CHAP, ii.] RESPIRATIOX. 339 



of the great respiratory process, and they teach us the meaning of 

 the change of colour which takes place when venous blood becomes 

 arterial or arterial venous. In venous blood, as it issues from the 

 right ventricle, the oxygen present is insufficient to satisfy the 

 whole of the haemoglobin of the red corpuscles ; much reduced hae- 

 moglobin is present, hence the purple colour of venous blood. 



When ordinary venous blood, diluted without access of oxygen, 

 is brought before the spectroscope, the two bands of oxyhsemo- 

 globin are seen. This is explained by the fact that in a mixture 

 of oxyhsemoglobin and (reduced) haemoglobin, the two sharp bands 

 of the former are always much more readily seen than the much 

 fainter band of the latter. Now in ordinary venous blood there is 

 always some loose oxygen, removable by diminished pressure or 

 otherwise ; there is always some, indeed a considerable quantity, of 

 oxyhsemoglobin as well as (reduced) haemoglobin. It is only in 

 the very last stages of asphyxia that all the loose oxygen of the blood 

 disappears ; and then the two bands of the oxyhaemoglobin vanish 

 too. So distinct are the two bands of even a small quantity of 

 oxyhaemoglobin in the midst of a large quantity of haemoglobin 

 that a solution of (completely reduced) haemoglobin may be used 

 as a test for the presence of oxygen. 



As the blood passes through the capillaries of the lungs, this 

 reduced haemoglobin takes from the pulmonary air its complement 

 of oxygen, all or nearly all the haemoglobin of the red corpuscles 

 becomes oxyhaemoglobin, and the purple colour forthwith shifts 

 into scarlet. 



The haemoglobin of arterial blood is saturated or nearly satu- 

 rated with oxygen. By increasing the pressure of the oxygen, 

 an additional quantity may be driven into the blood, but this, 

 after the haemoglobin has become completely saturated, is effected 

 by simple absorption. The quantity so added is extremely small 

 compared with the total quantity combined with the haemoglobin, 

 but its physiological importance is increased by its being present 

 at a high tension. 



Passing from the left ventricle to the capillaries, some of the 

 oxyhaemoglobin gives up its oxygen to the tissues, becomes 

 reduced haemoglobin, and the blood in consequence becomes once 

 more venous, with a purple hue. Thus the red corpuscles by 

 virtue of their haemoglobin are emphatically oxygen-carriers. 

 Undergoing no intrinsic change in itself, the haemoglobin combines 

 in the lungs with oxygen, which it carries to the tissues; these, 

 more greedy of oxygen than itself, rob it of its charge, and the 

 reduced haemoglobin hurries back to the lungs in the venous 

 blood for another portion. The change from venous to arterial 

 blood is then in part (for as we shall see there are other events as 

 well) a peculiar combination of haemoglobin with oxygen, while 

 the change from arterial to venous is, in part also, a reduction of 

 oxyhaemoglobin; and the difference of colour between venous and 



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