RESPIRATION 235 



naturally be expected, since even to the eye it varies greatly 

 according to the vein it is obtained from, the rapidity of the 

 circulation, and the .activity of the tissues which it has just 

 left. On the average, 



Volumes of 



O 2 C0 2 N 2 

 ioo volumes of arterial blood yield - - 20 40 1-2 



mixed venous blood (from right 



heart) yield - ... - 1012 45-50 1-2 



(reduced to o C. and 760 mm. of mercury). 



Average venous blood contains 7 or 8 per cent, by volume 

 less oxygen, and 7 or 8 per cent, more carbon dioxide, than 

 arterial blood. Thus, in the lungs the blood gains about 

 twice as many volumes of oxygen per cent, as the air loses, 

 and the air gains about half as many volumes of carbon 

 dioxide per cent, as the blood loses. And it is easy to see 

 that this must be so, for the volume of the air inspired in a 

 given time is about twice as great as that of the blood which 

 passes through the pulmonary circulation (pp. 197, 207, 224). 

 Even arterial blood is not quite saturated with oxygen ; it 

 can generally still take up one-tenth to one-fifteenth of the 

 quantity contained in it. Nor is venous blood nearly 

 saturated with carbon dioxide ; when shaken with the gas it 

 can take up about 150 volumes per cent. 



When the gases are not removed from blood immediately 

 after it is drawn, its colour becomes darker, and it yields 

 more carbon dioxide and less oxygen than if it is evacuated 

 at once (Pfluger). From this it is concluded that oxidation 

 goes on in the blood for some time after it is shed. The 

 oxidizable substances appear, however, to be confined to the 

 corpuscles, which suggests that ordinary metabolism simply 

 continues for some time in the formed elements of the shed 

 blood, and that the disappearance of oxygen is not due to 

 the oxidation of substances which have reached the blood 

 from the tissues. 



The Distribution of the Gases in the Blood. The oxygen is 

 nearly all contained in the corpuscles. A little oxygen can 

 be pumped out of serum (*i to *2 per cent, by volume), but 

 this follows the Henry-Dalton law of pressures; that is, it 



