THE CHEMISTRY OF RESPIRATION 1189 



the carbon dioxide is stated to be in combination as bicarbonate. 

 This is on account of the fact that in no case is the alkalinity of the 

 blood equal to that of a solution of sodium carbonate. On the other 

 hand, if we expose blood to a vacuum the whole of the carbon dioxide 

 is given off. If sodium bicarbonate be exposed to a vacuum, only 

 half of the carbon dioxide is evolved, sodium carbonate itself not 

 undergoing any decomposition in vacuo. If we attempt to extract the 

 carbon dioxide from blood-serum or blood-plasma, we may obtain 

 nearly all the carbon dioxide present, the last 5 per cent., however, 

 requiring the addition of a weak acid such as oxalic or phosphoric acid 

 in order that it may be given off. How are we to explain the difference 

 between the behaviour of blood and the behaviour of a solution of 

 sodium bicarbonate ? 



We may artificially make a fluid which behaves to carbon dioxide 

 in the same way as blood, by mixing together sodium carbonate 

 and sodium hydrogen phosphate Na 2 HP0 4 . From such a mixture 

 the whole of the carbon dioxide may be given off when exposed to a 

 vacuum. On the other hand, a large amount of carbon dioxide will 

 be taken up with a very small difference in tension of the gases. The 

 behaviour of the mixture is due to an interaction which occurs between 

 the acid radicals P0 4 and C0 3 . When the mixture is exposed to 

 a vacuum any sodium bicarbonate present will undergo dissociation, 

 carbon dioxide being given off and the carbonate Na 2 C0 3 formed. 



This then reacts with the sodium phosphate in the following way : 



2NaH 2 P0 4 + Na 2 C0 3 = 2Na 2 HP0 4 + C0 2 + H 2 0. 



In this way the whole of the sodium enters into combination with the 

 P0 4 radical and the carbon dioxide previously combined is given off. 

 On exposing the mixture to an atmosphere containing carbon dioxide, 

 the reverse change takes place, and we get once again sodium hydrogen 

 phosphate and sodium carbonate, and finally sodium bicarbonate. 

 It was formerly thought that in the blood-plasma phosphates were an 

 important factor in the evolution of the carbon dioxide. Blood- 

 plasma, however, contains the merest trace of phosphates, and the 

 role of a weak acid competing with the carbon dioxide for the sodium 

 is played chiefly by the proteins of the plasma. We can in fact, by 

 adding proteins to a solution of sodium carbonate and exposing the 

 mixture to a vacuum, obtain the evolution of practically all the carbon 

 dioxide previously in combination with the sodium. In the cor- 

 puscles 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 



