1194 



PHYSIOLOGY 



through the tube 7 and is collected in a vessel where it can be measured and 

 afterwards drawn off and reinjected into the 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 surrounding 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 

 gas in the blood. Clotting of the blood is prevented by the injection of 

 hirudin. 



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 



3O >tO iO 



to 20 ao <*> sa 



FIG. 501. 



Tensions of 2 and C0. 2 in alveoli compared with those in arterial 



blood of rabbit. 



The dotted lines represent the tensions in the alveolar air, the uninter- 

 rupted lines the tensions of the gases in the arterial blood. (KROGH.) 



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. 501. In 

 Fig. 502 A and B (Krogh) the composition of the alveolar air was 

 artificially altered by increasing the percentage of carbon dioxide 

 and of oxygen respectively. It will be seen in each case that 

 there was a corresponding alteration of the tension in the arterial 

 blood, the tension of carbon dioxide being higher and that of oxygen 

 lower in the blood than in the air throughout the experiment. We 

 have no direct determinations of the tensions of the gases in the blood 



