44 FROM FISH TO PHILOSOPHER 



body of certain types of substances which cannot be 

 utilized and the accumulation of which in the blood 

 would be injurious. It is remarkable, however, that many 

 synthetic substances which the organism has never en- 

 countered in its evolutionary history are excreted just 

 as eflBciently as are substances formed naturally in the 

 body, probably because of the presence of certain com- 

 mon, chemically reactive groups in the molecule. 



In the distal segment the greater part of the remaining 

 sodium is reabsorbed, leaving the corresponding quan- 

 tity of water free for excretion as what we may call 

 osmotically free water since it contains no salt, thus ful- 

 filling the function of the fresh-water kidney by excreting 

 water without loss of salt. Also, in the distal segment, 

 the urine is acidified and ammonia is added in exchange 

 for sodium and potassium, operations that permit the 

 excretion of acids without loss of these important salts, 

 and that conserve for the body its *alkaH reserve' (so- 

 dium bicarbonate) and maintain constant the hydrogen 

 ion concentration of the blood. So important is this 

 steady state that a very minute increase in the hydrogen 

 ion concentration of the blood induces the cHnical state 

 of coma by interfering with the function of the brain. It 

 is probably also by the distal segment that the excretion 

 of potassium, next in importance after sodium and the 

 hydrogen ion in the over-all composition of the body 

 fluids, is controlled. 



As might be anticipated, natural metabolic end-products 

 (urea, creatinine, creatine, uric acid, and others), as 

 well as foreign substances, are handled by the renal tu- 

 bules in di£Ferent animals in a variety of ways, and the 

 only generalization possible is that, in the formation of 

 urine in every glomerular animal, both tubular reabsorp- 

 tion and excretion play a part. 



This generalization poses one of the major problems 

 of renal physiology: what substances are excreted by 

 filtration only, and what substances by filtration supple- 



