RESPIRATION. 193 



Composition of Air which has been breathed. The changes effected by 

 respiration in the atmospheric air are: 1, an increase of temperature; 2, 

 an increase in the quantity of carbonic acid; 3, a diminution in the quan- 

 tity of oyxgen; 4, a diminution of volume; 5, an increase in the amount 

 of watery vapor; 6, the addition of a minute amount of organic matter 

 and of free ammonia. 



1. The expired air, heated by its contact with the interior of the 

 lungs, is (at least in most climates) hotter than the inspired air. Its 

 temperature varies between 97 and 99.5 F. (36 37 '5 C.), the lower 

 temperature being observed when the air has remained but a short time 

 in the lungs. Whatever may be the temperature of the air when inhaled, 

 it nearly acquires that of the blood before it is expelled from the chest. 



2. The Carbonic Acid in respired air is always increased; but the 

 quantity exhaled in a given time is subject to change from various cir- 

 cumstances. From every volume of air inspired, about 4 '8 per cent, of 

 oxygen is abstracted; while a rather smaller quantity, 4*3, of carbonic 

 acid is added in its place: the air will contain, therefore, 434 vols. of car- 

 bonic acid in 10,000. Under ordinary circumstances, the quantity of 

 carbonic acid exhaled into the air breathed by a healthy adult man amounts 

 to 1346 cubic inches, or about 636 grains per hour. According to this esti- 

 mate, the weight of carbon excreted from the lungs is about 173 grains 

 per hour, or rather more than 8 ounces in twenty-four hours. These 

 quantities must be considered approximate only, inasmuch as various cir- 

 cumstances, even in health, influence the amount of carbonic acid ex- 

 creted, and, correlatively, the amount of oxygen absorbed. 



Circumstances influencing the amount of carbonic acid excreted. The 

 following are the chief: Age and sex. 'Eespiratory movements. Ex- 

 ternal temperature. Season of year. Condition of respired air. Atmos- 

 pheric conditions. Period of the day. Food and drink. Exercise and 

 sleep. 



a. Age and Sex. The quantity of carbonic acid exhaled into the air 

 breathed by males, regularly increases from eight to thirty years of age; 

 from thirty to fifty the quantity, after remaining stationary for awhile, 

 gradually diminishes, and from fifty to extreme age it goes on diminish- 

 ing, till it scarcely exceeds the quantity exhaled at ten years old. In 

 females (in whom the quantity exhaled is always less than in males of the 

 same age) the same regular increase in quantity goes on from the eighth 

 year to the age of puberty, when the quantity abruptly ceases to increase, 

 and remains stationary so long as they continue to menstruate. When 

 menstruation has ceased, it soon decreases at the same rate as it does in 

 old men. 



b. Respiratory Movements. The more quickly the movements of 

 respiration are performed, the smaller is the proportionate quantity of 

 carbonic acid contained in each volume of the expired air. Although, 

 however, the proportionate quantity of carbonic acid is thus diminished 

 during frequent respiration, yet the absolute amount exhaled into the air 

 within a given time is increased thereby, owing to the larger quantity of 



VOL. I. 13. 



