180 Chapter XI 



blood. Fig. 96 A was the result of this process. Now the symmetry 

 has returned. 



I was troubled by the apparent waste of time which these curves 

 revealed, the blood acquired and lost most of its oxygen in so 

 small a fraction of the whole time spent taking up or losing the 

 whole. The acquisition of the last portion of oxygen (that acquired 

 between 85 and 94 per cent, saturation) was such a slow affair, and 

 similarly the reduction from 55 per cent, to 44 per cent. 



Then I applied the principle that the mechanism of respiration 

 must allow of a margin for activity. The probable significance of 

 these horizontal portions of the curve is that they can so easily be 

 dispensed with. The first effect of increased oxygen consumption 

 of the organ would be to lower the oxygen pressure in the organ. 

 The sharp bend in the reduction curve would then be far below 

 the blackened part which goes down to 44 per cent. The tail of the 

 curve would therefore be eliminated from the "working" portion 

 which would become like that shown in Fig. 96 B. 



This drop in oxygen pressure would have secured a rearrangement 

 of the time relations which would go far to meet the requirements of 

 some smallish muscle were it thrown into activity. For blood would 

 traverse this active muscle at a greatly accelerated speed, it would 

 then be thrown into the general circulation which would scarcely be 

 quickened. The blood corpuscle would traverse the muscle quickly 

 and the lung slowly. The comparative time relations of the reaction 



Hb0 2 ^ Hb + O 2 



taking place under the existing circumstances appear in Fig. 96 B 

 and are admirably adapted to the velocity of the corpuscle in the 

 lung and the tissue respectively. 



But my speculation carried me a stage further. Suppose that 

 the exercise was so extensive as to cause great acceleration of the 

 general circulation as well as a fall in the oxygen pressure of the 

 active tissues. Suppose the available time for the acquisition of 

 oxygen by the corpuscle is cut down to a quarter of its former 

 value. It would become about 85 per cent, saturated, with a little 

 time to spare. Of course enormously more oxygen would be leaving 

 the lung under these circumstances. The blood then reaches the 

 tissue 85 per cent, oxidised, the tissue extorts its 50 per cent, 

 of oxygen, and therefore reduces the blood to 35 per cent, satura- 

 tion. This it can do in approximately the same time as that in 



