RESPIRATORY QUOTIENT 129 



sis that the carbon dioxide is always a result of the direct action of 

 oxygen upon carbohydrates and fats. Such a definite relation between 

 these two gases can not be assumed to exist, in view of the fact that 

 carbon dioxide may be given off by the tissues in the absence of 

 oxygen. 



Many of the points that have been considered in relation to the 

 variations in the exhalation of carbon dioxide have been investigated in 

 Pettenkofer'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 this gas as existing in the blood in combination ; the greater 

 part being loosely united with certain other substances, and a small 

 quantity of that which is thrown off in the expired air being in a condi- 

 tion 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. An- 

 other and a smaller part probably is set free by the action of the 

 oxyhemoglobin, 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 it is more readily extracted from arterial 

 than from venous blood. The mechanism by which 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 as dissociation. It is certain that the oxyhemoglobin, which is 

 constantly forming in the lungs, assists materially in this process, pos- 

 sibly acting as an acid. 



Respiratory Quotient. The result of the division of the carbon 

 dioxide exhaled by the oxygen taken up by the blood is called the 

 respiratory quotient. Under ordinary conditions of respiration, this is 

 equal to about ninety (4.5 -s- 5). The greater the proportion of oxygen 

 consumed to the carbon dioxide exhaled, the less is the respiratory quo- 



