34 ORGANISM AND ENVIRONMENT 
pressure of 40 mm. in the alveolar air. In other words 
the oxygenation of the venous blood in the lungs helps 
to turn out the CO,—a fact long ago suspected by 
Ludwig, but of which the only evidence that could 
be obtained was negative until new and rapid methods 
of blood-gas analysis were introduced by Barcroft 
and myself. 
As regards the carriage of both oxygen and CO, 
it is thus the case that the blood is of such a nature 
that the pressures of these gases in the blood leaving 
the tissues may vary but little in spite of the varying 
amounts of gas carried. With respect to oxygen, a 
glance at the dissociation curve of oxyhaemoglobin 
shows that it matters but little to the saturation of the 
blood with oxygen whether the oxygen pressure in the 
alveolar air is a little higher or a little lower. With 
respect to CO,, however, variations in the alveolar 
CO, pressure will make a distinct difference to the 
CO, pressure in the blood leaving the tissues, so that 
it is intelligible that what governs the breathing is 
normally the CO, pressure, and not the oxygen pres- 
sure in the arterial blood. 
A further point about the curves for both oxygen 
and CO, is that for any one individual they are ex- 
traordinarily constant from day to day and month to 
month. Under normal conditions no difference can 
be detected in them, just as with the gas pressures in 
the alveolar air. The significance of this constancy 
is unmistakable; and to a mechanist who pointed out 
that the taking up and giving off of gases by the blood 
is a purely chemical and physical matter, a vitalist 
