SEC. 5. THE RESPIRATORY CHANGES IN THE TISSUES. 



In passing through the several tissues the arterial blood 

 becomes once more venous. A considerable quantity of the oxy- 

 hsemoglobin becomes 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 the presence of a large quantity of 

 reduced haemoglobin ; when the gland is active, the venous blood 

 in its colour, and in the amount of haemoglobin which it contains, 

 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 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 



