282 RESPIRATION 



pulmonary ventilation (the volume of air breathed per unit of time) 

 is chiefly regulated in ordinary breathing. 



For instance, an increase of as little as 0-2 per cent, of carbon 

 dioxide in the alveolar air, corresponding to an increase of 1-4 mm. 

 of mercury in the partial pressure (p. 247) of the gas, caused an 

 increase in the pulmonary ventilation of 100 per cent. The alveolar 

 oxygen pressure had to be diminished to 13 per cent, of an atmo- 

 sphere before any decided increase in the respiration occurred. 

 During moderate muscular work the percentage of carbon dioxide 

 in the alveolar air, and therefore in the hlpn^ 



causing an increase in the ventilation, and this is nnp of +HP 

 he hvperpncea associated with 



about. In_severe work lack of oxygen, with accumulation of lactic 

 acid and olher metabolic products, which increase the hydrogen-ion 

 concentration in the blood and thus stimulate the respiratory 

 centre or render it excitable by smaller pressures of carbon dioxide, 

 also plays a part. There is some evidence that in the case of 

 carbon dioxide also the actual excitation of the respiratory centre is 

 due to the increased hydrogen-ion concentration. Some physiolo- 

 gists hold the view that the respiratory centre reacts in the same 

 way to a given, increase in hvdro^en-ion r.nnrentra+im|_pr> matter 

 what the~acid may be, and that there is nothing specific in the 

 action of carbon dioxide. If it be assumed that changes in the 

 hydrogen-ion concentration of the blood are only caused by sub- 

 stances like carbon dioxide, which are eliminated by the lungs, 

 or by substances like lactic acid, which are either got rid of by oxi- 

 dation or are not liberated into the blood in the presence of a good 

 oxygen supply, no serious theoretical objection can be urged against 

 this conclusion. For increased activity of the respiratory centre 

 would favour the elimination of carbon dioxide and absorption of 

 oxygen, and in both ways would tend to maintain the normal 

 hydrogen-ion concentration in the blood. If, however, the hydrogen- 

 ion concentration can be influenced by substances which are not 

 directly affected by the respiratory process, it would seem unlikely 

 that the respiratory centre should blindly respond to such changes 

 just as if they had been produced by substances, the amount of 

 which it can effectively control. 



To sum up, the regulation of normal breathing is twofold a chemical 

 regulation (through the carbon dioxide, possibly by changes produced 

 in the hydrogen-ion concentration in the blood) of the amount of air 

 moved into and out of the lungs per unit of time ; and a nervous regu- 

 lation (chiefly through the vagi] of the rate and depth of the movements 

 necessary to effect the given amount of ventilation. 



When the vagi have been divided, an increase in the carbon 

 dioxide pressure within certain limits is responded to by an increase 

 in the total ventilation, just as in the normal animal, but the form 

 of the response is different. Whereas in the normal animal both 



