240 FUNCTIONS OF NUTRITION. 



prevent our arriving at an absolutely uniform standard. The average 

 result, derived from several of the most trustworthy experimenters, as 

 well as from our own observations, gives the amount of air taken into 

 and expelled from the lungs with each respiration as 320 cubic centi- 

 metres. This estimate is certainly not above the reality. If we take, 

 accordingly, eighteen respirations per minute as the mean rapidity 

 between the sleeping and waking hours, this would "amount to 5T60 

 cubic centimetres of inspired Jr per minute, 345,600 per hour, and 

 8,294,400 cubic centimetres, or 8294.4 litres per day. But as the 

 breathing is increased, both in rapidity and volume, by every muscu- 

 lar exertion, the daily quantity of air used in respiration is not less 

 than 10,000 litres, or 350 cubic feet. This is 140 times the bulk of 

 the entire body. 



Estimates of this kind are sometimes used to calculate the air-space 

 necessary for each inmate of a hospital or school-room. This alone, 

 however, can never be sufficient for the purpose. The successful 

 ventilation of a room depends not so much on the quantity of air 

 which it contains as on that introduced and expelled within a certain 

 period. The air of a small room, if properly renewed, may be amply 

 sufficient for respiration, while that of a large room, if it remain stag- 

 nant, will become unfit for use. A large air-space will render ventila- 

 tion more easy of accomplishment by ordinary methods, because the 

 air will not be so rapidly vitiated as if it were in smaller volume ; but 

 it must still be changed with a rapidity proportionate to its contamina- 

 tion, in order to maintain the apartment in a wholesome condition. 



Changes in the Air by Respiration. 



The atmospheric air is a mixture of oxygen and nitrogen in the pro- 

 portion, by volume, of about 21 parts of oxygen to 79 parts of nitro- 

 gen. It also contains .05 per cent, of carbonic acid, a varying quantity 

 of watery vapor, and some traces of ammonia. The last named ingre- 

 dients, so far as animal respiration is concerned, are insignificant in 

 comparison with the oxygen and nitrogen which form the principal 

 part of its mass. 



As discharged from the lungs in expiration, the air is found to 

 have become altered in the following particulars : first, it has lost 

 oxygen ; secondly, it has gained carbonic acid ; and thirdly, it has 

 absorbed the vapor of water. The most important of these changes 

 are its diminution in oxygen and its increase in carbonic acid. 



Diminution of Oxygen. According to Valentin, Yierordt, and Reg- 

 nault and Reiset, the air loses during respiration, in man, on an aver- 

 age, five per cent, of its volume of oxygen. At each inspiration, about 

 16 cubic centimetres of oxygen are removed from the air and ab- 

 sorbed by the blood ; and, as the daily quantity of air used in respira- 

 tion is about 10,000 litres, the oxygen consumed in twenty-four hours, 

 is not less than 500 litres, or seven times the bulk of the entire body. 



