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A TEXTBOOK OF PHYSIOLOGY 



After using the pump, air is introduced up the barometer tube AB, 

 and the froth-chambers cut from the condenser X. The detached 

 part can then be readily cleaned, sterilized, and resealed on the con- 

 denser. 



The chemical method employed is one in which oxygen is displaced 

 from the blood by the use of potassium ferricyanide, and the CO 2 is 

 displaced by the action of tartaric acid. The most convenient form 

 of apparatus for this purpose is that shown in Fig. 135. The 

 oxygen is liberated from the haemoglobin in one of the two bottles. 

 The pressures in these become unequal, and the difference in pressure 

 is indicated by the movement of fluid (clove-oil) in the manometer. 

 The difference in the level of the fluid surface on either side is read, 

 and by this means the amount of oxygen liberated is determined, 



FIG. 135. BARCROFT'S DIFFERENTIAL BLOOD-GAS APPARATUS. 



since a special calibration of the instrument has been made, and the 

 amount of oxygen corresponding to a difference in level is known. 

 By such means it is found that arterial blood has a gas content some- 

 where about 18 c.c. O 2 , 44 c.c. C0 2 , 1 c.c. N 2 ; and venous 12 c.c. O,, 

 50 c.c. C0 2 , 1 c.c. N 2 . The blood from the veins varies widely in its 

 gas content according to the activity of the part from which it comes. 

 Simultaneous analyses of the blood going to and coming from a part 

 give, when the amount of blood circulating through the part is 

 known, a knowledge of the internal respiration of that part (see 

 later, p. 320). 



The gases in the blood are in part dissolved according to the 

 physical laws of absorption, but by far the larger amount is in so- 

 called weak chemical combination. Water dissolves gases accord- 

 ing to the pressure, temperature, and nature of the gas, and the solu- 

 bility is lessened by the presence of salts, etc., in solution in the water. 



