242 RESPIRATION. 



air is found to have become altered in the following essential par- 

 ticulars, viz : 



1st. It has lost oxygen. 



2d. It has gained carbonic acid. And 



3d. It has absorbed the vapor of water. 



Beside the two latter substances, there are also exhaled with the 

 expired air a very small quantity of nitrogen, over and above what 

 was taken in with inspiration, and a little animal matter in a 

 gaseous form, which communicates a slight but peculiar odor to 

 the breath. The air is also somewhat elevated in temperature, by 

 contact with the pulmonary mucous membrane. 



The watery vapor, which is exhaled with the breath, is given off 

 by the pulmonary mucous membrane, by which it is absorbed from 

 the blood. At ordinary temperatures it is transparent and invisi- 

 ble ; but in cold weather it becomes partly condensed, on leaving 

 the lungs, and appears under the form of a cloudy vapor discharged 

 with the breath. According to the researches of Valentin, the 

 average quantity of water, exhaled daily from the lungs, is 8100 

 grains, or about l pounds avoirdupois. 



By far the most important part, however, of the changes suffered 

 by the air in respiration, consists in its loss of oxygen, and its 

 absorption of carbonic acid. 



According to the researches of Valentin, Vierordt, Regnault and 

 Reiset, &c., the air loses during respiration, on an average,five per 

 cent, of its volume of oxygen. At each inspiration, therefore, 

 about one cubic inch of oxygen is removed from the air and ab- 

 sorbed by the blood ; and as we have seen that the entire daily 

 quantity of air used in respiration is about 350 cubic feet, the entire 

 quantity of oxygen thus consumed in twenty-four hours is not less 

 than seventeen and a half cubic feet. This is, by weight, 7,134 

 grains, or a little over one pound avoirdupois. 



The oxygen which thus disappears from the inspired air is not 

 entirely replaced in the carbonic acid exhaled ; that is, there is less 

 oxygen in the carbonic acid which is returned to the air by expira- 

 tion than has been lost during inspiration. 



There is even more oxygen absorbed than is given off again in 

 both the carbonic acid and aqueous vapor together, which are 

 exhaled from the lungs. 1 There is, then, a constant disappearance 

 of oxygen from the air used in respiration, and a constant accumu- 

 lation of carbonic acid. 



1 Lehmann's Physiological Chemistry, Philada. ed., vol. ii. p. 432. 



