40 FROM FISH TO PHILOSOPHER 



absorption had to evolve in a parallel manner with in- 

 creased filtration, until the kidney came to operate as an 

 elaborate 'filtration-reabsorption' system working at top 

 speed all the time. 



It was through the evolution of this filtration-reab- 

 sorption system that the kidney came to be almost en- 

 tirely responsible for the composition of the internal en- 

 vironment of the body, manufacturing it, as we have 

 said, in reverse, by saving some things from the glomer- 

 ular filtrate and rejecting others. Among the substances 

 known to be reabsorbed by the tubxiles in man, for ex- 

 ample, are water, sodium, potassixim, calcium, chloride, 

 bicarbonate, phosphate, sulfate, glucose, fructose, a 

 large variety of amino acids, and several vitamins and 

 hormones. There are certainly many others on which no 

 quantitative observations are available. 



Chief in this list, however, was water: by filtering 

 large quantities of water and reabsorbing most of the 

 filtered water (99 per cent in man), large quantities of 

 waste products could be passively filtered through the 

 glomeruli along with the water and then excreted in a 

 urine that had been concentrated by the reabsorption 

 of water. Although a fine balance had to be maintained 

 between the reabsorption of salt and water in order to 

 maintain the proper proportions of these substances in 

 the body fluids, so long as the animal remained in fresh 

 water or had an excess of water available to it, the ex- 

 cretion of waste products posed no problem other than 

 the negative one of not reabsorbing them from the 

 filtrate. 



The filtration-reabsorption system served to excrete 

 waste products not so much because it was primarily 

 evolved for this purpose as because it automatically ac- 

 complished this result by letting some substances pass 

 into the urine while saving others. It presents no para- 

 dox, therefore, that the kidney was long coming into its 

 role as the sole excretory organ of the body: for ex- 

 ample, in the fishes the chief nitrogenous end-product 



