32 ORGANISM AND ENVIRONMENT 



of the combined CO 2 can be removed from blood by 

 exposing it to a vacuum, just as the whole of the loosely 

 combined oxygen can be removed. A strong acid 

 does not liberate any more. This is a very remarkable 

 fact ; for we cannot remove the CO 2 from x a sodium 

 carbonate solution by means of a vacuum, and sodium 

 is certainly combined with CO 2 in blood. Blood con- 

 tains an excess of alkali which is not combined with 

 any strong acid, and must be in part combined with 

 CO 2 . The explanation lies in the fact that haemoglo- 

 bin and other albuminous substances present in the 

 blood are capable of acting as very weak acids and so 

 partially preventing the CO 2 from combining with the 

 available alkali. When the pressure of CO 2 , and 

 therefore its "mass influence" is reduced, more and 

 more of it is driven out of combination, until with 

 the CO 2 pressure at zero none is left. 



From 100 volumes of human arterial blood about 

 50 volumes of CO 2 as gas are given off to a vacuum, 

 and average venous blood contains only about 4 vol- 

 umes more. The relations between pressure of CO 2 

 and the volume of CO 2 absorbed by human blood were 

 recently investigated by Christiansen, Douglas and 

 myself, and Figure 2 represents the results graphically. 

 We found that blood takes up considerably more CO 2 

 at a given pressure of the gas when the oxyhaemoglo- 

 bin is dissociated than when it is present as oxyhaemo- 

 globin. The oxyhaemoglobin thus acts as if it were 

 a more acid substance than dissociated or reduced 

 haemoglobin. The relation between pressure of CO 2 

 and its absorption by the blood in the living body is 



