164 THE BLOOD 



Reaction. The reaction of the blood as a whole, as well as that of 

 the plasma, differs with the indicator. The earlier determinations, 

 which have usually been made with glazed litmus paper, have given an 

 alkaline reaction, while the more recent determinations, for which 

 phenolphthalein has been employed, have shown that its reaction is 

 acid. Its tendency in either direction, however, is so slight that it is 

 really of very little importance. 



Physicochemical researches have shown that the acidity or alkalinity of a fluid 

 is dependent upon its content in hydrogen ions (+) and hydroxyl ions ( ). This 

 implies that acids are dissociated with a liberation of H ions, while bodies which 

 give rise to OH ions, behave like alkalies. In illustration of this statement might be 

 mentioned the dissociation of HC1 into H (+) and Cl ( ), and the dissociation of 

 NaOH into Na (+) and OH ( ). Besides, the acidity or alkalinity of the aqueous 

 solutions of these substances is directly proportional to the number of H ions or 

 OH ions contained therein. 



The number of OH ions contained in the blood and lymph has been determined 

 by an electrical process 1 and has been found to be very small, and hence, as they do 

 not exceed the H ions, the reaction cannot be decidedly alkaline, nor can it be acid 

 for the same reason. The number of the H ions in the blood remains very constant, 

 presumably on account of the presence in the plasma of the salts of carbonic acid, 

 phosphoric acid and protein which are all very weak in their action. When acid 

 is added to the blood, it reacts with the carbonates and phosphates. Carbonic 

 acid and acid phosphates are produced which leave the body, in the first case, 

 through the lungs and, in the second, through the kidneys. The tendency, 

 therefore, is to establish a faint degree of alkalinity as quickly as possible. 



Litmus paper gives a distinct alkaline reaction, because litmus acid which is the 

 indicator in this particular case, unites with the Na of the NaHCO 3 present in the 

 blood, and leaves the carbon dioxid uncombined. The subsequent dissociation of 

 the Na and litmus acid enables the litmus acid to generate the blue color. Phenol- 

 phthalein is not sufficiently strong to cause such a displacement of the carbon 

 dioxid. For this reason, the titration methods cannot give accurate results 

 concerning the reaction of the blood, but solely regarding the amount of alkalies 

 available for titration. 



In addition to such neutral salts as sodium and potassium chlorid and the 

 alkali salts of the proteins of the corpuscles and plasma, the blood also contains a 

 considerable quantity of sodium carbonate. This substance, however, cannot 

 give riseto a decided alkaline reaction, because it is continuously charged with the 

 carbon dioxid of the tissues. In this way, the sodium carbonate is retained in the 

 form of the bicarbonate, during the dissociation of which OH ions are not formed, 

 as is evident from the formula: NaHCOs = Na and HCOs. 



Like other living tissues, blood is practically neutral in reaction and performs its 

 function best when only very faintly alkaline, but naturally, as it serves as the 

 reservoir for the products of metabolism, large amounts of carbon dioxid and other 

 acids are constantly added to it. Under normal conditions, however, any tendency 

 on its part toward an acid reaction is quickly counteracted in the manner indi- 

 cated. This property of the blood to neutralize acids without acquiring an acid 

 reaction to litmus, is designated as its total alkalinity. 



Osmotic Pressure. As the osmotic pressure is the chief regulatory 

 mechanism by means of which the water content of the tissues is 

 safeguarded, it constitutes one of the most important properties of the 

 blood. Obviously, the normal concentration and composition of the 



1 Hober, Pfluger's Archiv, Ixxxi, 1900, 522; P. Franckel, ibid., xciv, 1903, 601; 

 Henderson, Am. Jour, of Physiol., xxi, 1908, 427, and Michaelis and Davidoff, Bioch. 

 Zeitschrift, xlvi, 1912, 131. 



