152 ELEMENTARY PHYSIOLOGY less. 



no exercise, in the course of twenty-four hours, and are 

 charged with carbonic acid, and deprived of oxygen, to 

 the extent of about four per cent. This amounts to about 

 450 litres (16 cubic feet) of the one gas taken in, 

 and of tlie other given out. Thus, if a man be shut uj) in 

 a close room, having the form of a cube seven feet in the 

 side, every particle of air in that room will have passed 

 through his lungs in twenty-four hours, and a fifth of the 

 oxygen it contained will be replaced by carbonic acid. 



The quantity of carbon eliminated in the twenty-four 

 hours is pretty nearly re{)resented by a piece of pure char- 

 coal weighing 225 grammes (eight ounces). 



The quantity of water given oft" from the lungs in the 

 twenty-four hours varies very much, but may be taken on 

 the average as rather less than 250 c.c. (half a pint, or 

 about nine ounces). It may fall below tliis amount, or 

 increase to double or treble the (juantity. 



The air expired during the first half of an expiration 

 contains less carbonic acid than that expired during the 

 second half. Further, when the frequency of respiration 

 is increased without altering the volume of each in- 

 spiration, though the percentage of carbonic acid in each 

 expiration is diminished, it is not diminished in the 

 same ratio as that in which the number of inspirations 

 increases ; and hence more carbonic acid is got rid of 

 in a given time. 



Thus, if the number of inspirations per minute is in- 

 creased from fifteen to thirty, the percentage of carbonic 

 acid evolved in the second case remains more than half 

 of what it was in the first case, and hence the total 

 evolution is greater. 



This does not imply that there is a greater formation of 

 carbonic acid in the tissues, but only that the carbonic 

 acid in tlie blood passes more rapidly into tlie alveolar air 

 and is in turn replaced by that in the tissues. Thus the 



