238 RESPIRATION. 



recurrence of these alternating movements of inspiration and expi- 

 ration, fresh portions of air are constantly introduced into and 

 expelled from the chest. 



The average quantity of atmospheric air, taken into and dis- 

 charged from the lungs with each respiratory movement, is, ac- 

 cording to the results of various observers, twenty cubic inches. At 

 eighteen respirations per minute, this amounts to 360 cubic inches 

 of air inspired per minute, 2.1,600 cubic inches per hour, and 518,400 

 cubic inches per day. But as the movements of respiration are 

 increased both in extent and rapidity by every muscular exertion, 

 the entire quantity of air daily used in respiration is not less than 

 600,000 cubic inches, or 350 cubic feet. 



The whole of the air in the chest, however, is not changed at each 

 movement of respiration. On the contrary, a very considerable 

 quantity remains in the pulmonary cavity after the most complete 

 expiration ; and even after the lungs have been removed from the 

 chest, they still contain a large quantity of air which cannot be 

 entirely displaced by any violence short of disintegrating and dis- 

 organizing the pulmonary tissue. It is evident, therefore, that only 

 a comparatively small portion of the air in the lungs passes in and 

 out with each respiratory movement ; and it will require several 

 successive respirations before all the air in the chest can be entirely 

 changed. It has not been possible to ascertain with certainty the 

 exact proportion in volume which exists between the air which is 

 alternately inspired and expired, or "tidal" air, and that which 

 remains constantly in the chest, or " residual" air, as it is called. 

 It has been estimated, however, by Dr. Carpenter, 1 from the reports 

 of various observers, that the volume of inspired and expired air 

 varies from 10 to 13 per cent, of the entire quantity contained in 

 the chest. If this estimate be correct, it will require from eight to 

 ten respirations to change the whole quantity of air in the cavity 

 of the chest. 



It is evident, however, from the foregoing, that the inspiratory 

 and expiratory movements of the chest cannot be sufficient to 

 change the air at all in the pulmonary lobules and vesicles. The 

 air which is drawn in with each inspiration penetrates only into 

 the trachea and bronchial tubes, until it occupies the place of that 

 which was driven out by the last expiration. By the ordinary 

 respiratory movements, therefore, only that small portion of the 



1 Human Physiology, Philada. ed., 1855, p. 300. 



