RESPIRATORY CHANGES IN THE BLOOD. 357 



to which the reduced haemoglobin is added, the single band immediately 

 gives way to the two bands of oxy-haemoglobin. 



As the venous 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 oxy-haemo- 

 globin, and the purple color forthwith shifts into scarlet. For careful 

 observations show that the haemoglobin 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 oxy- 

 haemoglobin gives up some of its oxygen to the tissues, becoming, in part, 

 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 oxy-hsemoglobin ; and the difference of 

 color between venous and arterial blood depends almost entirely on the fact 

 that the reduced haemoglobin of the former is of purple color, while the oxy- 

 hsemoglobin of the latter is of a scarlet color. 



There may be other causes of the change of color, 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. 



292. We have spoken of the combination of haemoglobin with oxygen 

 as being a peculiar one. The peculiarity consists in 'the facts that the oxy- 

 gen may be associated and dissociated, without any general disturbance of 

 the molecule of haemoglobin, and that dissociation may be brought about 

 very readily. Haemoglobin combines in a wholly similar manner with other 

 gases. If carbonic oxide (monoxide) be passed through a solution of 

 haemoglobin, a change of color takes place, a peculiar bluish tinge making 

 its appearance. At the same time the spectrum is altered ; two bands are 

 still visible, but on accurate measurement it is seen that they are placed more 

 toward the blue end than are the otherwise similar bands of oxy-haemoglobin 

 (see Fig. 97, 6) ; their centres corresponding respectively to about wave- 

 lengths 572 and 533, while those of oxy-haemoglobin, as we have seen, cor- 

 respond to 578 and 539. When a known quantity of carbonic oxide gas is 

 sent through a haemoglobin solution, it will be found on examination that a 

 certain amount of the gas has been retained, an equal volume of oxygen ap- 

 pearing in its place in the gas which issues from the solution. If the solution 

 so treated be crystallized, the crystals will have the same characteristic color 

 and give the same absorption spectrum as the solution ; when subjected to 



