1066 PHYSIOLOGY 



one hundred cubic centimetres of arterial blood from the dog would yield 

 normally about 40 c.c. of carbon dioxide. These 40 c.c. would be divided 

 as follows : 



In simple solution in the plasma and corpuscles . 1 -9 c.c. 



((a) in corpuscles . . . .6-81 _ 

 As sodium bicarbonate .... 12 . = 



In organic combination with haemoglobin in corpuscles . 7-5 1 _ 

 In organic combination with proteins of the plasma . . 11 '8 / ~ 



It will be noted that although we are dealing here with arterial blood, 

 in which the tension of carbon dioxide is comparatively low, 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 bicar- 

 bonate 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 HPO 4 + C0 2 + H 2 0. 



In this way the whole of the sodium enters into combination with the P0 4 

 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 



