240 MANUAL OF PHYSIOLOGY. 



of pressure (20 mm. mercury) the oxygen comes off almost com- 

 pletely. 



Carbon Dioxide (CO 2 ). The amount of carbon dioxide also 

 varies more in venous than in arterial blood, for under certain 

 circumstances (suffocation) it may rise to over 60 volumes per 

 cent., although ordinary venous blood on an average contains 

 only 46 volumes in every 100. On the other hand, the amount 

 of this gas in arterial blood varies little from 39 volumes per 

 cent. 



The larger proportion of carbon dioxide exists in the plasma, 

 where it appears to be chemically combined with soda salts, 



Nitrogen. The amount of nitrogen does not vary much, being 

 in both venous and arterial blood about 1.5 volume per cent., 

 and it would appear to be simply absorbed. 



For further details about arterial and venous blood, see Respira- 

 tion. 



