CHANGES OF AIR IN RESPIRATION. 173 



partly into the branches of the bronchial vein, and thence to 

 the right auricle, and partly into the small branches of the 

 pulmonary artery, or, more directly, into the pulmonary capil- 

 laries, whence, being with the rest of the blood arterialized, it 

 is carried to the pulmonary veins and left side of the heart. 



Changes of the Air in Respiration. 



By their contact in the lungs the composition of both air 

 and blood is changed. The alterations of the former being 

 manifest, simpler than those of the latter, and in some degree 

 illustrative of them, may be considered first. 



The atmosphere we breathe has, in every situation in which 

 it has been examined in its natural state, a nearly uniform 

 composition. It is a mixture of oxygen, nitrogen, carbonic 

 acid, and watery vapor, with, commonly, traces of other gases, 

 as ammonia, sulphuretted hydrogen, &c. Of every 100 vol- 

 umes of pure atmospheric air, 79 volumes (on an average) 

 consist of nitrogen, the remaining 21 of oxygen. The propor- 

 tion of carbonic acid is extremely small ; 10,000 volumes of 

 atmosperic air contain only about 4 or 5 of carbonic acid. 



The quantity of watery vapor varies greatly, according to 

 the temperature and other circumstances, but the atmosphere 

 is never without some. In this country, the average quantity 

 of watery vapor in the atmosphere is 1.40 per cent. 



The changes produced by respiration on the atmospheric air 

 are, that, 1, it is warmed ; 2, its carbonic acid is increased ; 

 3, its oxygen is diminished ; 4, its watery vapory is increased ; 

 5, a minute amount of organic matter and of free ammonia is 

 added to it. 



1. The expired air, heated by its contact with the interior 

 of the lungs, is (at least in most climates) hotter than the in- 

 spired air. Its temperature varies between 97 and 99^, the 

 lower temperature being observed 'when the air has remained 

 but a short time in the lungs, rather than when it is inhaled 

 at a very low temperature ; for whatever the temperature when 

 inhaled may be, the air nearly acquires that of the blood be- 

 fore it is expelled from the chest. 



2. The carbonic acid in respired air is always increased; but 

 the quantity exhaled in a given time is subject to change from 

 various circumstances. 'From every volume of air inspired, 

 about 4? per cent, of oxygen are abstracted ; while a rather 

 smaller quantity of carbonic acid is added in its place. It 

 may be stated, as a general average deduced from the results 

 of experiments by Valentin and Brunner, that, under ordi- 

 nary circumstances, the quantity of carbonic acid exhaled 



15 



