THE CHEMISTRY OF RESPIRATION 



1067 



dioxide previously in combination with the sodium. In the corpuscles both 

 haemoglobin and proteins play the part of a weak acid. When plasma 

 is exposed to a vacuum it is necessary, as we have seen, to add a little acid 

 in order to obtain the last traces of carbon dioxide from the fluid. Instead 

 of adding a weak acid, haemoglobin or red blood- corpuscles may be employed. 

 In the latter case it seems probable that the action on the carbonates of the 

 plasma is due not so much to the haemoglobin as to an interchange of acid 

 radicals between the corpuscles and the plasma. If carbon dioxide be 

 passed into defibrinated blood the alkalinity of the plasma increases 

 while the chlorides diminish, and it is probably the reverse interchange 

 of radicals between corpuscles and plasma which is responsible for the 

 evolution of carbon dioxide on the addition of corpuscles to plasma in 

 vacuo. 



THE ALKALINITY OF BLOOD. Blood-plasma is generally described as 

 slightly alkaline, and its alkalinity is measured in terms of deci- or centi- 

 normal acid. The term alkalinity is relative. Caustic alkali owes its 

 alkalinity to the presence of OH ions. The neutrality of distilled water is 

 due to the presence of practically equal amounts of H and OH ions in the 

 fluid. We may measure the concentration of H or OH ions in a fluid 

 electrically.* If this electrical method be applied to blood or blood- plasma 

 it reveals either of these fluids as practically neutral, i.e. there is little or no 

 greater concentration of H or OH ions in blood than in distilled water. The 



* By the use of different indicators we may arrive at some conclusion as to the 

 approximate concentration of hydrogen ions in any given liquid. In the following 

 Table are set out, from a paper by Roaf, the colours of a number of different indicators, 

 and the degree of acidity which is sufficient to change their colours : 



It should be remembered that in distilled water of the highest state of purity the 

 concentration of H and OH ions respectively is about 1 x 10-7. 



