Excreiion - 375 



Often the reabsorption of useful substances 

 is not complete, however, since always it is 

 a selective process. During hyperglycemia, 

 for example, the excess of glucose is left in 

 the filtrate and appears in the urine. In other 

 words, the tubule tends to stop reabsorbing 

 a substance when a proper amount has been 

 put back into circulation. Thus many sub- 

 stances do not appear in the urine unless 

 their concentration in the blood exceeds a 

 certain threshold value. Such so-called 

 threshold substances include many of the 

 inorganic salts, which thus are retained in 

 proper amounts in the blood stream. In fact, 

 the selective nature of the reabsorptive proc- 

 esses in large measure accounts for the ability 

 of the renal organs to maintain a beautifully 

 regulated homeostatic control of the body 

 fluids. 



In salvaging glucose, amino acids, and other 

 substances from the filtrate, the tubules must 

 expend energy. Initially the concentration of 

 glucose, for example, is equal in the filtrate 

 and in the blood. Consequently there is no 

 tendency for glucose to be reabsorbed spon- 

 taneously, except to a small extent, by virtue 

 of the fact that glucose in the filtrate may 

 become more concentrated as the reabsorp- 

 tion of water proceeds (see below). The kid- 

 ney cells derive the energy for this work from 

 their oxidative metabolism; and if a kidney 

 is deprived of oxygen (for example, by treat- 

 ment with cyanide), reabsorption quickly 

 ceases. Such a kidney still forms urine, but 

 the urine is extremely copious and dilute, 

 possessing practically the same composition 

 as the filtrate. 



Reabsorption of Water. The salvaging of 

 water from the nephric filtrate is a major 

 kidney function. In fact, the human kidney 

 produces about 100 liters of filtrate while it 

 is forming just one liter of urine. In other 

 words, the kidney usually reabsorbs about 99 

 liters of water in the process of forming one 

 liter of urine; and if the kidneys lose their 

 capacity to reabsorb water, they put forth 

 very large volumes of highly dilute urine 

 (p. 412). 



In order to calculate the filtrate/urine vol- 

 ume ratio and thus to determine the true ex- 

 tent of water reabsorption by the kidney, it 

 was necessary to find a substance capable of 

 meeting four precise specifications: (1) The 

 substance must be nontoxic when measurable 

 amounts are injected into the blood stream. 

 (2) It must be completely impervious to any 

 reabsorption by the tubules. (3) It must be 

 altogether nonsusceptible to secretional ac- 

 tivity (see below) in the tubules. (4) It must 

 be freely filtered into Bowman's capsule. 

 Such a substance was finally found in the 

 form of inulin, a relatively short-chain poly- 

 saccharide compound; and the studies of 

 Homer W. Smith and co-workers at New 

 York University, dealing with the excretion 

 of inulin, contributed greatly to the progress 

 of renal physiology during the second quar- 

 ter of the present century. The heavy reab- 

 sorption of water accounts mainly for the 

 high concentrations of urea and other wastes 

 that are found in the urine. In fact, any 

 solute originally present in the filtrate be- 

 comes highly concentrated in the urine, un- 

 less this solute is reabsorbed while water is 

 being reabsorbed. In the human urine, urea 

 usually reaches a concentration about 60 

 times greater than in the blood plasma, and 

 were it not for the fact that small quantities 

 of urea are reabsorbed along with the larger 

 quantities of water, this difference of con- 

 centration would be even greater. 



The quantity of water reabsorbed by the 

 human kidney varies quite widely, depend- 

 ing on the needs of the body. When large 

 quantities of water are drunk, and while the 

 blood tends to be diluted, the reabsorption 

 of water remains at a minimum and a large 

 volume of dilute urine is formed by the kid- 

 ney. But if the water intake is restricted, re- 

 absorption reaches a maximum, so that the 

 kidney conserves a maximum of water for 

 the body. 



The water-reabsorbing capacity of the kid- 

 ney is a good measure of an animal's capacity 

 to conserve water. In aquatic and semiaquatic 

 vertebrates, especially fresh-water fish and 



