312 THE RESPIRATION 



sume a more inflated condition between breaths or, as it has been clum- 

 sily styled, a greater mid-capacity. These changes may serve as a 

 physiologic method for ^increasing the efficiency of alveolar ventilation 

 so as to meet the greater needs of the body. This is partly because the 

 pulmonary vessels become dilated and the bloodflow through the lungs 

 is favored, and partly because of the influence of the reserve and sup- 

 plemental airs on the tension of the arterial blood gases during the res- 

 piratory cycle. For example, if the lungs were completely depleted 

 of air during expiration, the blood leaving them at the end of this act 

 would be entirely venous. On the other hand, if the amount of air left 

 in the lungs at the end of expiration were above the normal amount, 

 each increment of C0 2 given off from the blood, or of 2 absorbed by 

 it would produce less change in the pressure of the C0 2 or 2 . 



The importance of these influences will be seen from the following 

 figures. If the residual and supplemental air amounts to 2000 c.c., and 

 the percentage of C0 2 in the alveolar air at the end of expiration is 

 5 per cent, then 100 c.c. of C0 2 must be present in the lungs. In a con- 

 dition of bodily rest about 20 c.c. of this gas is excreted during a res- 

 piratory cycle, so that if the breath were held during this period, the 

 percentage of C0 2 would rise from 5 to 6 per cent, and an inspiration of 

 400 c.c. would be required to bring the air in the lungs back to 5 per 

 cent of C0 2 . On the other hand, if the residual and supplemental air 

 amounted to 3000 c.c. with 5 per cent of C0 2 in the alveolar air at the 

 end of the expiration, there would be 150 c.c. of C0 2 in the lungs at 

 the end of the expiration, so that holding the breath for the time of the 

 respiratory cycle would raise the percentage of 00 2 only to 5.66 (pro- 

 vided the production of C0 2 was the same as in the first case), and an 

 inspiration of 600 c.c. would be necessary to reduce it to the normal 

 expiratory figure. Or, putting it another way, the production of CO., 

 can be increased 50 per cent in the time of a respiratory cycle without 

 affecting the tension of gases in the lungs, provided the residual and 

 supplemental air and the volume of the respiration are increased 50 

 per cent. If only one of the factors is changed, however, then the bal- 

 ance of the respiration must be disturbed, and the greater variation 

 in the tension of the gases in the arterial blood must occur at the dif- 

 ferent phases of the respiratory cycle. Bohr and Siebeck have shown 

 that the residual air is invariably increased in emphysema and that the 

 mid-capacity of the lungs is likewise increased; and it would appear 

 from Siebeck 's data that a similar condition must be present in cases of 

 decompensated heart. 



Patients suffering from dyspnea, particularly those suffering from 



