106 RESPIRATION 



normally through a tube provided with valves so arranged 

 that the exhaled and inhaled air are kept separate. At 

 the termination of each expiration the last fraction of air 

 expired is collected in a side-tube. This method is said 

 to give better results than Haldane's when strenuous 

 exercise is being performed. 



The Tension of Oxygen in the Blood 



This can be determined in animals with a high degree 

 of accuracy by means of Krogh's Microtonometer. 



This is shown in Fig. 23. 



The blood enters from the proximal end of the cut artery 

 by the inner tube 1 (Fig. 23 a) and returns to the circulation 

 by the tube 7. The stream issuing from 1 plays upon a 

 small gas bubble 2 in such a manner as to agitate it violently. 

 From time to time the bubble is drawn up into the graduated 

 capillary tube by means of the screw tap 4 and its length 

 measured. When this no longer changes, it shows that 

 blood and bubble are in equihbrium. The bubble is now 

 analysed in the apparatus by exposure in turn to caustic 

 soda and pyrogalHc acid. 



The advantages of this apparatus he in the relatively 

 slight disturbance of the blood, in the small amount of air 

 used — a bubble of only 2 mm. diameter^and in the rela- 

 tively large surface exposed to the blood. Its disadvan- 

 tages are that it is inapphcable to man, and that even in 

 animals it cannot be used to determine the effect of exercise 

 and other natural conditions. A further disadvantage is 

 that in animals one cannot determine the composition of 

 the alveolar air. Krogh had to be content with taking a 

 sample of air from the bifurcation of the trachea. 



Experiments conducted with this apparatus, so far as 

 they go, show that the pressure of oxygen is always higher 

 in the lungs than in the arterial blood — thus supporting 

 the diffusion theory. 



