CHAPTER XLII 



THE CONTROL OF RESPIRATION (Cont'd) 

 THE ALVEOLAR C0 2 -TENSION. ANOXEMIA. 



The Constancy of the Alveolar C0,-Tension Under Normal Conditions 



Since a close relationship exists between the alveolar C0 2 -tension and 

 the respiratory activity, it is to be expected that the two would bear a 

 strict proportionality to each other, and since the breathing under normal 

 conditions does not vary much, the C0 2 -tension should also be constant. 

 Many observations show this to be the case. The tension is remarkably 

 constant from day to day and even from month to month in the same 

 individual, provided the physiological conditions, are the same. A slight 

 seasonal variation is said to exist, a rise in the temperature of the en- 

 vironment of the individual causing a slight depression in the C0 2 -ten- 

 sion, while a fall in temperature causes a slight rise (Haldane). These 

 changes are independent of any demonstrable change in rectal temper- 

 ature and, therefore, are probably due to the influence of the temperature 

 on the skin. 



Since it is the number of molecules of C0 2 in a given volume of alve- 

 olar air (i.e., the partial pressure or tension) that is of importance, it 

 is only when the barometric pressure is the same that the percentage of 

 C0 2 in the sample of alveolar air can be constant. To allow for this, 

 all results are reduced to standard barometric pressure (760 mm. Hg). 

 If the barometric pressure is lowered, there will have to be a higher 

 percentage of C0 2 in the sample in order that there may be the same 

 tension of this gas in the air of the alveoli ; and vice versa when the bar- 

 ometric pressure is raised. The equation by which this tension, ex- 

 pressed in millimeters of mercury, is determined is: 100:760::a:p, where 

 a is the percentage actually found in the air of the sampling tube and p 

 the tension. A correction must also be introduced in this equation to 

 allow for the vapor tension of the air in the alveoli, for of course H 2 

 molecules will behave like C0 2 molecules in causing a partial pressure. 



When reduced to this standard, it has been found that the tension of 

 CO 2 in the alveolar air tends to remain constant under the varying baro- 

 metric conditions that obtain at sea level or at moderate departures from 

 this. This is shown in the following table : 



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