THE CONTROL OF THE RESPIRATION 371 



bear no relation to that which they bear to other acids. It may be, however, 

 that the more toxic effect of the C0 2 is dependent upon the greater rate 

 at which it penetrates cell membranes than other acids. It would there- 

 fore enter the cells of the respiratory center and by dissociation cause 

 alteration of C H in the protoplasm. In the light of this possibility it is of 

 interest that whereas other acids cause cessation of the movements of flag- 

 ella in protozoa with no visible changes in the interior of the cells, C0 2 

 has little effect on the flagella but causes marked alterations in the intra- 

 cellular activities. 



Relationship Between Alveolar C0 2 and Respiratory Activity. Vari- 

 ations in the respiratory hormone, whatever this may be, are associated 

 with changes in the C0 2 content of the alveolar air. Increase in the 

 alveolar C0 2 immediately stimulates respiration unless under certain 

 conditions which will be discussed later. Indeed the respiratory center 

 is so very sensitive towards this stimulus that whenever the percentage 

 of C0 2 in the inspired air tends to rise, pulmonary ventilation is excited to 

 a degree which is just sufficient to maintain the tension of* C0 2 in the 

 alveolar air at the normal level. There is therefore, no better method for 

 testing the excitability of the center than to observe the magnitude of 

 pulmonary ventilation, when known percentages of C0 2 are added to the 

 inspired air. If the amount of C0 2 in the inspired air is sufficient to raise 

 the C0 2 in the alveoli, in spite of the greater breathing ; thus, it has been 

 found that an increase of from 0.2-0.3 per cent in alveolar C0 2 in man 

 causes a doubling in the alveolar ventilation, or, more precisely stated, an 

 increase of ten liters in the air entering and leaving the alveoli per minute 

 results from raising the alveolar C0 2 tension by 2.2 to 3.1 mm. Hg (Doug- 

 las 24 ). 



The relationship between breathing and alveolar C0 2 is by no means al- 

 ways so simple as in the instances just described. In these hyperpnea is sec- 

 ondary to an increase in alveolar C0 2 but there are many cases where the 

 reverse relationship obtains namely, where decreased alveolar C0 2 is sec- 

 ondary to hyperpnea caused by stimulation of the respiratory center by 

 some other agency than increase in C0 2 tension of the blood. These agen- 

 cies include afferent nerve stimulation, lowering of the 2 -tension, or in- 

 crease in CH of the blood brought about by other acids than C0 2 such 

 as occurs in clinical cases of acidosis (see page 654). 



The whole question is very closely linked with that of the control of the 

 reaction of the body fluids and with the etiological factors in acidosis. 

 When it is fully answered, many obscure clinical conditions in which respi- 

 ratory disturbances occur will be much better understood than they are at 

 present. On account of the great importance of the subject, considerable 



*The tension is found by the equation given on page 374. 



