CHAPTER XIV 

 General Conclusions. 



ON looking back at the results reached in successive chapters of 

 this book certain points of general physiological significance 

 emerge. The present chapter will be devoted to their discussion. 



It is evident that within the limits of health the breathing 

 represents the lung ventilation required to keep the reaction 

 and the pressure of oxygen in the blood supplying the re- 

 spiratory center constant within certain narrow limits, and that 

 the breathing increases or diminishes in accordance with the 

 quantity of air needed to produce this effect. The "chemical" and 

 "nervous" stimuli acting on the respiratory center cooperate in 

 bringing about the constancy. The circulation is, in the main, 

 similarly regulated so as to maintain a normal reaction and 

 oxygen pressure in each of the various organs, although other 

 factors may also determine the local circulation rate to some 

 extent. 



The quantity of respired air required to keep the arterial blood 

 normal varies with the very variable consumption of oxygen and 

 output of carbonic acid by the whole of the living tissues. In 

 different individual parts of the body the variations in consump- 

 tion of oxygen and output of carbonic acid are still more striking; 

 and meeting these variations there are equally striking variations 

 in the local circulation rates. 



What is regulated by the breathing and circulation is not pri- 

 marily the consumption of oxygen and formation of carbonic acid, 

 but the partial pressures, or diffusion pressures, of these sub- 

 stances. If their diffusion pressures become more than slightly 

 abnormal the result is, not a mere slowing or quickening of physio- 

 logical activity, but totally abnormal activity and abnormal change 

 in structure. What is immediately effected is the maintenance of 

 these pressures. The supply of oxygen and removal of carbonic 

 acid are such as to keep them approximately steady. We have also 

 seen that it is simply as an acid that carbonic acid is of physio- 

 logical importance, so that in reality a normal reaction, or normal 

 diffusion pressure of hydrogen and hydroxyl ions, and not merely 

 a normal diffusion pressure of carbonic acid, is maintained. 



