EESPIBATION. 273 



given out by the lungs constitutes about 4. 3 per cent, of the expired air ; but 

 if the air which is breathed be previously impregnated with carbon dioxide 

 (as is the case when the same air is frequently respired) , then the quantity of 

 carbon dioxide exhaled becomes relatively much less. 



/. Hygrometric State of Atmosphere. The amount of carbon dioxide exhaled 

 is considerably influenced by the degree of moisture of the atmosphere, much 

 more being given off when the air is moist than when it is dry. 



g. Period of the Day. During the day-time more carbon dioxide is exhaled 

 than corresponds to the oxygen absorbed ; while, on the other hand, at night 

 very much more oxygen is absorbed than is exhaled in carbon dioxide. There is, 

 thus, a reserve fund of oxygen absorbed by night to meet the requirements of the 

 day. If the total quantity of carbon dioxide exhaled in 24 hours be repre- 

 sented by 100, 52 parts are exhaled during the day, and 48 at night. While 

 similarly, 33 parts of the oxygen are absorbed during the day, and the remain- 

 ing 67 by night. 



h. Food and Drink. By the use of food the quantity is increased, while 

 by fasting it is diminished ; it is greater when animals are fed on farinaceous 

 food than when fed on meat. The effects produced by spirituous drinks de- 

 pend much on the kind of drink taken. Pure alcohol tends rather to increase 

 than to lessen respiratory changes, and the amount therefore of carbon dioxide 

 expired ; rum, ale, and porter, also sherry, have very similar effects. On the 

 other hand, brandy, whiskey, and gin, particularly the latter, almost always 

 lessened the respiratory changes, and consequently the amount of the gas 

 exhaled. 



i. Exercise. Bodily exercise, in moderation, increases the quantity to about 

 i more than it is during rest : and for about an hour after exercise the volume 

 of the air expired in the minute is increased nearly 2,000 ccm., or 118 cubic 

 inches : and the quantity of carbon dioxide about 125 ccm., or 7.8 cubic inches 

 per minute. Violent exercise, such as full labor on the tread- wheel, still fur- 

 ther increases the amount of the acid exhaled. 



A larger quantity is exhaled when the barometer is low than when it is 

 high. 



3. The oxygen is diminished. Pettenkofer and Voit have found that 

 the mean consumption of oxygen during 24 hours, by a man weighing 

 70 kilos, is about 700 grins., or 490 litres. The quantity of oxygen ab- 

 sorbed increases with muscular exercise, and falls during rest. In gen- 

 eral terms the quantity absorbed varies with the activity of the metabolic 

 processes. 



4. The volume of air is diminished (allowance being made for the ex- 

 pansion in heating), the loss being due to the fact that a portion of the 

 oxygen absorbed is not returned in the form of carbon dioxide. Since 

 the oxygen of a given volume of carbon dioxide would have the same 

 volume as the carbon dioxide itself at a given temperature and pressure, 

 a portion of the oxygen absorbed must be used for other purposes than 

 the formation of carbon dioxide. In fact, some of it is used in the 

 formation of urea, some in the formation of water, etc. The oxygen in 



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