148 RESPIRATION 



The capacity of the trachea and larger bronchi is known as 

 the bronchial capacity, and amounts to about 8 cubic inches. 



The quantity of air in the small bronchioles and air vesicles 

 is increased by inspiration and decreased by expiration; it is 

 called alveolar capacity, and at the end of ordinary expiration 

 amounts to about 150 cubic inches. Quiet inspiration increases 

 it to about 1 80 cubic inches. 



All these estimates, of course, represent only an average. 

 The vital capacity is increased by stature, by any occupation 

 which calls for active physical work and by various other 

 conditions. 



Composition of Air. Ordinary atmospheric air contains, in 

 round numbers, about 21 parts of oxygen to 79 parts of nitrogen. 

 These two gases make up the main bulk of the atmosphere. In 

 addition, the atmosphere always contains a little carbon dioxide 

 (about .04 per cent.), ammonia, moisture, organic material, 

 dust, nitric acid, etc. All except the oxygen and nitrogen are of 

 minor importance in respiration when they are not present in 

 amounts beyond the usual. It will be seen that the striking 

 difference between inspired and expired air is in the proportions 

 of oxygen and carbon dioxide. 



Diffusion in the Lungs. The expired air contains much 

 more CO 2 and much less O than the inspired air. The inter- 

 change of gases between the alveolar air and the blood is re- 

 sponsible for the difference. 



The question is what forces cause the O of the air to enter the 

 alveoli and the CO 2 to leave it. As might be supposed, the air 

 escaping during the first part of expiration differs very little in 

 composition from the inspired air, for it has been occupying the 

 upper air passages where no interchange occurs. The bronchial 

 capacity is only about one-third large enough to accommodate the 

 tidal air, and consequently the greater part of it must come from 

 lower down in the lung structure, and the CO 2 in the expired air 

 continuously increases until the end of the act. At each inspira- 



