The Evolution of the Kidney 65 



and over three pounds of sodium chloride per day, not to 

 mention quantities of phosphate, amino acids and other sub- 

 stances must be saved from being lost in the urine by being 

 reabsorbed from the tubular stream. There is enough waste 

 motion here to bankrupt any economic system — other than 

 a natural one, for Nature is the only artificer who does not 

 need to count the cost by which she achieves her ends. 



The chief waste product which the kidney is called upon 

 to excrete is urea. The glomeruli remove each minute such 

 urea as is contained in 125 cc. of blood, but because of the 

 way the tubules are put together fifty per cent of this urea 

 diffuses back into the blood again, so that in terms of the total 

 renal blood flow (1300 cc. per minute) the over-all efficiency 

 of the excretion is only about 5 per cent. There are certain 

 foreign substances, however (diodrast, hippuran, phenol red, 

 etc. ) , which have been synthetized only within the past few 

 years, which the kidney excretes with almost 100 per cent 

 efficiency. Is it not strange that, in spite of the fact that it 

 has never before encountered them, the kidney should be 

 able to excrete such artificial synthetic compounds twenty 

 times as efficiently as it excretes the principle nitrogenous 

 waste product naturally formed in the body, and which it 

 has been excreting for millions and millions of years? 



The kidney is receiving more attention today than ever 

 before. These scientific problems range from local organic 

 pathology to such subtle matters as the relation of the in- 

 ternal environment and its multiplicity of chemical factors 

 to personality and mental disease. Certainly, mental integrity 

 is a sine qua non of the free and independent life. As inter- 

 mittent rays of hght blend into moving images on the cine- 

 matographic screen, so the multiform activities within the 



