IV THE GASES OF BLOOD 125 



Arterial Blood. Venous Blood. 



Oxygen 20 vols 8-12 vols. 



Carbonic acid ... 40 ,, 46 ,, 



Nitrogen 1-2 , 1-2 ,, 



This difference in their gaseous contents is the most 

 obvious difference between venous and arterial blood, as 

 may be demonstrated experimentally. For if venous 

 blood be shaken up with oxygen, or even with air, it 

 gains oxygen, loses carbonic acid, and takes on the colour 

 and properties of arterial blood. Similarly, if arterial 

 blood be treated with carbonic acid so as to be thoroughly 

 saturated with that gas, it gains carbonic acid, loses 

 oxygen, and acquires the true properties of venous blood ; 

 though, for a reason to be mentioned below, the change 

 does not take place so readily nor is it so com4jlete in this 

 case as in the former. The same result is attained, though 

 more slowly, if the blood, in either case, be received into 

 a bladder, and then placed in the oxygen, or carbonic acid ; 

 the thin moist animal membrane allowing the change to 

 be effected with perfect ease, and offering no sei-ious im- 

 pediment to the passage of either gas. 



Practically we may say that the most important differ- 

 ence between venous and arterial blood is not so much 

 the relative quantities of carbonic acid as that the red 

 corpuscles of venous blood have lost a good deal of oxygen, 

 are reduced, and ready at once to take up any oxygen 

 offered to them. 



Similarly the loss of oxygen by the red corpuscles is 

 the chief reason why the scarlet arterial blood turns 

 of a more purple or claret colour in becoming venous. 

 It has indeed been urged that the red corpuscles are 

 rendered somewhat flatter by oxygen gas, while they 

 are distended by the action of carbonic acid. Under 

 the former circumstances they may, not improbably, 



