

 transformation of energy and of which we can calculate the 

 efficiency or relative amount of useful work compared to the 

 total energy used. 



Cushny suggests that the glomerulus filters the crystalloids 

 from blood and that the tubule absorbs the ideal composition 

 of the crystalloids of serum, but this amounts to the same 

 thing so far as the energy requirements are concerned. Brodie 

 and Cullis stated that both glomerulus and tubule have a 

 selective secretory power. { 



Nussbaum jtied the renal arteries in frogs, and by micro- 



* W. v. Schroeder, Zeit. physiol. Chem., 1890, vol. 14, p. 576. 

 f J. Barcroft and H. Stranb, ]ouvn. Physiol., 1911, vol. 41, p. 145. 

 J T. G. Brodie and W. C. Cullis, Journ. Physiol., 1906, vol. 34, 

 p. 224. 



