RESPIRATION 37 



with the general expansion of the lungs. In addition, they are more 

 completely washed out by fresh air during inspiration. It also fol- 

 lows that the "effective or virtual dead space" is neither a definite 

 anatomical space nor a fixed dead space in any sense, but a value 

 dependent on several variable factors. These factors include the 

 rates at which CO 2 passes outwards and oxygen passes inwards 

 between the air and blood at different points in the alveolar sys- 

 tem. For this reason the "effective dead space" is different for 

 oxygen and CO 2 . The over-ventilation of the atria, etc., removes 

 from the blood circulating round them an extra proportion of 

 carbon dioxide, but cannot, for a reason which will be discussed 

 later, give to the blood any appreciable extra amount of oxygen. 

 During inspiration this extra proportion of CO 2 passes on to the 

 saccular alveoli, but not during expiration. The "respiratory 

 quotient," or ratio between the volume of carbon dioxide given 

 off and of oxygen absorbed, is thus abnormally high in the air 

 expired from the atria, etc., and as a consequence abnormally 

 low in the air sacs, so that the "effective dead space," as calculated 

 from deficiency of oxygen in the expired air, compared with that 

 in the "alveolar air," is greater than when the dead space is calcu- 

 lated from the relative CO 2 percentages. The respiratory quotient 

 for the "alveolar air" is also below the correct value as calculated 

 from the composition of the mixed expired air. 



The following table, giving results on myself, shows the varia- 

 tions in the "effective dead space" with varying depth of breathing 

 as calculated both from CO 2 and from oxygen, and also the differ- 

 ences between the respiratory quotient as calculated from the 

 expired air and from the alveolar air. Using a slightly different 

 method, Henderson, Chillingworth, and Whitney got similar re- 

 sults. 



It will be seen from this table how enormously the apparent 

 dead space varies with the depth of breathing and how much 

 greater the dead space calculated from the oxygen is than that 

 calculated from the CO 2 . A further point which comes out is that 

 with deep breathing the difference between the alveolar CO 2 

 percentages at the beginning and end of expiration is far less than 

 the difference between the oxygen percentages. This is mainly 

 because the extra CO 2 washed out of the alveolar ductules and 

 atria passes on into the saccular alveoli during inspiration. A 

 further point is that the true respiratory quotient is about a sixth 

 higher than the alveolar respiratory quotient. The fact that the 

 alveolar respiratory quotient is a good deal lower than the true 



