188 HANDBOOK OF PHYSIOLOGY. 



some. In this country, the average quantity of watery vapor in the 

 atmosphere is 1.40 per cent. 



Composition of Air ivhich 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 

 quantity of oxygen ; 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 in- 

 haled, it nearly acquires that of the blood before it is expelled from the 

 chest. 



2. The Carbonic Acid is always increased; but the quantity exhaled 

 in a given time is subject to change from various circumstances. 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 carbonic 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 estimate, 

 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 quan- 

 tities must be considered approximate only, inasmuch as various circum- 

 stances, even in health, influence the amount of carbonic acid excreted, 

 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. 

 External temperature. Season of year. Condition of respired air. 

 Atmospheric conditions. Period of the day. Food and drink. Exer- 

 cise 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 a while, 

 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, 



