THE GASES OF THE BLOOD 



261 



of them could not be accurately compared with the corresponding 

 gas tensions in the alveolar air. The discussion of the table is 

 none the less instructive to the student who desires to learn how 

 knowledge is won, and how frequently an advance in knowledge is 

 dependent upon an advance in technique. 



As regards the venous blood, we have already learnt that very 

 considerable variations in the content of oxygen and of carbon 

 dioxide are associated with the varying functional activity of the 

 tissues from which the blood comes. This factor, of course, is also 

 not without influence upon the gas tensions of the venous blood. 

 The carbon dioxide tension of arterial blood is affected by variations 

 in the amount of the pulmonary ventilation, which affect the partial 

 pressure of the carbon dioxide in the alveolar air and thus alter the 

 steepness of the slope of pressure between the two sides of the pul- 

 monary membrane. 



It is chiefly the enormous differences in the recorded oxygen 

 tensions of the arterial blood which excite surprise. To some ex- 

 tent, indeed, these also may depend upon differences in the partial 

 pressure of the oxygen in the alveoli, and it has been shown experi- 

 mentally (by the aerotonometer) that with increasing oxygen tension 

 of the inspired air the oxygen tension of the arterial blood increases 

 (Fredericq). Still, the differences which can possibly have existed 

 in the partial pressure of the oxygen in the alveoli in the various 

 series of observations can only to a small extent account for the 

 differences in the results. The main reason for the great range of 

 values lies unquestionably in the different experimental procedures 

 * The numbers in brackets are averages. 



