CHAPTER II. 

 THE TISSUES AND MECHANISMS OF RESPIRATION. 



WE have already seen (Introduction, p. 3) that one particular item 

 of the body's income, viz. oxygen, is peculiarly associated with one 

 particular item of the body's waste, viz. carbonic acid, the means 

 which are applied for the introduction of the former being also used 

 for the getting rid of the latter. Both are gases, and in consequence 

 the ingress of the one as well as the egress of the other is far more 

 dependent on the simple physical process of diffusion than on any 

 active vital processes carried on by means of tissues. Oxygen 

 passes from the air into the blood mainly by diffusion, and mainly 

 by diffusion also from the blood into the tissues ; in the same way 

 carbonic acid passes mainly by diffusion from the tissues into the 

 blood, and from the blood into the air. Whereas, as we have seen, 

 in the secretion of the digestive juices the epithelium-cell plays an 

 all-important part, in respiration the entrance of oxygen from the 

 lungs into the blood, and from the blood into the tissue, and the 

 passage of carbonic acid in the contrary direction, are affected, if at 

 all, in a wholly subordinate manner, by the behaviour of the pul- 

 monary, or of the capillary epithelium. What we have to deal with 

 in respiration then is not so much the vital activities of any par- 

 ticular tissue, as the various mechanisms by which a rapid inter- 

 change between the air and the blood is effected, the means by 

 which the blood is enabled to carry oxygen and carbonic acid to and 



