468 RESPIRATION. 



When carbonic acid is formed by the blood, after it is drawn 

 from the body, it is immediately exhaled, at least in part. 

 When blood is in contact with a certain quantity of air, oxy- 

 gen is consumed and carbonic acid is exhaled. The fact that 

 carbonic oxide, which has such a remarkable affinity for the 

 corpuscles, displaces oxygen almost exclusively, is another 

 argument in favor of the view that the carbonic acid is con- 

 tained mainly in the plasma. 



A portion of the carbonic acid which is formed by the 

 system, unites with the carbonates in the blood, particularly 

 the carbonate of soda, to form bicarbonates, is carried to the 

 lungs, and there set free by the pneumic acid. It here exists 

 in so loose a condition of combination, that it may be dis- 

 engaged by treating the blood with inert gases, or putting it 

 under the receiver of an air-pump. 



The carbonic acid which is formed in the tissues, and taken 

 up by the blood in its passage through the capillaries, exists 

 in this fluid in two forms : one, in simple solution, chiefly in 

 the plasma ; and the other, in a state of such loose chemical 

 combination in the bicarbonates, that it may be disengaged 

 by displacement by another gas, and is readily set free by 

 pneumic acid. This gas is a product of excretion, and is not 

 engaged in any of the vital functions ; while oxygen, which 

 has an all-important function to perform, unites immediately 

 with the blood-corpuscles, and is not easily disengaged, 

 except when it undergoes transformation in the process of 

 nutrition. It is certain that all the carbonic-acid in the blood 

 is not in combination with bases, for the proportion of salts 

 is not sufficient to account for all the carbonic acid that can 

 be disengaged. 



In addition to this excrementitious carbonic acid, there 

 is another portion which is a permanent constituent of the 

 blood, in the carbonates, and cannot be set free without the 

 use of reagents. 



Nitrogen exists in the blood in the same condition of solu- 

 tion in the plasma as carbonic acid. 



