252 A MANUAL OF PHYSIOLOGY 



the volume of the air inspiaed in a given time is about twice as 

 great as that of the blood which passes through the pulmonary 

 circulation (pp. 209, 220, 242). 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 

 (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 of the Gases in the Blood. The oxygen is 

 nearly all contained in the corpuscles. A little oxygen can be 

 pumped out of serum (o-i to 0-2 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 is being pumped out, very little oxygen comes 

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

 sphere. At about a third of an atmosphere, if the blood is nearly 

 at body temperature, the oxygen begins to escape a little more 

 freely ; and when the pressure has fallen to about one-sixth 

 of an atmosphere (corresponding to a partial pressure of oxygen 

 of 25 to 30 mm. of mercury), it is disengaged with a burst. This 

 shows that it is not simply absorbed, but is united by chemical 

 bonds to some constituent of the blood. The same thing is seen 

 when defibrinated blood is saturated at body temperature with 

 oxygen at different pressures. The quantity taken up lessens 

 but slowly as the pressure is reduced, till at about 25 to 30 mm. 

 of mercury an abrupt diminution takes place. It is found that 

 a solution of pure haemoglobin crystals behaves towards oxygen 

 somewhat differently from blood containing the same proportion 

 of blood-pigment ; and although there is no doubt that the body 

 in blood with which the oxygen is loosely united is intimately 

 related to the haemoglobin, which can be artificially prepared from 

 it, there are good reasons for believing that they are not identical. 

 Some writers for this reason prefer to give the special name 

 haemochrome to the native blood-pigment as it exists within the 

 unaltered corpuscles, reserving the term haemoglobin for the more 

 or less artificial though, perhaps, only slightly altered product. 



