80 



SCIENCE 



[N. S. Vol. XLVI. No. 1178 



slightly increased. (Under normal cir- 

 cumstances this change is probably in- 

 finitesimal.) The kidney now removes 

 relatively a siill larger amount of acid than 

 of alkaline phosphate, perhaps on account 

 of changes in the blood bicarbonate rather 

 than in the phosphate, and thus restores 

 the ratio of base to acid in the blood. Here 

 Ihe essential factor is the ability of the kid- 

 ney widely to vary the ratio of acid to al- 

 kaline phosphate without large variation 

 of the hydrogen ion concentration of the 

 urine. This very important fact once more 

 depends upon the favorable value of k for 

 acid phosphate. 



It is because, in the normal individual, 

 both the production of ammonia and the 

 ratio of acid to alkaline phosphate in the 

 urine are variable within wide limits, and 

 can be made to conform exactly to the 

 varying ingestion and production of acid 

 in he body, that the fundamental physico- 

 chemicsl apparatus can be kept intact and 

 accurately adjusted. 



A further factor in the process is the 

 activity of the lung in excreting carbonic 

 acid. This substance is the chief excretory 

 product of the organism. As such it must 

 be eliminated promptly and completely. 

 Moreover, in that it leaves the body not 

 in aqueous solution and as an acid, but 

 almost exclusively in the form of gaseous 

 cai'bon dioxide, there is no possibility of 

 any variation of the permanent effect pro- 

 duced upon the reaction of the body by 

 the elimination of a definite amount of it. 

 In the final regulation by excretion it is 

 not, therefore, concerned. And yet it has, 

 in the process of excretion, a very impor- 

 tant role in regulating the reaction of the 

 body. This depends upon the fact that 

 carbonic acid is not only a waste product, 

 but also a normal constituent of the blood, 

 and, as such, a principal factor in the 

 physico-chemical regulation. Thus, if the 



ratio of caiflbonic acid to bicarbonates in a 

 normal individual were 1 : 15, a large pro- 

 duction of acid might cause a destruction 

 of a third part of all the bicai^onates, pro- 

 ducing in itsj)lace an equivalent amount 

 of free caribonic acid. This, if nothing else 

 occurred, would reduce the relative amount 

 of bicarbonates from 15 to 10, and simulta- 

 neously increase the free carbonic acid 

 from 1 to 6. The ratio would now be 6 : 10, 

 and since the hydrogen ion concentration is 

 porportional to this ratio, this ion would 

 suffer a nearly ten-fold increase of con- 

 centration. But at this point, or, more 

 strictly speaking, continuously during the 

 process, the excretory function intervenes. 

 There is a tendency for the respiratory 

 process to hold the tension of carbonic di- 

 oxide in the blood nearly constant. This 

 is the reason why carbonic acid has some- 

 times been thought the respiratory hor- 

 mone. Assuming that the exact quantity 

 of carbonic acid set free by the reaction of 

 neutralization were thus eliminated, the 

 ratio would be reduced to 1 : 10, and the 

 hydrogen ion concentration would rise but 

 one third above its original value. More 

 recent investigations, however, have shown 

 that a tendency to acidity is accompanied 

 'by a lowering of the tension of carbon di- 

 oxide. Let us suppose that in this case 

 the tension was lowered one third. The 

 free carbonic acid of the blood would then 

 become 0. 67 instead of 1.00, and the ratio 

 of acid to salt 0.67 : 10, which is exactly 

 equal to 1 : 15, the original ratio. Accord- 

 ingly, the hydrogen ion concentration 

 would be restored exactly to its original 

 value, and the regulation by excretion 

 would be quite perfect. Now there is 

 abundant evidence to show that something 

 very muc,h like this is always occurring in 

 the body, and, on the whole, I believe that 

 the most delicate of all means to regulate 

 the reaction of the body is to be found in 



