RESPIRATION 403 



To calculate the oxygen evolved from 100 cc. of blood, allowance must 

 be made for the fact that a 20 cc. pipette does not deliver 20 cc. of blood, 

 but only about 19.6 cc. The actual amount of shortage can easily be 

 determined by weighing. A further slight correction is needed on ac- 

 count of the fact that the air in the bottle at the end of the operation is 

 richer in oxygen than at the beginning, so that, as oxygen is about a 

 third more soluble than air, slightly more gas will be in solution. With a 

 bottle of 120 cc. capacity and 20 per cent of oxygen in the blood, the air 

 in the bottle will evidently contain about 26 per cent of oxygen, so that, 

 assuming that the coefficients of absorption of oxygen and nitrogen in 

 the 54 cc. of liquid in the bottle are nearly the same as in water, the 

 correction will amount at 15° to .03 cc. in the reading of the burette, if 

 the oxygen capacity is normal, or 0.75 per cent of the oxygen given off. 



In order to make quite sure that no oxyhaemoglobin remains in the 

 solution owing to a reshrinkage of corpuscles on adding the ferricyanide, 

 and consequent escape of some of the oxyhaemoglobin from the action 

 of the ferricyanide, the liquid in the bottle can afterwards be examined 

 as follows. Part of it is diluted with 0.8 per cent salt solution, shaken up 

 in a test tube with expired air so as to render the solution just acid, and 

 examined spectroscopically. Any trace of oxyhaemoglobin left in incom- 

 pletely laked corpuscles is shown by the presence of the characteristic 

 absorption bands. These are completely absent if only methaemoglobin 

 is present, as ought to be the case. If they are present the result will be 

 too low, and the experiment must be repeated with saponin added. 



If the blood is saturated with CO instead of oxygen the reaction is 

 slower, but gives precisely the same result. The correction for physically 

 dissolved gas is, however, scarcely appreciable, as CO is very little more 

 soluble than air. If the blood has begun to decompose, owing to bacterial 

 action, the result will of course be too low, and this can easily be detected, 

 because of the fact that each successive reading of the burette will be 

 lower, owing to the disappearance of oxygen.* There is no appreciable 

 error, owing to the tension of ammonia vapor in the air ; and the method 

 is one of extreme accuracy and certainty. Different determinations ought 

 not to differ by more than i /200th of the quantity measured. On com- 

 paring the results with those from the pump, after allowance in the case 

 of the pump for oxygen in simple solution in the blood, or adhering to 



* Under certain abnormal conditions even fresh mammalian blood, as Douglas 

 iJourn. of Physiol., p. 453, 1910) has shown, may in presence of the ferricyanide 

 absorb an appreciable amount of oxygen before a determination is complete : in 

 which case the quicker method described below is greatly preferable. An appreciable 

 absorption can also be detected in normal fresh human blood left for an hour or 

 two in the apparatus. 



