I5O RESPIRATION 



respiratory tract; but this is never done during life because 

 the partial pressure above is being continually increased by 

 the introduction of new O, and below is being continually 

 diminished by the removal of that gas from the alveoli by 

 the blood. 



In case of CO2 opposite conditions prevail. This gas is 

 being continually introduced into the alveolar air from the 

 blood, and consequently it is present there in much larger 

 quantities than in the trachea and bronchi, which contain 

 newly inspired air. The partial pressure, therefore, of CO2 

 in the alveoli is much higfrer than in the upper respiratory 

 passages, and a continual current of it diffuses upward to 

 equalize the pressure; this is never accomplished, however, 

 for reasons of similar nature to those keeping up the con- 

 stantly unequal pressure of O. 



These three factors respiratory and cardiac movements 

 and the natural diffusion of gases are, therefore, in con- 

 tinual operation to get O to and CO2 away from the alveoli. 

 Under their influence the composition of the alveolar air re- 

 mains fairly uniform. 



Alterations of Air in the Lungs. These are chiefly : (a) 

 Loss of oxygen, (b) gain of carbon dioxide, (c) elevation 

 of temperature, (d) gain of water, (e) gain of ammonia, (/) 

 gain of organic matter, (g) gain of nitrogen, (h) loss of 

 (actual) volume. The capital changes are loss of O and 

 gain of CO2. 



(a) Loss of Oxygen. The air in passing through the 

 lungs loses of O nearly 5 per cent, of its total volume. That 

 is, whereas on entering it contains 21 parts, on leaving it 

 contains only about 16 parts per hundred of this gas. Nearly 

 25 per cent, of the total volume of O inspired, therefore, is 

 lost in the lungs. 



When the respirations are 18 to the minute, and 20 cu. in. 

 of air are inspired at each breath, the amount inspired in an 

 hour will be 21,600 cu. in. Since a little more than one-fifth 

 of this air is O, and since only 'one-fourth of the inspired O 



