148 HUMAN PHYSIOLOGY. 



Chemistry of Respiration. As the inspired air undergoes a 

 change in composition during its stay in the lungs which renders it 

 unfit for further respiration, it becomes requisite, for the correct 

 understanding of respiration, to ascertain the composition of both 

 inspired and expired air. 



Composition of Air. Chemic analysis has shown that every 

 100 volumes of air contain 20.81 volumes of oxygen, 70^19 volumes 

 of nitrogen, and 0.03 volume of carbonic acid. Aqueous vapor is 

 also present, though the quantity is variable. The higher the tem- 

 perature, the greater the amount. 



The changes in the air effected by respiration are: 

 Loss of oxygen, to the extent of five cubic inches per 100 of air, or 



one in twenty. 

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



air, or 0.93 inch in twenty. 

 Increase of water-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The total quantity of oxygen withdrawn from the air and con- 

 sumed by the body in twenty-four hours amounts to fifteen cubic 

 feet, and can be readily estimated from the amount consumed at 

 each respiration. Assuming that one cubic inch of oxygen remains 

 in the lungs at each respiration, in one hour there are consumed 

 1080 cubic inches, and in twenty- four hours 25,920 cubic inches, 

 or fifteen cubic feet, weighing eighteen ounces. To obtain this 

 quantity, 300 cubic feet of air are necessary. 



The quantity of oxygen consumed daily is subject to considerable 

 variations. It is increased by exercise, digestion, and lowered tem- 

 perature, and decreased by the opposite conditions. 



The quantity of carbonic acid exhaled in twenty-four hours varies 

 greatly. It can be estimated in the same way. Assuming that an 

 individual exhales 0.93 + cubic inch at each respiration, in one 

 hour there are eliminated 1008 cubic inches, and in twenty- four 

 24,192 cubic inches, or fourteen .cubic feet, containing seven ounces 

 of pure carbon. 



The exhalation of carbonic acid is increased by muscular exer- 

 cise, nitrogenous food, tea, coffee, and rice, age, and by muscular 

 development; decreased by a lowering of temperature, repose, gin 

 and brandy, and a dry condition of the air. 



