THE GASES OF THE BLOOD 



257 



kept in contact with the gas for a quarter of an hour or longer, so as 

 to insure equilibrium. Finally, in Krogh's mi crotonomeLer the gas space 

 is reduced to the smallest possible dimensions, being composed merely 

 of an air-bubble 2 mm. in diameter, which is exposed to the contact of 

 a stream of blood from an artery or vein. Equilibrium is established 

 so quickly that it is indifferent whether a bubble of air or of pure 

 nitrogen is employed. The bubble is analyzed at the end of the obser- 

 vation, and its composition gives the tension of the blood gases. 



Suppose that the gaseous mixture which is in equilibrium with the 

 blood contains 10 per cent, of oxygen and 5 per cent, of carbon dioxide, 



Fig. 122. Krogh's Microto- 

 nometer. The apparatus, 

 which is filled with salt solu- 

 tion, consists of a graduated 

 capillary tube, 3, the lower 

 part of which, with its ex- 

 panded lower end, is shown 

 on a much enlarged scale in 

 A. The rest of the capillary 

 tube, surrounded by a water- 

 jacket to control the tempera- 

 ture, is shown on a smaller 

 scale in B. 2 is a gas-bubble, 

 against which blood flowing 

 from the very narrow mouth 

 of the tube i plays, i is con- 

 nected by a rubber tube with 

 a cannula in a bloodvessel. 

 The blood forces its way up 

 above the gas-bubble, which 

 is pressed a little down by the 

 current, and kept oscillating 

 rapidly. The blood flows off 

 through the tube 5, and is 

 collected drop by drop and 

 measured. By means of the 

 screw 4, shown in B, which 

 moves in mercury, the gas- 

 bubble can be drawn into the 

 capillary for measurement. 

 The upper end of the capil- 

 lary tube also expands into a 

 funnel-shaped cavity, which 

 is closed by a stopper, and 

 is only used for cleaning the 

 apparatus. 



the tension of oxygen in the blood would be one-tenth of an atmosphere 

 (i.e., of 760 mm. of mercury), or 76 mm., and the tension of the carbon 

 dioxide in the blood one -twentieth of an atmosphere, or 38 mm. of 

 mercury. 



Another method by which the tension of the gases in the venous 

 blood passing from the right heart through the lungs has been estimated 

 depends upon the use of the pulmonary catheter. This consists of two 

 tubes, one within the other. The inner tube, which is a fine elastic 

 catheter, projects free from the other for a little distance at its lower 

 end. The outer tube terminates in a thin india-rubber balloon, through 

 which the inner tube passes without communicating with the balloon. 



