246 MANUAL OP PHYSIOLOGY. 



manner which facilitates further analysis. Together they are 

 found to measure about 60 volumes for every 100 volumes of 

 blood. 



Oxygen. The amount of oxygen in the blood is found to vary 

 much with circumstances. In arterial blood the quantity is much 

 more constant, and always exceeds that in venous blood. It is 

 estimated (at C. and 760 mm. pressure) that every 100 vol- 

 umes of arterial blood yield 20 volumes of oxygen, while the 

 volume of oxygen in venous blood varies from 8 to 12 per cent. 



The oxygen which comes off in the Torricellian vacuum exists 

 in the blood in two distinct states : (1) a very small quantity 

 simply absorbed about as much as water absorbs under atmos- 

 pheric pressure ; (2) chemically combined, in which state nearly 

 all the oxygen exists, and forms with the haemoglobin a loose 

 combination called oxyhsemoglobin. This oxygen, therefore, does 

 not follow the laws of absorption by leaving the blood in propor- 

 tion as the pressure is reduced, but at a certain point of reduction 

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

 pletely. 



Carbon Dioxide (C0 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 Respi- 

 ration. 



