148 RESPIRATION 



any circumstances be called into use, and consequently the 

 vital capacity is equal to the total capacity minus the residual 

 air (100 cubic inches), or 230 cubic inches. It is the volume 

 which can be expelled by the most forcible expiration after 

 the most forcible inspiration. 



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 vesi- 

 cles is increased by inspiration and decreased by expiration; 

 it is called alveolar capacity, and at the end of ordinary ex- 

 piration amounts to about 150 cubic inches. Quiet inspira- 

 tion increases it to about 180 cubic inches. 



All these estimates, of course, represent only an aver- 

 age. The vital capacity is increased by stature, by any oc- 

 cupation 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 at- 

 mosphere. In addition, the atmosphere always contains a 

 little carbon dioxide (about .04 per cent.), ammonia, mois- 

 ture, 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 diox- 

 ide. 



Diffusion in the Lungs. The expired air contains much 

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

 change of gases between the alveolar air and the blood is 

 responsible for the difference. 



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

 the alveoli and the CCte 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 



