CHAP. IL] RESPIRATION. 595 



quantity of oxygen be present so distinct are the two bands that a 

 solution of completely reduced haemoglobin may be used as a test 

 for the presence of oxygen; if oxygen be present in any fluid 

 to which the reduced haemoglobin is added, the single band 

 immediately gives way to the two bands of oxyhaemoglobin. 



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

 this reduced haemoglobin takes from the pulmonary air its comple- 

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

 corpuscles becomes oxy haemoglobin, and the purple colour forthwith 

 shifts into scarlet. For careful observations shew that the haemo- 

 globin of arterial blood is saturated or nearly saturated with 

 oxygen; it probably falls short of complete saturation by about 

 1 vol. of oxygen in 100 vols. of blood. 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. 



Passing from the left ventricle to the capillaries of the tissues 

 the oxyhaemoglobin gives up some of its oxygen to the tissues, 

 becoming, in part, reduced haemoglobin, and the blood in conse- 

 quence 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 reduc- 

 tion of oxyhaemoglobin ; and the difference of colour between 

 venous and arterial blood depends almost entirely on the fact that 

 the reduced haemoglobin of the former is of purple colour, while 

 the oxyhaemoglobin of the latter is of a scarlet colour. 



There may be other causes of the change of colour, but these 

 are wholly subsidiary and unimportant. When a corpuscle swells, 

 its refractive power is diminished, and in consequence the number 

 of rays which pass into and are absorbed by it are increased at the 

 expense of those reflected from its surface; anything therefore 

 which swells the corpuscles, such as the addition of water, tends to 

 darken blood, and anything, such as a concentrated saline solution, 

 which causes the corpuscles to shrink, tends to brighten blood. 

 Carbonic acid .has apparently some influence in swelling the 

 corpuscles, and therefore may aid in darkening the venous blood. 



349. We have spoken of the combination of haemoglobin 

 with oxygen as being a peculiar one. The peculiarity consists in 

 the facts that the oxygen may be associated and dissociated, with- 



