32 ORGANISM AND ENVIRONMENT 
of the combined CO, 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, from a sodium 
carbonate solution by means of a vacuum, and sodium 
is certainly combined with CO, 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,. 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 CO,, and 
therefore its “mass influence” is reduced, more and 
more of it is driven out of combination, until with 
the CO, pressure at zero none is left. 
From 100 volumes of human arterial blood about 
50 volumes of CO, 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, 
and the volume of CO, 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, 
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, 
and its absorption by the blood in the living body is 
