CHAPTER II. 



RESPIRATION. 



SEC. 1. THE STRUCTURE OF THE LUNGS AND 

 BRONCHIAL PASSAGES. 



314. 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, in as much as the means which are 

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

 getting rid of the latter. Both are gases, and 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 

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

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

 particular tissue, as the various mechanisms by which a rapid 

 interchange between the air and the blood is effected, the means 

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

 to and from the tissues, and the manner in which the several 

 tissues take oxygen from and give carbonic acid up to the blood. 

 We have reasons for thinking that oxygen can be taken into the 

 blood, not only from the lungs, but also to a certain small extent 



