564 VERTEBRATE LIFE AND ORGANIZATION 



bule; additional water molecules are reabsorbed from Henle's loop and 

 the distal tubule. Inorganic ions are reabsorbed from both the proximal 

 and distal tubule. Reabsorption involves both the passive diffusion of 

 materials back into the capillaries surrounding the tubule, and the 

 active uptake of materials by the tubular cells and their secretion into 

 the blood against a concentration gradient. About 85 per cent of the 

 water, in the glomerular filtrate, for example, diffuses back into the 

 blood simply because the blood contains more osmotically active solutes 

 than the filtrate, but the balance of the water is actively reabsorbed 

 and involves work on the part of the tubular cells. Passive reabsorption of 

 water occurs in the proximal tubule, and the active reabsorption takes 

 place in the loop of Henle and distal tubule. 



In addition to a selective reabsorption of materials, certain of the 

 tubular cells can secrete wastes into the tubule— a process known as 

 augmentation. In certain teleost fishes, which have lost the renal cor- 

 puscles, this is an important way of eliminating waste products, but 

 relatively little is added to the filtrate by augmentation in mammals. 

 Creatinine, ammonia, hydrogen ions and various drugs (penicillin) are 

 among the few substances eliminated in this way. 



The fluid that reaches the end of the tubules is known as urine. 

 In man, the volume of urine is only about 1 per cent of the volume of 

 the glomerular filtrate, and its composition is quite different from that 

 of the filtrate, for a great many substances have been reabsorbed and 

 others have been added. As a result of these processes the waste products 

 are concentrated in the urine. The most important nitrogenous wastes 

 in the urine are urea, ammonia, uric acid and creatinine. The yellowish 

 color of the urine is due to the presence of urochrome, a pigment 

 derived from the breakdown of hemoglobin and hence related to the 

 bile pigments. 



Kidney Regulation of Body Fluids. Although some of the urea 

 present in the filtrate diffuses back into the blood, most of the wastes 

 present in the glomerular filtrate are excreted by the kidneys, for these 

 substances are not actively reabsorbed. Those materials that can be 

 actively reabsorbed are taken back in varying amounts depending 

 upon their concentration in the blood. If the concentration of one of 

 these materials in the blood and glomerular filtrate rises above a certain 

 level, known as the renal threshold, not all of it will be reabsorbed 

 into the blood from the tubule, and the amount present in excess of 

 the renal threshold is excreted. The quantitative value of the renal 

 threshold differs for different substances. In diabetes mellitus, for ex- 

 ample, in which impaired cellular utilization of glucose leads to a 

 high concentration of glucose in the blood, the renal threshold for 

 glucose (about 150 mg. of glucose per 100 ml. blood) is exceeded and 

 the sugar appears in tlie urine in large amounts. The osmotic pressure 

 of the body fluids is controlled by the amount of salts, and the pH by 

 the amount of hydrogen ions, that are taken back into the blood 

 from the glomerular filtrate. 



The volume of the body fluids is also regulated by the kidneys. If 

 an excess of water is present in the body fluids, the blood volume and 



