OXYH^EMOGLOBIN. 355 



tion of this varies considerably in venous, but not very much in 

 arterial blood. An average is given in the following table : 



O per cent. vols. COa per cent. vols. N per cent. vols. 



Arterial, . . 20 39 1-2 



Venous, . . . 8-10 (about) 46 1-2 



The more rapidly the gases are removed the greater is the pro- 

 portion of O that can be obtained ; as delay allows some of it to 

 combine with easily oxidized substances in the blood itself. The 

 amount of oxygen varies in different parts of the venous system. 

 In the blood of an animal which has died of slow asphyxia only 

 traces of oxygen can be found. 



The proofs that O is, for the most part, in chemical combina- 

 tion with the haemoglobin of the red blood corpuscles, and not 

 merely absorbed, as one might be led to suppose from its coming 

 away when the pressure is removed, are numerous and satisfac- 

 tory. 



First. When arterial blood is submitted to gradual diminution 

 of pressure in the mercurial air pump, the oxygen does not come 

 away in accordance with the established law of the absorption of 

 gases ( Henry -Dalton) by coming off in proportion to the diminu- 

 tion of the pressure, as at first only traces appear (probably the 

 small amount really dissolved), and when the pressure has been 

 reduced to a certain point the oxygen comes off suddenly ; after 

 which little more can be obtained by further reduction of pres- 

 sure. Haemoglobin combines with O in the same way, very 

 rapidly at first when the pressure is low, and then with a much 

 higher pressure a smaller quantity is taken up. 



Secondly. If the oxygen were only in a state of absorption, the 

 blood, while passing through the pulmonary capillaries, could 

 only take up about 0.4 volume per cent., which would be inade- 

 quate for life. We know that the quantity of O going to the 

 blood from the air in the alveoli cannot well be explained on 

 physical grounds alone ; and, moreover, when an animal is v 

 allowed to die of asphyxia in a limited space, all the O of the 

 air in the space is absorbed. Since the partial pressure of the O 

 in this case must fall to zero, it cannot be the pressure which 

 makes the pass into the blood. 



