592 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



height of the barometer seven hundred and sixty millimeters. If the 

 gas so collected be analyzed it will be found that in the gaseous mixture 

 so obtained from the arterial blood there will be more oxygen than can 

 be obtained from the venous blood ; while, on the other hand, the gases 

 so abstracted from the venous blood will contain more carbon dioxide 

 than those obtained from the arterial blood. Further, it may be repeated 

 that if venous blood be agitated with air or with oxygen, it will at once 

 assume the arterial hue. It may, therefore, be concluded that the differ- 

 ence in the relative amounts of these gases present in the blood consti- 

 tutes the real difference between arterial and venous blood, and all other 

 differences, such as difference in color, are dependent upon this funda- 

 mental fact. The amount of gas which may be abstracted from blood 

 has been placed at sixty volumes for each one hundred volumes of blood. 

 The following table represents the proportions of these gases which 

 may be obtained from one hundred volumes of arterial and venous 

 blood : — 



Oxygen. Carbon Dioxide. Nitrogen. 



Arterial blood, . 20 volumes. 36 volumes. 1 to 2 volumea. 



Venous blood, . 8 to 12 " 46 " 1 to 2 



All measured at 760 millimeters and 0° C. 



The question is now raised as to the manner in which these gases 

 are held in the blood. In the chapter on the diffusion of gases it was 

 pointed out that every liquid possesses the power of absorbing gases, 

 and that the co-efficient of absorption varies with the nature of the 

 liquid, the nature of the gas, pressure of the gas, and the temperature 

 to which it was subjected. Blood, therefore, like every other liquid, 

 possesses the power of absorbing gases. When, however, we measure 

 the quantity of oxygen which may be extracted from the blood by 

 exposing it to a vacuum, we are struck by the fact that the amount so 

 removed is greatly in excess of that which may be held in solution in 

 the blood, regarding it merely as a fluid, by exposure to a gaseous 

 medium in which the oxygen is in no higher tension than it is in the 

 atmosphere. 



Again, the fact is worthy of notice that the absorption of oxygen 

 by the blood and its release from the blood are not governed by 

 variations of pressure. If we expose blood containing little oxygen to 

 a successive series of atmospheres in which the oxygen tension gradually 

 increases, we shall find that at first there is a very rapid absorption of 

 oxygen, and that this suddenly ceases; while if we submit blood highly 

 charged with oxygen to an atmosphere containing decreasing amounts of 

 oxygen, we shall find that at first but little oxygen will diffuse out from, 

 the blood until the pressure has been reduced almost to that of a vacuum. 

 Oxygen then suddenly almost totally escapes from the blood. Again, if 



