THE INDIVIDUAL GROUPS OF PROTEINS 509 



Determination of the Difference in the Oxygen Content be- 

 tween Arterial and Venous Blood. 



This determination is one of the most useful for studying the gaseous 

 metabolism of organs and is very simple, provided that the arterial blood is 

 saturated or nearly saturated with oxygen, as is usually the case. 



Samples of blood from the artery and the vein are drawn into i c.c. 

 pipettes containing a little hirudin solution to prevent coagulation. 



Ferricyanide is not required as the determination depends upon the 

 measurement of the difference between the amounts of oxygen absorbed by 

 the two bloods when they are shaken to saturation with air. 



The two bottles should have as nearly as possible the same constant, i.e. 

 be of equal volume. Since the bottles are rarely quite equal, equality is 

 obtained by placing 2 c.c. of ammonia in the smaller bottle and adding to 

 the other bottle as much ammonia as is necessary to leave an air space of the 

 same volume as in the smaller bottle. 1 



Beneath the ammonia in each bottle is put i c.c. of arterial blood in the 

 left-hand and i c.c. of venous blood in the right-hand. The bottles are fitted 

 on the stoppers without disturbing the blood, the temperature is equalised 

 and the taps are closed to the air. The blood is laked and both bottles are 

 equally saturated by shaking until a maximum difference of pressure is 

 observed. (The oil ascends on the side of the venous blood.) The difference 

 (d) multiplied by the constant (k] of the smaller bottle gives the difference in 

 oxygen per c.c. of blood in cmm. There is no correction for temperature and 

 pressure. 



Determination of the Percentage Saturation of Blood with 

 Oxygen. 



This determination is often required for the construction of dissociation 

 curves for blood. It may be carried out in two ways : 



(1) The oxygen content is determined and the oxygen capacity is deter- 

 mined by'adding ferricyanide to the saturated blood. 



If/! and/ 2 are respectively the two readings, the percentage saturation is 

 given by 



/! x TOO 



ft 



(2) The difference in oxygen content between the blood and saturated 

 blood is determined and then the total capacity is determined by adding ferri- 

 cyanide to one side. 



If the difference in pressure after complete saturation be /, and if it 

 be p' after the action of ferricyanide, p'-p is the pressure produced by the 

 oxygen originally present in the unsaturated blood if it alone were liberated. 

 The percentage saturation is 



i>' - i) 



*L -J- x 100. 



P 



The apparatus for -i c.c. of blood is used in an exactly similar way as the 

 larger apparatus but with '2 c.c. of ammonia solution, *i c.c. of blood in the 

 lower part and a drop of ferricyanide in the upper bulb. 



With the older form of apparatus (see p. 502) there are some slight differ- 

 ences in technique. The pressure screw must not be altered from its initial 

 position and it is better to substitute a short piece of glass rod for the pipette 

 and screw. Thus modified it is used in the same way as the later form of 

 apparatus. 



The volumes of the bottles may be determined from the formula : 

 V (cmm.) = 10,000 (K - area of manometer tubing in sq. mm.) where K 

 is the differential constant. The area of the manometer tubing is given by 



volume _ 'i 

 pressure /j 



1 The difference in the volumes (Vj - V 2 ) of the two bottles may be found from theii 

 constants K, and K 2) thus 



V, - V 2 = 10,000 (Kj - K 2 ) cmm. 



