CHAPTER XVI. 



THE DISSOCIATION CURVE FOR OXYGEN AND THE CO 2 - 

 COMBINING POWER OF BLOOD. 



For accurate determination of the relative amounts of reduced 

 and oxvhemoglobin in blood exposed to atmospheres containing 

 varying partial pressures of oxygen no method surpasses that of 

 Barcroft and his coworkers. The principle of this method is to 

 expose a small quantity of blood in a thin film on the walls of a 

 relatively large cylindrical vessel (tonometer) containing a mixture 

 of nitrogen and oxygen gases until equilibrium has become estab- 

 lished between the partial pressure of the oxygen in the atmosphere 

 and the absorption of oxygen by the blood. Some of the blood is 

 transferred to a small bottle connected with a differential mano- 

 meter and shaken with dilute ammonia water, whereby it becomes 

 laked, and by taking up oxygen, causes shrinkage in the volume of 

 air in the bottle, the degree of which is indicated by the manometer. 

 The oxygen-saturated blood is then shaken with ferricyanide of 

 potassium which dislodges the oxygen and causes the pressure in 

 the bottle to rise. From the relative displacement of the fluid in 

 the manometer in the two observations, the percentage saturation 

 of the blood with oxygen is readily calculated. 



For use by a class of students we have found it more expedient 

 to expose the blood to a partial vacuum instead of a mixture of 

 gases, the partial pressure of oxygen being readily calculated 

 from the degree to which the tonometer is evacuated as measured 

 by a barometer. This simplifies the technique by making it un- 

 necessary to analyse the gas in the manometer. For very accurate 

 work, however, mixtures of Q% and N are preferable. After exposure 

 to the partial vacuum, the blood is transferred to a differential 

 blood gas manometer. 

 Experiment 53. The following apparatus is required: 



THE TONOMETER consists of a wide glass tube (the tonometer T, Fig. 36) 

 of fairly stout glass, tapering down to narrow tubes at both ends. The capacity 



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