SEC. 5. THE RESPIRATORY CHANGES IN THE TISSUES. 



289. In passing through the several tissues the arterial 

 blood becomes once more venous. The oxyhaemoglobin becomes 

 considerably reduced, and a quantity of carbonic acid passes 

 from the tissues into the blood. The amount of change varies 

 in the various tissues, and in the same tissue may vary at differ- 

 ent times. Thus in a gland at rest, as Ave have seen, the venous 

 blood is dark, shewing that the haemoglobin is to a large extent 

 in the reduced condition ; when the gland is active, the venous 

 blood in its colour, and in the extent to which the haemoglobin 

 is in the condition of oxyhsemoglobin, resembles closely arterial 

 blood. The blood therefore which issues from a gland at rest 

 is more ' venous ' than that from an active gland ; though owing 

 to the more rapid flow of blood which, as we saw in an earlier 

 section, accompanies the activity of the gland, the total quan- 

 tity of oxygen taken up from and of carbonic acid discharged 

 into the blood from the gland in a given time may be greater 

 in the latter. The blood, on the other hand, which comes from 

 an active, i.e. a contracting muscle, is, in spite of the more 

 rapid flow, not only richer in carbonic acid, but also, though not 

 to a corresponding amount, poorer in oxygen than the blood 

 which flows from a muscle at rest. 



In all these cases the question which first comes up for our 

 consideration is this : Does the oxygen pass from the blood into 

 the tissues, and does the oxidation take place in the tissues, giv- 

 ing rise to carbonic acid, which passes in turn away from the 

 tissues into the blood? or do certain oxidizable reducing sub- 

 stances pass from the tissues into the blood, and there become 

 oxidized into carbonic acid and other products, so that the chief 

 oxidation takes place in the blood itself? 



There are, it is true, reducing oxidizable substances in the 

 blood, but these are small in amount, and the quantity of car- 

 bonic acid to which they give rise when the blood containing 

 them is agitated with air or oxygen, is so small as scarcely to 

 exceed the errors of observation. 



We may add, that the oxidative power which the blood 

 itself removed from the body is able to exert on substances 



467 



