THE EXCRETION OF URINE 



549 



glomerulus, so that the results may be due to filtration through the 

 capsule. Microscopical examination of the glomeruli has revealed the 

 important fact that all are not equally active at the same time. As judged 

 from the blood flow in them, the glomeruli would appear to work in 

 relays. 



The Reabsorption Process, It is generally accepted that the proof that 

 the capsule acts as a filter is fairly complete. Unfortunately such decisive 

 experimental facts can not be offered to prove the assumption that the epi- 

 thelium of the tubules reabsorbs the excess of water and salts which are fil- 

 tered off through the capsule. If the modern theory of urine excretion is cor- 

 rect, the cells of the tubules must not only absorb large amounts of water, 



Renal 

 arfery 



Malpighian 

 corpuscle 



Renal-portal vein 



Fig. 1/2. Diagram of blood supply of Malpighian corpuscle and of convoluted tubules in amphibian 



kidney. (Redrawn from Cushny.) 



but they must also allow for the reentrance into the blood, either completely 

 or partially, of certain salts, while they must reject others entirely. 



We have called attention above to the fact that the glomerular filtrate is 

 very different from the urine that is finally passed. The urine contains a 

 very high percentage of small molecules, and the proportion in which they 

 are present is entirely different from that in the blood plasma or in the 

 glomerular filtrate. 



This is shown in the following table, in which the figures in the first two columns 

 represent the average number of grams of urea, uric acid, chlorine, and glucose in 

 100 c.c. of protein-free blood plasma and in 100 c.c. of urine. In the third column is 

 given the change in concentration which must occur in the kidney. 



