SEC. 6. THE RESPIRATORY CHANGES IN THE TISSUES. 



358. 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 different 

 times. Thus in a gland at rest, as we 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 haemo- 

 globin 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 

 quantity 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 great question which 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, giving rise 

 to carbonic acid, which passes in turn away from the tissues into 

 the blood ? or do certain oxidizable reducing substances 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 carbonic 

 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. 



