RESPIRATION 



207 



As was shown above, a difference of .012 in the PH of the blood 

 is sufficient to double the resting breathing, or cause apnoea. This 

 difference in PH corresponds to a difference of only about one 

 part by weight of ionized hydrogen in a million million parts of 

 blood. A continued difference of o. I in PH would in all probability 

 cause danger to life. This is a much lower limit than has commonly 

 been assumed. By forced breathing we can, it is true, produce a 

 greater difference in the PH of arterial blood, and maintain this 

 difference for an hour or more without loss of consciousness. The 

 difference, however, applies only to the arterial blood. As will be 

 shown in Chapter X, slowing of the circulation protects the tissues 

 to a large extent from great rises in PH. It is possible, also, that ac- 

 tive secretion of CO 2 by the lungs, as well as quickening of the cir- 

 culation, protects similarly against fall in the PH of the tissues. 

 Nevertheless, as Yandell Henderson has so clearly shown, when 

 efficient forced respiration is kept up in animals for a sufficient 

 time, not only do coma and progressive failure of circulation ensue, 

 but so much damage is done that it is impossible to recover the ani- 

 mal on restoring the PH of the blood by administering CO 2 , just as 

 it is impossible to recover a patient who has suffered for a sufficient 

 time from acute anoxaemia. That progressive and often irrepar- 

 able damage ensues also during a condition of excessive acidosis 

 is suggested by the phenomena of CO 2 poisoning and clinical 

 acidosis. To what extent the damage during alkalosis is due di- 

 rectly to the rise in PH, or to the accompanying anoxaemia, we 

 cannot at present say; and perhaps the question is at bottom 

 merely academic. When the forced breathing is of oxygen instead 

 of air the effects are much less marked, as mentioned above; but 

 this may be because the circulation can be shut down more effec- 

 tively when oxygen is breathed, and that hence the rise in PH in 

 the tissues is diminished. 



