190 The Vertebrate Organ Systems 



After the efferent blood vessel leaves the glomerulus, a second 

 capillary bed is formed in the region of the tubule. Here within this 

 convoluted and looped tubule, much of the filtrate collected within 

 the Bowman's capsule is reabsorbed. This reabsorption is a selective 

 process. About 98 per cent of the water is reabsorbed and glucose is 

 completely reabsorbed, while other substances are only partially re- 

 turned to the blood. In this way, urine is formed due to the more or 

 less double process of reabsorption and concentration. 



This process of urine formation has been experimentally verified. 

 By means of micropipettes, portions of fluid have been withdrawn for 

 analysis from both the Bowman's capsule and the tubules. It was defi- 

 nitely shown that the composition of the filtrate was the same as that 

 of the blood plasma minus the large protein molecules. Thus the kid- 

 ney tubules filter all of the blood except the formed elements and 

 the large protein constituents. It has been estimated that the amount 

 of filtrate passing through the Bowman's capsules may approach 180 

 liters a day. This volume is reduced to one or one and one-half 

 liters by the reabsorption taking place in the tubules. 



Urine. — Urine when voided is a yellow, clear liquid with a definite 

 odor. It is usually acid in its reaction and has a specific gravity slightly 

 greater than that of water. 



Whether urine is acid or basic in its reaction depends upon the 

 diet of the animal. The end products of the metabolism of vegetable 

 materials are basic while those of proteins are acid; thus herbivorous 

 animals usually have urine with a basic pH. 



While all terrestrial animals have the problem of conserving water, 

 those dwelling on deserts have the greatest necessity for utilizing even 

 the water produced during metabolic reactions. Some desert rodents, 

 for example, excrete nearly dry nitrogeneous wastes. 



In addition to water and salts, urine normally has certain charac- 

 teristic waste materials of protein metabolism. These and their origins 

 are as follows : 



Creatinine, a complex protein, apparently originates from the creatine of striated 

 muscle. Creatine is normally found in striated muscle and is involved in its 

 activity. 



Urea, which is produced by the liver cells from ammonium salts, forms about 

 one-half of the solids excreted in the urine. The ammonium salts are produced 

 during protein metabolism. 



