94 



RESPIRATION 



capillaries is an important factor in facilitating the unloading of 

 oxygen from the blood ; and this line of argument has been further 

 elaborated by Barcroft. The actual shifting is, however, very 

 small under normal conditions, and of much less physiological 

 importance than the effect of the shifting of the CO 2 absorption 

 curve in consequence of reduction of oxyhaemoglobin. 



Figure 28. 



The thick line shows the dissociation curve of oxyhaemoglobin in the blood of 

 J. S. H. and C. G. D. in the presence of 40 mm. pressure of COa. The thin line 

 represents the dissociation curve of oxyhaemoglobin in the blood of J. S. H. and 

 C. G. D. within the body. 



We are now in a position to interpret much more completely 

 the facts concerning the regulation of breathing by small varia- 

 tions in the alveolar CO 2 pressure. How very small the mean 

 variations are, we have already seen. On the other hand the 

 breathing is constantly being interrupted or interfered with in 

 one way or another during ordinary occupations, such as speak- 

 ing or singing, and the breath can be held for a few seconds with- 

 out any noticeable air hunger being produced. During these 

 interferences the alveolar CO 2 pressure must be constantly rising 

 and falling on either side of the normal limit, but the physiological 

 effect seems almost nil, and to popular imagination it seems as 

 if the breathing, instead of being regulated so rigorously as was 

 shown to be the case in the second chapter, is hardly regulated at 



