METABOLISM 189 



traction of muscles, which, however, is suspended by the pres- 

 ence in the solution of certain (not all) dyad ions like Ca or Mg. 

 Numerous other examples of such antagonistic actions of ions 

 are known, such as those observed by Loeb, for example, in 

 the development of the egg. In general it may be said that cell 

 activities are dependent among other things upon a suitable 

 ionic concentration of various elements in their surroundings, 

 and it is a striking and interesting fact that the so-called physi- 

 ological salt solutions in which living organs may be kept func- 

 tionally active for a longer or shorter time contain the various 

 salts in approximately the same proportions as are found in 

 sea water. 



Another example of the influence of ionic concentration is 

 afforded in the case of the digestive enzyms. Ptyalin, for 

 example, is sensitive to a very slight excess of hydrogen ions. 

 Pepsin, on the other hand, is most active in the presence of 

 hydrogen ions, while trypsin acts best in the presence of an 

 excess of OH ions. 



271. Maintenance of neutrality. Closely connected with 

 the foregoing topic and constituting indeed a special case of it, 

 is that of the maintenance of neutrality in the body fluids. A 

 fluid is neutral in the chemical sense when it contains 

 no excess of H nor of OH ions, an excess of the former being 

 equivalent to acidity and an excess of the latter to alkalinity. 

 It has been shown that the blood serum, as a representative of 

 the body fluids, is very nearly neutral, its content of H and 

 OH ions being approximately 0.4 X io~ 7 and 7.2 X io~ 7 , 

 i.e., it has an alkalinity equivalent to about 0.000012 gram 

 NaOH per liter. 1 



The body katabolism is continually producing acids, espe- 

 cially carbonic, phosphoric and sulphuric acids (256, 259), which 

 tend to increase the acidity of the blood. These acids are in 

 part neutralized by the ammonia produced in the katabolism of 

 protein (233), but it has been shown by the investigations of 

 L. J. Henderson that the salts of the blood serum, especially 

 the sodium phosphates and bicarbonates, play an important 

 part in maintaining its neutrality. They are present in such 



1 Blood is commonly said to be alkaline because it gives an alkaline reaction to 

 ordinary indicators, such as litmus. Such a reaction, however, gives no definite 

 measure of the true alkalinity or acidity. 



