1070 



PHYSIOLOGY 



animal if necessary. Since the total pressure of the gases in the blood is nearly always 

 negative, it is necessary to keep the pressure in the tonometer also negative.* This is 

 accomplished by means of a mercury valve and can be regulated to any desired pressure. 

 During the course of a tonometric experiment the volume of the gas bubble is 

 measured from time to time by drawing it up into the graduated tube, and the pressure 

 is regulated until the volume of the bubble remains constant. After five minutes 

 gaseous equilibrium will have been established between the gas bubble and the surround- 

 ing blood, and it is only necessary then to draw it up into the graduated tube and analyse 

 it in order to determine the tension of the gases in the blood. Clotting of the blood is 

 prevented by the injection of hirudin. 



B 



s i 



FIG. 502. A, Krogh's microtonometer. B, upper part of microtonometer showing 

 capillary tube into which the bubble is returned for measurement and analysis. 



In the experiments the tension of the air in the alveoli of the animal's 

 lungs or in the bifurcation of the trachea was determined by taking samples 

 of the air. The results of the experiments show that the tension of the 

 gases in arterial blood follows closely the tension of the corresponding gases 

 in the alveolar air. The tension of carbon dioxide in arterial blood is either 

 identical with or very slightly above the tension of the gas in the alveolar 

 air. The oxygen tension of the blood is always lower than the alveolar 

 oxygen tension, and the difference is generally 1 to 2 even 3 to 4 per cent, 

 of an atmosphere. The results of a series of determinations of the tensions 

 of the gases in the blood and alveolar air respectively are given in Fig. 503. 

 In Fig. 504 A and B (Krogh) the composition of the alveolar air was artificially 

 * Otherwise the whole bubble would gradually go into solution and disappear. 



