250 RESPIRATION 



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. 

 It is easy to see that this must be so, for the volume of air inspired 

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

 passes through the pulmonary circulation (pp. 223, 234). Even 

 arterial blood is not quite saturated with oxygen; it can still take 

 up a variable small amount. The percentage saturation with 

 oxygen of the arterial blood of a normal woman from whom 

 blood was being transfused into a patient was directly determined. 

 The blood proved to be 94 per cent, saturated i.e., it could still 

 have taken up about one-sixteenth 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, it yields more carbon dioxide and less oxygen than if it 

 is evacuated at once (Pfliiger). From this it is concluded that 

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

 oxidizable substances are, however, 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 and Condition of the Oxygen in the Blood. The 

 oxygen is nearly all contained in the corpuscles. A little oxygen 

 can be pumped out of serum (0-2 or 0-3 per cent, by volume), but this 

 follows the Henry-Dalton law of pressures that is, it comes off in 

 proportion to the reduction of the partial pressure of the oxygen in 

 the pump, and is simply in solution. 



When blood at body temperature is shaken up with air at the 

 ordinary pressure, corresponding to a partial pressure of oxygen 

 of a little over one-fifth of an atmosphere (in round numbers 160 mm. 

 of mercury), the blood-pigment becomes saturated with oxygen or 

 nearly so. When the blood is now pumped out, very little oxygen 

 comes off till the pressure has been reduced to about half an atmo- 

 sphere, corresponding to a pressure of oxygen of about 80 mm. 

 At about 70 mm. partial pressure the dissociation is somewhat 

 greater. At a third to a quarter of an atmosphere (50 to 40 mm.) 

 the amount of oxygen liberated is markedly increased, and the 

 dissociation becomes more and more rapid as the pressure falls 

 towards zero. This behaviour shows that the oxygen is not simply 

 absorbed, but is united, as a dissociable compound, to some con- 

 stituent of the blood. The same thing is, of course, seen when 



