146 FROM FISH TO PHILOSOPHER 



freely available to the mammals during their evolution- 

 ary history; and to excrete a urine no more concentrated 

 than is the blood is uneconomical, in that it entails ex- 

 cessive water loss. Here is where the mammahan ca- 

 pacity for concentrating the urine to an osmotic level 

 above that of the plasma comes into operation. 



The quantitative importance of this concentrating 

 process can be illustrated by a simple calculation cm an 

 individual who is dehydrated and in whom, therefore, 

 all antidiuretic mechanisms are operating maximally. 

 Out of each 100 cc. of filtrate let us say that 97.6 cc. of 

 water and its contained salt can be reabsorbed without 

 raising the osmotic concentration of the urine to a level 

 greater than that of the blood. Such substances as are 

 not reabsorbed by the tubules will not have been con- 

 centrated 100/2.4, or forty-two times, but their excre- 

 tion involves water loss at a rate of 2.4 cc. per minute, or 

 3456 cc. per day. In the final process of water reabsorp- 

 tion, however, let us suppose that an additional 1.8 cc. 

 of water is recovered by the tubules, raising the urine 

 osmotic concentration to a value 4.0 times (2.4/0.6) 

 that of the plasma (the maximal osmotic urine/plasma 

 concentration ratio in man ranges from 3.8 to 4.2) . After 

 this operation the minimal urine flow becomes 0.6 cc. 

 per minute or 884 cc. per day, a saving of 2572 cc. per 

 day. Unreabsorbed waste products would now be con- 

 centrated 100/0.6 or 167-fold, and, at a urine flow of 

 884 cc. per day, the water saved by concentrating the 

 urine would— so long as the kidneys alone are considered 

 —prolong life in a man wholly deprived of water by 

 three days. Yet even when water is freely available, the 

 urine flow in man averages only about 1500 cc. per day, 

 so that his kidneys are generally operating on the 

 moderately 'concentrated' side. 



