CHAPTER LXXXIII 



THE PHYSIOLOGICAL PRINCIPLES OF VENTILATION 



The well-being of a conscious animal in relationship to its environment 

 constitutes the main problem of the study of ventilation. In the case of 

 animals leading an outdoor life, it is a problem of relatively little im- 

 portance, but for those like man which spend much of their time in 

 confined spaces, it is a problem of great importance, because it becomes 

 necessary to determine the limits within which the outside influences may 

 be altered without detriment to health or comfort. This problem is con- 

 sidered here because it is closely related to that of the body temperature. 



The Relationship Between the Chemical Composition of the Air and 

 the Well Being of the Body. When our knowledge of the function of 

 breathing became developed to the extent of showing that an animal 

 requires the oxygen of the air for the living processes of its body, and 

 as a result of these processes that it produces carbonic acid, which is 

 then added to the air, it was natural to suppose that the unfavorable ef- 

 fect of overcrowded confined spaces was due either to the using up of 

 the available oxygen or to a poisonous action of the carbonic acid. 



It needs only a few words to point out how utterly erroneous were 

 these earlier explanations. That deficiency of oxygen is no factor is in- 

 dicated by the facts, first, that this gas is seldom reduced by more than 

 one per cent, even in the most crowded places; and secondly, that people 

 live a normal existence at altitudes at which the oxygen percentage, meas- 

 ured at sea level, is reduced to less than two-thirds the normal. 



It is not altogether easy to understand why excess of C0 2 was thought 

 to be responsible for the evil effects of vitiated atmospheres. No doubt 

 the chief reason was that the percentage of this gas is often raised in 

 such atmospheres, but this is nothing more than coincidence, for on the 

 one hand most unsuitable conditions may exist when the percentage of 

 C0 2 is normal, and on the other, air loaded with almost a hundred times 

 the percentage found even in the most polluted atmosphere can be breathed 

 for indefinite periods of time without any unfavorable symptoms. 



As a matter of fact, even in the open, we are constantly taking into the pulmonary 

 alveoli large percentages of CO 2 , for obviously with each inspiration the first air to be 

 drawn in is that which remains over in the air passage from the preceding expiration. 

 This air contains somewhere about 5 per cent of CO,, and in quiet breathing it amounts 

 in volume to about one-third of all the air that is drawn in from the outside. This in 

 itself indicates that CO, per se can not be poisonous, and when we consider further the 



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