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 CO2 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 CO2 

 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 CO2, however, variations in the alveolar 

 CO2 pressure will make a distinct difference to the 

 CO2 pressure in the blood leaving the tissues, so that 

 it is intelligible that what governs the breathing is 

 normally the CO2 pressure, and not the oxygen pres- 

 sure in the arterial blood. 



A further point about the curves for both oxygen 

 and CO2 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 



