CIRCULATION, RESPIRATION AND EXCRETION 141 



The chief stimulus to the secretion of water by the kidneys 

 is the water content of the blood, the kidneys acting as regu- 

 lators of this important factor and eliminating more or less water 

 as the blood contains a larger or smaller percentage of it. 



As regards the excretion of dissolved matter, very interesting 

 relations exist. With one important exception (hippuric acid) 

 the kidneys do not manufacture the excretory products. Their 

 essential function is to maintain the composition of the blood 

 constant. For each substance capable of being excreted at all 

 in the urine there exists a certain minimum concentration in 

 the blood above which it begins to pass through the kidneys into 

 the urine. For the normal excretory products, as well as for 

 foreign substances, this minimum approaches zero, that is, only 

 very minute amounts of these substances can be retained in 

 the blood. For dextrose the limit is approximately 0-2-0-3 

 per cent, for sodium chlorid 0-6 per cent, etc. So long as 

 the percentage of one of these substances in the blood does not 

 exceed its own particular limit, none of it is excreted through the 

 kidneys. On the other hand, & slight rise above this limit 

 causes an excretion of the substance concerned. This function 

 of the kidneys has been likened to the working of an overflow 

 valve on a tank. It should be added that each particular sub- 

 stance has its own minimum, independent to a large degree of 

 all the others. 



The functions of the kidneys, however, in this respect are 

 not so simple as those of an overflow valve for the reason that 

 the concentration of the excreted substances is greater in the 

 urine than in the blood. In other words, the kidney does its 

 work by transferring substances from a fluid of lower to a fluid 

 of higher osmotic pressure and the expenditure of energy in 

 this work is not inconsiderable. This is notably true o urea 

 and the other nitrogenous waste products, of which only traces 

 can be detected in the blood. 



In addition to the nitrogenous substances excreted in the 

 urine there are present in the feces, as already noted (154), 

 excretory products which represent a certain fraction of the 

 organic body waste. Finally, small amounts of urea and 

 other nitrogenous substances are excreted in the perspiration. 



199. Excretion of ash ingredients. Being non-volatile 

 the ash ingredients are excreted chiefly through the feces or 



