33 ORGANISM AND ENVIRONMENT 



of the combined CO2 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 CO2 from a sodium 

 carbonate solution by means of a vacuum, and sodium 

 is certainly combined with CO2 in blood. Blood con- 

 tains an excess of alkali which is not combined with 

 any strong acid, and must be in part combined with 

 CO2. 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, from combining with the 

 available alkali. When the pressure of CO2, and 

 therefore its "mass influence" is reduced, more and 

 more of it is driven out of combination, until with 

 the CO2 pressure at zero none is left. 



From 100 volumes of human arterial blood about 

 50 volumes of CO2 as gas are given off to a vacuum, 

 and average venous blood contains only about 4 vol- 

 umes more. The relations between pressure of CO2 

 and the volume of CO2 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 CO2 

 at a given pressure of the gas when the oxy haemoglo- 

 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 CO2 

 and its absorption by the blood in the living body is 



