II 



THE READJUSTMENTS OF REGULATION IN 

 ACCLIMATISATION AND DISEASE 



We have seen that under ordinary conditions the 

 regulation of breathing is dependent on very small 

 variations in the degree to which the arterial blood 

 leaving the lungs is saturated with CO 2 , and that a 

 normal CO 2 pressure of about 40 mm. is maintained 

 in the alveolar air of the lungs during rest. Never- 

 theless this normal pressure may become altered. Thus 

 if the oxygen percentage or pressure in the lung air 

 becomes very low in consequence of great deficiency 

 in the oxygen percentage of the air breathed, or from 

 the barometric pressure being very low, as at great 

 altitudes, the breathing is increased and the alveolar 

 CO 2 pressure falls. A similar fall occurs after mineral 

 acids have been taken, or in diseases in which abnor- 

 mal quantities of acid are discharged into the blood, 

 or after severe muscular exertion. To understand 

 how the breathing is affected under these various con- 

 ditions, and on what the normal conditions of breath- 

 ing ultimately depend, it is necessary to consider the 

 blood, and particularly the gases contained in it. 



When a liquid is brought into intimate contact with 

 a gas the liquid takes up the gas in solution until a 

 point is reached at which equilibrium or saturation 

 occurs. At this point as many molecules of gas are 

 being given off from the liquid as enter it, and the 



