1 50 RESPIRATION 



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 CO 2 in the alveoli is much 

 higher than in the upper respiratory passages, and a continual 

 curreht of it diffuses upward to equalize the pressure; this is 

 never accomplished, however, for reasons of similar nature to 

 those keeping up the constantly unequal pressure of O. 



These three factors respiratory and cardiac movements and 

 the natural diffusion of gases are, therefore, in continual opera- 

 tion to get O to and CO 2 away from the alveoli. Under their 

 influence the composition of the alveolar air remains fairly 

 uniform. 



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

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

 perature, (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 CO 2 . 



(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 is consumed, 

 the total amount necessary for an hour will be about 1,100 cu. in. 

 This allows, however, for no muscular, digestive or other ac- 

 tivity, and the amount actually necessary is larger than this. 



The circumstances which call for an increase in O almost in- 

 variably cause an increase in the output of CO 2 . 



(b) Gain of Carbon Dioxide. The amount of CO 2 in inspired 

 air is about .04 part per hundred (4/100 per cent.); the amount 



