Gases of the Blood 627 



proportion of C0 2 in the alveolar air, lessens it at the same time in 

 the blood. 



This carbonic acid which remains thus in the lungs represents 

 the regular excess of the carbonic acid formed in our tissues over 

 that which is exhaled through the trachea. This quantity will not 

 vary, if no change is made in the conditions of metabolism and of 

 pulmonary ventilation. Now this seems to be the case during 

 respiration in compressed air, at least for the phenomena that are 

 gross and apparent. If then there is produced during the same 

 time the same quantity of CO,, the quantity which will remain in 

 the air of the lungs will be the same; but as this quantity is com- 

 pressed, its volume decreases inversely as the pressure, and it is 

 clear that then its percentage in the air of the lungs, whose total 

 volume does not change, will diminish directly as the volume. So 

 if the air in the lungs of an animal at normal pressure contained 

 6 per cent of C0 2 , at 2 atmospheres it will contain only 3 per cent, 

 at 3 atmospheres 2 per cent, at 6 atmospheres 1 per cent, etc. 



Now since the pressure exerted by this carbonic acid upon the 

 carbonic acid of the blood evidently has as a measure the product 

 of the percentage by the barometric pressure, it will be expressed 

 in the different cases mentioned above: at normal pressure, by 

 6x1 = 6; at 2 atmospheres, by 3x2 = 6; at 3 atmospheres by 

 2x3 = 6; etc., that is, its value always remains the same. It is 

 therefore not astonishing that the carbonic acid content of the blood 

 does not vary either. 



But why does it diminish in very low pressures? In this case, 

 the same reasoning, the same conclusions, apparently. But here 

 the question becomes complicated. First, if we assume that the 

 animal is at a pressure of a half -atmosphere, the proportion of car- 

 bonic acid in the lungs will rise to 12 per cent; the oxygen content 

 of the air in the pulmonary vesicles is thus diminished, and the 

 animal is forced to maintain a more active ventilation which, les- 

 sening the tension 12 x % = 6, lets more acid escape from the blood. 



But the principal reason lies elsewhere than in the decrease of 

 barometric pressure; we shall see later, in fact, that the carbonic 

 acid content of the blood diminishes merely from breathing an air 

 with a smaller oxygen content. It is therefore in the troubled 

 chemical conditions of the formation of CO, that we must seek the 

 most important cause of this diminution. No doubt the same thing 

 is true of the diminution which coincides with pressures above one 

 atmosphere. 

 3. Nitrogen. For this last gas, matters should take place with a 



