200 HUMAN PHYSIOLOGY 



AMOUNT OF GASES PER HUNDRED VOLUMES OF BLOOD. 



Arterial Blood. Venous Blood. 



Oxygen 20 12 



Carbon dioxide . 40 46 



There are two ways in which gases may be held in such a 

 fluid as the blood 



1st. In simple solution. 



2nd. In chemical combination. 



Oxygen. At the temperature of the body the blood can 

 hold in solution less than 1 per cent, of oxygen. Now the 

 amount of oxygen actually present is about 20 per cent. So 

 that by far the greater quantity of the gas is not in solution. 

 We have already seen that it is in loose chemical union with 

 hsemoglobin. 



Carbon dioxide. In the animal body the blood can dis- 

 solve about 2J per cent, of carbon dioxide. But it may con- 

 tain as much as 46 per cent. Hence the greater part of the 

 gas must be in chemical combination. Since the proportion 

 of carbon dioxide is greater in the plasma than in entire 

 blood, this gas must be held in the plasma. Analysis of the 

 ash of the plasma shows that the sodium is more than 

 sufficient to combine with the chlorine and phosphoric acid, 

 and is thus available to take up carbon dioxide. Carbonate 

 of soda and basic phosphate of soda are therefore present 

 together in the plasma, and if a stream of carbon dioxide be 

 passed through a solution of basic phosphate of soda, it 

 appropriates a certain amount of soda and leaves the neutral 

 phosphate. On the other hand, if the amount of carbon 

 dioxide is small, the phosphoric acid again seizes on the soda, 

 and turns out the carbon dioxide. Thus the soda is the 

 carrier of carbon dioxide in the blood plasma. The blood, 

 in passing through the tissues which are loaded with this 

 gas, gains carbon dioxide, and some of the basic phosphate 

 of s*oda is changed to the less alkaline neutral phosphate. 

 When the lungs are reached the blood is exposed to air poor 

 in carbon dioxide, and the phosphoric acid is able to turn 

 out the carbon dioxide. In fact, the carriage of carbon 



