148 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



tions in the exhalation of carbon dioxide have been investigated in Petten- 

 kofer's chamber, and the results very nearly correspond with the observations 

 quoted from Scharling, Edward Smith and others. 



Sources of Carbon Dioxide in the Expired Air. All the carbon dioxide 

 in the expired air comes from the venous blood, where it exists in two forms ; 

 in a condition either of simple solution or of association with certain of the 

 constituents of the plasma, and in union with bases, forming the carbonates 

 and bicarbonates. The fact that carbon dioxide, as regards the quantity 

 absorbed by the blood, does not obey, in all regards, the laws which regulate 

 the absorption of gases by liquids under different conditions of pressure, has 

 led some physiologists to regard all of this gas as existing in the blood in 

 chemical combination ; the greater part being very loosely united with cer- 

 tain other substances, and a small quantity of that which is thrown off in 

 the expired air being in a condition of union much more stable. 



The greater part of the carbon dioxide exhaled comes from the plasma, 

 where it is in a condition of what is known as association. Another and a 

 smaller part is probably set free by the action of the oxyhaemaglobine, which 

 is distinctly acid. It has been shown that more carbon dioxide can be 

 extracted by means of a vacuum from the entire blood than from the serum ; 

 and this gas is more readily extracted from arterial than from venous blood. 

 The mechanism by which the carbon dioxide is discharged from the venous 

 blood is probably the following : 



Carbon dioxide is carried from the tissues to the lungs, in the venous 

 blood. Here it exists mainly in the plasma, a small quantity, only, existing 

 in the corpuscles. As the venous blood passes through the lungs, the greater 

 part of the carbon dioxide of the plasma either simply diffuses from the blood 

 into the air-cells or passes out by a process known to chemists as dissociation 

 (Deville). It is certain that the oxyhaemaglobine, which is constantly form- 

 ing in the lungs, assists materially in this process. 



There can be no doubt with regard to the existence of an acid of some 

 kind in the lungs, which possibly decomposes a portion of the bicarbonates 

 of the blood, in ordinary respiration. When sodium bicarbonate is injected 

 into the jugular of a living animal, a rabbit, for example, it is decomposed as 

 fast as it gets to the lungs, and carbon dioxide is evolved. This experiment 

 produces no inconvenience to the animal when the bicarbonate is introduced 

 slowly ; but when it is injected in large quantity, the evolution of gas in the 

 lungs is so great as to fill the pulmonary structure and even the heart and 

 great vessels, and death is the result (Bernard). 



Exhalation of Watery Vapor. From a large number of observations on his 

 own person and upon eight others, collecting the water by sulphuric acid, 

 Valentin made the following estimates of the quantities of water exhaled 

 from the lungs in twenty-four hours : 



In his own person the exhalation in twenty-four hours was 5,934 grains 

 (384-48 grammes). 



In a young man of small size the quantity was 5,401 grains (350 

 grammes). 



