286 RESPIRATION 



21 of oxygen. The proportion of carbon dioxide is extremely small; 10,000 

 volumes of atmospheric air contain only about 4 of that gas. 



The quantity of watery vapor varies greatly according to the tempera- 

 ture and other circumstances, but the atmosphere is never without some. 

 In this country the average quantity of watery vapor in the atmosphere 

 varies greatly according to the re'gion. In some of our Western arid plains 

 in the dry season the air is almost free of moisture. 



Character and Composition of Air which has been Breathed. 

 The changes effected by respiration in the atmospheric air are: i, an increase 

 of temperature; 2, a diminution in the quantity of oxygen; 3, an increase in 

 the quantity of carbon dioxide; 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. 



Temperature of the Expired Air. Expired air, after its contact with the 

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

 its temperature varies between 36 and 37.5 C. (97 and 99.5 F.), 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 acquires nearly that of the blood before it is expelled from the chest. 



The Oxygen of Expired Air. Pettenkofer and Voit have found that the 

 mean consumption of oxygen during 24 hours by a man weighing 70 kilos 

 is about 700 grams or 490 liters. The quantity of oxygen absorbed increases 

 with muscular exercise, and falls during rest. In general terms the quantity 

 absorbed varies with the activity of the metabolic processes, following very 

 closely the variation of carbon dioxide under the conditions outlined below. 



The Carbon Dioxide of Expired Air. The percentage of carbon dioxide 

 is increased in expired air, but the total quantity of carbon dioxide exhaled 

 in a given time is subject to change from various circumstances. From 

 every volume of air inspired 4 to 5 per cent, of oxygen is abstracted; while 

 a rather smaller quantity, 4.38 per cent., of carbon dioxide is added in its 

 place. The expired air will contain, therefore, 438 volumes of carbon di- 

 oxide in 10,000. The total quantity of carbon dioxide exhaled into the air 

 breathed by a healthy adult, calculating that 15.4 c.c. of the 350 c.c. of the 

 average air exhaled at each expiration consists of carbon dioxide, and that 

 the rate of respiration per minute is on an average 16, would be about 400 

 liters in twenty-four hours. From actual experiment this amount seems 

 to be a trifle too great, since from the average of many investigations the 

 total amount of carbon dioxide excreted per day by the entire body has been 

 found to be about 400 liters, weighing 800 grams, and consisting of 218 

 grams of carbon, and 582 grams of oxygen. From the 218 grams of carbon 

 must be deducted about 10 grams excreted in other ways than by the lungs, 

 which leaves about 215 grams as the amount of carbon excreted by the aver- 

 age healthy man by respiration each day and night. These quantities 



