THE CONTROL OF THE RESPIRATION 



359 



oratory and in clinical work, a special chapter has been devoted to the 

 subject, giving in detail the more recent methods devised by R. G. Pearce. 



Lastly, it should be noted that several observers believe that a more 

 reliable estimate of the alveolar tension of C0 2 (and of 2 ) can be made 

 by analyzing a sample of ordinary expired, air and calculating the per- 

 centages of C0 2 and 2 in the alveolar air by allowing a constant dead- 

 space capacity of 140 c.c. (Krogh, etc.). 



If we compare the C0 2 tension of arterial blood, as measured by the 

 Krogh method, with that of alveolar air, we shall find that there is a 

 remarkable correspondence, indicating, therefore, that, when the arterial 



^ % 02 in. it spired &ir 



20 30 itO SO 



20 30 40 



Fig. 127. Same as Fig. 126, except that in this case the tension of CO 2 in the alveolar air was 

 experimentally altered. (From A. and M. Krogh.) 



blood leaves the alveoli, its partial pressure or tension of C0 2 is exactly 

 equal to that in the alveolar air. This is shown in the accompanying 

 curves of experiments performed by Krogh. The dotted line in these 

 curves represents the tension of C0 2 or 2 in alveolar air, and the con- 

 tinuous line, these tensions in arterial blood. Close correspondence 

 will be observed between the C0 2 curves even when sudden changes in 

 alveolar C0 2 w^ere induced by artificial means. In the case of the 2 

 tensions, however, that of the blood is always lower than that of the 

 alveolar air, the differences being especially marked when the 2 ten- 

 sion in the alveoli is raised (Figs. 126 and 127). 



Tension of C0 2 in Venous Blood, If we examine the C0 2 tension of 

 the venous blood coming to the lungs, we shall find that it is distinctly 



