296 Appendix I. On methods 



Determination of the oxygen in unsaturated blood with the 



differential apparatus. 



Apparatus as above, with the unsaturated blood in addition. 



Place 2 c.c. of ammonia in each bottle, put 1 c.c. of the unsaturated blood in the 

 left bottle and 1 c.c. of saturated blood in the right bottle. The blood must in each 

 case be run from the pipette gently to the bottom of the bottle so that it lies in 

 a layer underneath the ammonia and is protected by it from oxidation by the air. 



Grease the stoppers, place the bottles on the stoppers, place potassium ferri- 

 cyanide in the left reservoir. With the tap open place the apparatus in the water 

 bath ; after five minutes read the meniscus. Suppose it to be 



Left Eight 



120 119-5 



Shut the taps, see that the meniscus does not move in the next two minutes. 

 Then shake thoroughly so as to lake both samples of blood. The level of the clove 

 oil will have changed, as the unsaturated blood will have taken up some oxygen. 

 This change may be disregarded, however, as this extra oxygen combined with the 

 haemoglobin will be turned out again by the ferricyanide producing a corresponding 

 change in the opposite direction. When the blood is completely laked turn the 

 bottle so as to let down the ferricyanide, shake as in the above example. Let the 



final reading be 



Left Eight p 



110 129-5 19-5+ -5 = 20 mm. 



The volume of gas as measured therefore is pxk: 20 x 3'03 = 60-6 cubic mm. or 

 0'06 c.c. This must be corrected in several ways before a correct result is obtained. 

 The considerations which must be borne in mind are as follows: (1) temperature, 

 (2) pressure, (3) the calibration of the pipette, (4) the fact that the plasma is not 

 saturated with oxygen, (5) the temperature at which the blood was in equilibrium 

 with oxygen and nitrogen. Taking the first three together and using the values 

 given in the determination of the total oxygen capacity the corrected reading would 

 be 0'059 c.c. 



As unsaturated blood is usually taken directly from the body or from a tonometer 

 at body temperature we will suppose that it lias been exposed to gases at 37 C. 



In the lung the partial pressure of nitrogen is about 560 mm., in the air in the 

 blood gas bottle about 590. The question then is how much nitrogen will blood 

 which has been exposed to 560 mm. nitrogen at 37 C. take up when exposed to 

 590 mm. at say 15 C. 



Exposed to 760 mm. N 2 pressure at 15 C. blood take&up 0-016 c.c. 



760 ,, ,, 37 C. ,, 0-011 c.c. 



590 ,, 15 C. 0-012 c.c. 



therefore ,, 560 ,, ,, 37 C. ,, 0-008 c.c. 



The blood in the apparatus will therefore take up nitrogen from the air to the 

 extent of 0'012-0'OOS c.c. = 0'004 c.c. This must decrease the amount of oxygen 

 which appears to have been given out by this amount, therefore 0'004 c.c. must be 

 added to the answer given above, namely 0'059 c.c. ; it therefore becomes 0'063 c.c. 

 Blood which is unsaturated at room temperature may be regarded as having no 



