316 VENTILATION 



vesicles, or air cells, of the lungs, is but slightly altered in 

 composition by each inflow of tidal air. Considering these 

 facts, authorities agree that the amount of carbon dioxide in the 

 residual air can not be less than 6 per cent. Now this residual 

 air in the lung cells, or vesicles, is the air which receives the 

 carbon dioxide from the blood and gives up oxygen to the 

 blood. In a very true sense, it is almost wholly this residual 

 air upon which the efficiency of our respiration depends. 



From the preceding facts we see that the lungs contain con- 

 stantly over 200 cu. in. of air (100 cu. in. being reserve air 

 and 100 cu. in. being residual air) containing, at least, some 

 5 per cent, of carbon dioxide. It is evident, therefore, that it 

 matters little whether the 20 to 30 cu. in of tidal air contains 0.04 

 per cent, or 0.06 per cent, or even 4 per cent, of carbon dioxide. 



345. Same Reasoning Applied to the Other Constituents of 

 the Air. Fresh air is about 21 per cent, oxygen. Exhaled 

 air is known to be about 16 per cent, oxygen. Evidently the 

 oxygen in the residual air in the lungs can not be more than 

 about 15 per cent, oxygen. Now, experimenters have shown 

 that a person feels no discomfort when breathing air containing 

 no more than 15 per cent, oxygen. We are, therefore, forced 

 to the conclusion that it is not the effect of breathing air with an 

 increased percentage of carbon dioxide, nor with a decreased 

 percentage of oxygen, nor of breathing air containing u crowd 

 poison'' which constitutes the chief cause of the evil effects of 

 living in an atmosphere of vitiated air. Investigators have 

 come to believe that other factors have quite as much bearing 

 upon the problems of ventilation. 



346. Relation of Humidity of Ventilation. We have already 

 seen that the rate of evaporation from any surface depends 

 upon temperature and the degree of saturation of the air 

 surrounding that surface (Art. 247). We also know that 

 when evaporation takes place much heat is absorbed. In fact, 

 it requires 1037 British thermal units of heat to evaporate 1 Ib. 

 of water at the temperature of human blood, 98,4F. (com- 

 pare with Arts. 150 and 244). 



