THE MECHANICS OF PULMONARY RESPIRATION. 333 



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 depend- 

 ent on the simple physical process of diffusion than on any active vital pro- 

 cesses 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 epithelial 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 behavior 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 from the skin, and, as we have seen, from the alimen- 

 tary canal also ; and carbonic acid certainly passes away from the skin, and 

 through the various secretions, as well as by the lungs. Still the lungs are 

 so eminently the channel of the interchange of gases between the body and 

 the air, that in dealing at present with respiration, we shall confine our- 

 selves entirely to pulmonary respiration, leaving the consideration of the 

 subsidiary respiratory processes till we come to study the secretions of which 

 they respectively form part. 



THE MECHANICS OF PULMONARY RESPIRATION. 



267. The lungs are placed, in a state which is always one of disten- 

 tion, sometimes greater, sometimes less, in the air-tight thorax, the cavity 

 of which they, together with the heart, great bloodvessels, and other organs, 

 completely fill. By the contraction of certain muscles the cavity of the 

 thorax is enlarged. The lungs must follow this enlargement and be them- 

 selves enlarged, otherwise the pleural cavities would be enlarged ; but this 

 is impossible so long as the walls are intact. The enlargement of the lung 

 consists chiefly in an enlargement or expansion of the pulmonary alveoli, 

 the air in which becomes, by the expansion, rarefied. That is to say, the 

 pressure of the air within the lungs becomes less than that of the air out- 

 side the body, and this difference of pressure causes a rush of air through 

 the trachea into the lungs until an equilibrium of pressure is established 

 between the air inside the lungs and that outside. This constitutes inspir- 

 ation. On relaxation of the respiratory muscles (the muscles whose con- 

 tractions have brought about the thoracic expansion), the elasticity of the 

 lungs and chest-walls, aided, perhaps, to some extent by the contraction of 

 certain m.uscles, causes the chest to return to its original size ; in consequence 

 of this the pressure within the lungs becomes greater than that outside, and 

 thus air rushes out of the trachea until equilibrium is once more established. 

 This constitutes expiration ; the inspiratory and expiratory act together form- 

 ing a respiration. The fresh air introduced into the upper part of the pul- 

 monary passages by the inspiratory movement contains more oxygen and less 

 carbonic acid than the old air previously present in the lungs. By diffusion 

 the new or tidal air, as it is frequently called, gives up its oxygen to, and 

 takes carbonic acid from, the old or stationary air, as it has been called, 



