RESPIRATION. 147 



Amount of Air Exchanged in Respiration, and Capacity of Lungs. 



The tidal or breathing volume of air, that which passes in and 

 out of the lungs at each inspiration and expiration, is estimated at 

 from twenty to thirty cubic inches. 



The complemental air is that amount which can be taken into the 

 lungs by a forced inspiration, in addition to the ordinary tidal vol- 

 ume, and amounts to about no cubic inches. 



The reserve air is that which usually remains in the chest after 

 the ordinary efforts of expiration, but which can be expelled by 

 forcible expiration. The volume of reserve air is about 100 cubic 

 inches. 



The residual air is that portion which remains in the chest and 

 cannot be expelled after the most forcible expiratory efforts, and 

 which amounts, according to Dr. Hutchinson, to about 100 cubic 

 inches. 



The vital capacity of the chest indicates the amount of air that 

 can be forcibly expelled from the lungs after the deepest possible 

 inspiration, and is an index of an individual's power of breathing 

 in disease and during prolonged severe exercise. The combined 

 amount of the tidal, the complemental, and the reserve air, 230 

 cubic inches, represents the vital capacity of an individual five feet 

 seven inches in height. The vital capacity varies chiefly with 

 stature. It is increased eight cubic inches for every inch in height 

 above this standard, and diminishes eight cubic inches for each inch 

 below it. 



The tidal volume of air is carried only into the trachea and large 

 bronchial tubes by the inspiratory movements. It reaches the 

 deeper portions of the lungs in obedience to the law of diffusion of 

 gases, which is inversely proportionate to the square root of their 

 densities. 



The ciliary action of the columnar cells lining the broncial tubes 

 also assists in the interchange of air and carbonic acid. 



The entire volume of air passing in and out of the thorax in 

 twenty- four hours is subject to great variation, but can be readily 

 estimated from the tidal volume and the number of respirations a 

 minute. Assuming that an individual takes into the chest twenty 

 cubic inches at each inspiration, and breathes eighteen times a 

 minute, in twenty-four hours there would pass in and out of the 

 lungs 518,400 cubic inches, or 300 cubic feet. 



