Effect of exercise on the dissociation curve of blood 241 



mind that the extra-capillary oxygen pressure is, in the case of muscle, 

 never high, being perhaps 20 mm. or less and at most 27 mm. 



In the capillaries we have then a most beautiful mechanism for 

 rapid unloading of the red corpuscles during their speedy transit 

 through the active tissues. 



It might at first sight appear that what is gained in the tissue is 

 lost in the lung. The decreased value of K tends to make each 

 corpuscle take up oxygen less quickly in the lung, other things being 

 equal ; but so far as the lung is concerned other things are not equal. 



FIG. 114. Showing relation of percentage saturation of Roberts' blood at 

 17 mm. oxygen pressure to acid added. 



For the meionectic condition of the blood stimulates the respiratory 

 centre and causes increased ventilation of the lung. We have seen 

 that there is a fall in the carbonic acid in the alveolar air. The 

 counterpart of this is that there is a rise in the pressure of oxygen in 

 the alveoli. The rate at which the oxygen is taken up, other things 

 again being equal, depends upon the pressure of oxygen. 



Apart from the fact that nature can more than compensate for 

 the diminished value of K by increasing the respirations, it must be 



B. R. P. 16 



