THE RESPIRATORY SYSTEM. 



253 



be measured. The hsemoglobin then takes up an equivalent amount 

 of oxygen from the reagents, being converted into methsemoglobin. 

 The probable course of the reaction is represented by the equation 



Hb0 2 + 4K 3 Fe(C N) 



Oxjrhiemoglobin 



4KOH = Hb0 2 + 4K 4 Fe(C N) 6 + 2H 2 + 2 . 



Methremoglobin 



Barcroft's differential apparatus consists of a manometer, of which the bore 

 is 1 mm., and which is provided with a scale graduated in millimetres. 

 Attached to each limb is a small bottle, which is detachable, and by means of 

 taps each bottle can be connected with or shut off from the outer air. The 

 manometer is partially filled with clove oil. 



(1) To determine the oxygen capacity of a sample of blood, 2 c.c. of dilute 

 ammonia and 1 c.c. of the blood to 



be examined are placed in each 



bottle, and the bottles are shaken so 



that the blood is thoroughly laked 



and saturated with oxygen. The 



stoppers are carefully greased, and 



O2 c.c. of a saturated solution of 



potassium ferricyanide is placed in 



the reservoir in the stopper of one 



bottle A. The bottles are then 



attached to the manometer and 



placed in a water bath, the taps 



being open. When the reading of 



the manometer becomes constant, 



the taps are closed, thereby exclud- 



ing the bottles and the manometer 



from the outer air, and the apparatus 



is tilted so that the ferricyanide runs 



into the blood in A, which gives off 



its oxygen. The level of the clove 



oil falls in the limb attached to the 



bottle A and rises in the opposite 



limb, and the bottles are replaced in 



the water bath until the readings 



become constant. If the difference 



of level on the two sides is 60 mm., 



this difference, multiplied by the 



constant of the apparatus (which may be taken as 3*0), represents the amount 



of oxygen given off by the blood; thus 60 mm. x 3'0 = 180 c.mm. oxygen. 



Since 1 c.c. of blood gives oft' 0'18 c.c. of oxygen, 100 c.c. of blood will give off 



18'0 c.c. oxygen, and this is its oxygen capacity. We have taken 3*0 as the 



constant of the apparatus, but it must be remembered that each apparatus has 



its own constant, which may be slightly greater or less than 3 - 0. 



(2) To determine the amount of oxygen present in a given sample of blood, 

 2 c.c. of dilute ammonia are. placed in each bottle; 1 c.c. of the blood to be 

 examined is carefully placed under the ammonia at the bottom of one bottle A, 

 and thus kept from contact with the air. 1 c.c. of fully oxygenated (saturated) 

 blood is placed in the other bottle B. The stoppers are greased, the bottles 

 are attached to the manometer and placed in a water bath, the taps being open. 

 When the reading of the manometer becomes constant, the taps are closed and 

 the apparatus is shaken in order to lake the blood in each bottle. The blood 

 in bottle B, being already fully saturated, takes up no more oxygen when 

 brought into contact with the air in the bottle. If the blood in the bottle A 

 is already fully saturated with oxygen, it takes up no mpre oxygen, and the 



FIG. 96. Barcroft's blood-gas apparatus. 

 (From Barcroft, Respiratory Function 

 of the Blood.} 



