CHANGES EFFECTED IN THE BLOOD. 387 



half as much. Consequently, when the chest is completely filled with 

 air, the real quantity of oxygen included in it, is only half of that 

 which is drawn in by a corresponding inspiration at the earth's surface. 



698. With regard to the absorption and exhalation of Nitrogen, it 

 se'ems probable that both these processes are constantly going on ; but 

 that their relative activity varies under different conditions. Thus it 

 has been ascertained by MM. Regnault and Reiset, that although 

 warm-blooded animals, when subjected to their ordinary regimen, usu- 

 ally increase the amount of nitrogen in the atmosphere, yet that, when 

 food is withheld, or the animals are fed upon a diet to which they are 

 unaccustomed, an absorption of nitrogen takes place ; this being parti- 

 cularly remarkable in the hybernating Mammalia, in which the gain in 

 weight by the absorption of oxygen and nitrogen even exceeds the loss 

 occasioned by the exhalation of carbon. 



699. Having thus considered the changes produced by the Respira- 

 tory function, in the air submitted to it, we have next to inquire into 

 converse series of changes effected by it in the blood. The nature of 

 these cannot be well stated with precision, as they have not yet been 

 fully determined. It was formerly supposed, that the venous blood 

 arrives at the lungs charged with carbon, and that this carbon is united 

 with the oxygen of the air in the lungs themselves. Numerous facts, 

 however, go to prove, that the blood comes to the lungs charged with 

 carbonic acid ; and that it gives out this ready formed, and receives 

 oxygen in its stead. Thus it has been already shown, that there is a 

 positive disappearance of oxygen, more of that element being withdrawn 

 from the atmosphere, than is restored to it in the condition of carbonic 

 acid ; so that we know that the surplus must be received into the blood. 

 Further, cold-blooded animals may be made to breathe nitrogen or 

 hydrogen for a sufficient length of time, to cause a large quantity of 

 carbonic acid to be disengaged ; and this must have been brought to the 

 lungs ready formed, since no oxygen was present there to generate it. 

 Lastly, it can be shown by experiment, that oxygen, carbonic acid, and 

 nitrogen exist in a free state in blood, arterial as well as venous ; but 

 that the proportion of oxygen is greater in arterial than in venous 

 blood, whilst that of carbonic acid is less. The following table ex- 

 presses the percentage of each kind of gas in the two sorts of blood 

 respectively, as deduced from the experiments of Magnus. 



Arterial Blood. Venous Blood. 



Carbonic acid, .... 62-3 71-6 



Oxygen, . . . . . 23-2 15-3 



Nitrogen, 14-5 13-1 



Thus it appears that the quantity of nitrogen is very nearly the same 

 in both, as would be anticipated from what has been already stated in 

 regard to its non-participation in the respiratory process ; whilst about 

 one-third of the free oxygen of arterial blood disappears during its cir- 

 culation in the systemic capillaries, to be replaced by an equivalent 

 amount of carbonic acid ; and a converse change takes place in the pul- 

 monary capillaries, this additional portion of free carbonic acid being 

 set free, and replaced by oxygen. 



