EFFECTS OF CHANGES IN AIR BREATHED 1101 



so as to obtain a sufficiency of this gas for the lowered requirements of the 

 body. That the adaptation is effective is shown by the fact that most 

 individuals, if they remain at a height, gradually recover from the mountain 

 sickness and may finally be able to carry out muscular movements with 

 almost as great precision and force as they could previously on the plains. 

 The mechanism by which increased ventilation of the lungs is attained is that 

 already mentioned in dealing with the effects of lack of oxygen, namely, the 

 production of acid substances in the body. The respiratory centre is thus 

 stimulated by these acid substances, especially lactic acid, as well as by 

 the carbon dioxide tension of the blood, and the joint action of these two 

 substances (which probably co-operate in raising the hydrion concentration of 

 the blood) determines the marked increase in the lung ventilation. Since 

 the carbon dioxide is no longer the sole factor responsible for the ventilation, 

 the tension of this gas in the alveolar air is diminished. 



ACAPNIA. This diminution of carbon dioxide tension in the blood and alveolar air 

 has been regarded by Mosso as the essential factor in the causation of mountain sickness 

 and has been designated acapnia. It may be absent, however, in the most marked 

 cases of mountain sickness, where the respiratory centre has failed to respond to the 

 additional acid stimulation, and may be present to a marked degree in individuals who 

 are experiencing none of the ill effects of this disorder. 



Another important means of rapid adaptation is by means of the circula- 

 tion. This is noticeable even in the case of persons sitting quietly in a 

 gas chamber who are subjected to gradually lower pressures. It is evident 

 that a deficient passage of oxygen from the alveoli to the blood may, so 

 far as the tissues and heart are concerned, be accommodated for by increasing 

 the rapidity of the circulation, and this is effected by a quickening of the 

 pulse-rate. The following Table shows the changes in the pulse-rate caused 

 by exposure to varying pressures in a gas chamber : 



PULSE IN GAS CHAMBER 



Pressure Pulse 



720 64 



650 72 



424 ....... 84 



This quickening of the pulse is to be observed also in the trained mountain 

 soldier, in individuals in whom there is no lowering of the alveolar carbon 

 dioxide tension, so that apparently in such cases the whole adaptation to 

 altered conditions is by means of the circulation. In cases where adaptation 

 fails it is in the circulation that the failure is most marked, so that the 

 symptoms of severe mountain sickness resemble closely those produced by 

 rapid heart- failure. Dilated heart, cyanosis, muscular weakness, vomiting, 

 mental torpor, inco-ordination, delirium, may all be observed in both cases. 

 The disturbance of the central nervous system is shown by the almost 

 invariable occurrence at great heights of Cheyne Stokes breathing. 



If the animal is able to withstand the immediate effects of exposure to a 

 rarefied atmosphere, a process of adaptation comes into play which finally 



