RESPIRATION. 59 



be expelled after the most forcible expiratory efforts, and which amounts, 

 according to Dr. Hutchinson, to about loo 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 pro- 

 longed severe exercise. The combined amounts of the tidal, the comple- 

 mental and reserve air, 230 cubic inches, represents the vital capacity of an 

 individual 5 feet 7 inches in height. The vital capacity varies chiefly with 

 stature. It is increased 8 cubic inches for every inch in height above this 

 standard, and diminishes 8 cubic inches for each inch below it. 



The Tidal Volume of air is carried only into the trachea and larger 

 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 bronchial tubes also 

 assists in the interchange of air and carbonic acid. 



The entire volume of air passing in and out of the thorax in 24 hours is 

 subject to great variation, but can be readily estimated from the tidal 

 volume and the number of respirations per minute. Assuming that an 

 individual takes into the chest 20 cubic inches at each inspiration, and 

 breathes 18 times per minute, in 24 hours there would pass in and out of 

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



Composition of Air : Oxygen, 20.81 parts; nitrogen, 79.19, forming a 

 mechanical mixture in which exist traces of carbonic acid and watery 

 vapor. 



The changes in the air effected by respiration are 



Loss of oxygen, to the extent of 5 cubic inches per 100 of air, or I in 



20. 

 Gain of carbonic acid, to the extent of 4.66 cubic inches per 100 of 



air or .93 inch in 20. , 



Increase of watery vapor and organic matter. 

 Elevation of temperature. 

 Increase and at times decrease of nitrogen. 

 Gain of ammonia. 



The total quantity of oxygen withdrawn from the air and consumed by 

 the body in 24 hours amounts to 15 cubic feet, and can be readily esti- 

 mated from the amount consumed at each respiration. Assuming that one 

 inch of oxygen remains in the lungs at each respiration, in one hour there 

 are consumed 18 inches, and in 24 hours, 25,920 cubic inches or 15 cubic 



