1120 PHYSIOLOGY 



gases is thus very slow, and it is difficult to be certain at any time that 

 the blood and the gas with which it is in contact are really in equilibrium. 

 Krogh therefore adopted an ingenious device of limiting the volume of air 

 to a small bubble, the superficial area of which is large in proportion to 

 its bulk. This bubble, after it has been in a stream of blood for some minutes, 

 is transferred to a special capillary tube in which its analysis can be carried 

 out with a fair degree of accuracy. 



The performance of a tonometer may be expressed by the ratio of the surface of 

 blood exposed to the volume of the air used. The * specific surface ' of an aerotonometer 



is represented by , The specific surface of Pfluger's instrument is only 



volume in c.c. 



3*3 and of Bohr's only 5'2. In Krogh's microtonometer the absolute volume of air 

 employed is reduced to a bubble of about 2 mm. in diameter, having a volume of -004 c.c. 

 and a surface of 0*125 sq. cm., so that its specific surface is 30. In such a bubble the 

 equalisation of the tensions takes place with extreme rapidity and only a minute 

 quantity of fluid is necessary. The microtonometer consists of the tonometer proper 

 and the apparatus for the micro-analysis of the gas bubble. In the latter the measure- 

 ment of the gas bubble is carried out in a capillary tube, the absorption of carbon dioxide 

 and of oxygen being effected in the usual way with potash and with pyrogalh'c acid. 

 The tonometer is represented in Fig. 515. It is kept in a small water-bath at the tem- 

 perature of the blood to be examined. The tonometer is filled with saline solution and 

 contains the gas bubble 2, which can be drawn up by means of the screw 4 into the 

 narrow graduated tube 3, where its volume is measured. The blood from the artery or 

 vein, in which we wish to examine the tension of the gases, passes by a cannula through 

 the tube 1, and enters the tonometer as a fine jet. It forces its way up above the gas 

 bubble, which is pressed a little down by the current, and kept oscillating with great 

 rapidity. From the tonometer the blood flows back 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. 1 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 sur- 

 rounding blood, and it is necessary then only 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. 



In these 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 Figs. 516 



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



