100 



RESPIRATION 



The very powerful steadying influence on the CO 2 pressure of 

 the capacity of the tissues for taking up CO 2 is evident from these 

 figures. In consequence of this influence, and in a much less degree 

 that of the reserve of air in the lungs, variations of short duration 

 in the alveolar CO 2 pressure hardly count, although even the 

 slightest variations of a more prolonged character count a great 

 deal. 



2 4 



T/ne in Minutes 



Figure 33. 



Approximate variations in COa pressure of arterial 

 and venous blood during and after holding the breath 

 for 130 seconds with oxygen. 



On examining Figure 32 it will be seen that, although the 

 venous CO 2 pressure is below that of the alveolar air during most 

 of the apnoea, CO 2 is being given off all the time into the alveolar 

 air. This is due to the effect of oxygenation in decreasing the 

 capacity for CO 2 and thus raising its pressure in the blood. This 

 effect is explained by the fact that the thick line of Figure 26 will 

 be inclined to the left, as very little CO 2 is being given off by the 

 tissues, impoverished as they are of CO 2 by the forced breathing. 



In order to realize how important the steadying influence just 

 mentioned is, we have only to turn to what happens when want 

 of oxygen, instead of CO 2 , is exciting the center. Oxygen is no 



