CHAP, ii.] KESPIRATION. 441 



The quantity of nitrogen in the expired air is sometimes 

 found to be slightly greater than, as in the table above, but some- 

 times equal to, and sometimes less than, that of the inspired air. 



In a single breath the air is richer in carbonic acid (and 

 poorer in oxygen) at the end than at the beginning of the 

 breath. Hence the longer the breath is held, the greater the 

 (artificial) pause between inspiration and expiration, the higher 

 the percentage of carbonic acid in the expired air. Thus by 

 increasing the interval between two expirations to 100 seconds, 

 the percentage may be raised to 7*5. When the rate of breath- 

 ing remains the same, by increasing the depth of the breathing 

 the percentage of carbonic acid in each breath is lowered, but 

 the total quantity of carbonic acid expired in a given time is 

 increased. Similarly, when the depth of breath remains the 

 same, by quickening the rate the percentage of carbonic acid 

 in each breath is lowered, but the quantity expired in a given 

 time is increased. 



Taking, as we have done, the amount of tidal air passing in 

 and out of the chest of an average man at 500 c.c., such a 

 person will expire about 22 c.c. of carbonic acid at each breath ; 

 this, reckoning the rate of breathing at 17 a minute, would 

 give over 500 litres of carbonic acid for the day's production. 

 Actual determinations however give a rather smaller total than 

 this ; thus in a series of experiments of which we shall have to 

 speak hereafter, the total daily excretion of carbonic acid in an 

 average man was found to be 800 grms., i.e. rather more than 

 400 litres (406), containing 218-1 grms. carbon, and 581*9 

 grms. oxygen, the oxygen which actually disappeared from the 

 inspired air at the same time being about 700 grms. This 

 amount it should be said represents, owing to the manner in 

 which the experiment was conducted, the gases given out and 

 taken in, not by the lungs only, but by the whole body ; but 

 the amount of carbonic acid given out by other channels than 

 the lungs is, as we shall see, very slight (10 grms. ' or even 

 less), so that 800 grms. maybe taken as the average production 

 of carbonic acid by an average man. The quantity however, 

 both of oxygen consumed and of carbonic acid given out, is 

 subject to very wide variations; thus in the observations of 

 which we are speaking the daily quantity of carbonic acid 

 varied from 686 to 1285 grms., and that of the oxygen from 

 594 to 1072 grms. These variations and their causes will be 

 discussed when we come to deal with the problems of nutrition. 



271. When the total quantity of tidal air given out at 

 any expiration is compared with that taken in at the corre- 

 sponding inspiration, it is found that, both being- dried and 

 measured at the same temperature and pressure, the expired 

 air is less in volume than the inspired air, the difference 

 amounting to about ^th or -^th of the volume of the latter. 



