THE BLOOD GASES. 370 



amount to about 72 volumes for every 100 volumes of blood, 

 and in the two cases are as follows 



Venous Blood. Arterial Blood. 



Oxygen 1Q 20 - 



Carbon dioxide 60 50 



Nitrogen 2 2 



It is important to bear in mind that while arterial blood 

 contains some carbon dioxide that can be removed by the 

 air-pump, venous blood also contains some oxygen remova- 

 ble in the same way; so that the difference between the- 

 two is only one of degree. When an animal is killed by 

 suffocation, however, the last trace of oxygen which can be 

 yielded up in a vacuum disappears from the blood before 

 the heart ceases to beat. All the blood of such an animal 

 is what might be called suffocation blood; and has a far 

 darker color than ordinary venous blood. 



The Cause of the Bright Color of Arterial Blood. The 

 color of the blood depends on its red corpuscles, since pure 

 blood plasma or blood serum is colorless, or at most a very 

 faint straw yellow. Hence the color change which the blood 

 experiences in circulating through the lungs must be due 

 to some change in its red corpuscles. Now, minute solid 

 bodies suspended in a liquid reflect more light when they 

 are more dense, other things being equal; and the first 

 t thing that suggests itself as the cause of the change in 

 color of the blood is that its red corpuscles have shrunk in 

 the pulmonary circulation, and so reflect more light and 

 give the blood a brighter look. This idea gains some 

 support from the fact that, as seen under the microscope, 

 the red blood corpuscles of some animals, as the frog, do 

 expand somewhat when exposed to carbon dioxide gas and 

 shrink up a little in oxygen. But that this is not the chief 

 K t, cause of the color change is readily proved. By diluting 

 blood with water the coloring matter of the red corpuscles 

 can be made to pass out of them and go into solution in the 

 plasma (p. 46) and it is found that such a solution, in which 

 there can be no question as to the reflecting powers of 

 colored solid bodies suspended in it, is brighter red when 

 supplied with oxygen than when deprived of that gas. 



