CHANGES IN AIR AND BLOOD IN RESPIRATION. 



623 



the stopcock b and to trickle down the sides of the tube. Diffusion 

 relations are set up between the blood and the known atmosphere, 

 and after equilibrium has been established the gas is driven out 

 through a into a convenient receiver and analyzed. If two aero- 

 tonometers are used, one containing the gas at somewhat higher 

 pressure than that expected, and the other at a somewhat lower 

 pressure, an average result is ob- 

 tained which expresses with suffi- 

 cient accuracy the pressure of the 

 given gas in the blood. 



It is important not to confuse 

 the tension at which a gas is held 

 in a liquid with the volume of the 

 gas. Thus, blood exposed to the 

 air contains its oxygen under a 

 tension of 152 mms. Hg, but the 

 amount of oxygen is equal to 20 

 volumes per cent. Water exposed 

 to the air contains its oxygen un- 

 der the same tension, but the 

 amount of gas in solution is less 

 than 1 volume per cent. Tensions 

 of gases in liquids are expressed 

 either in percentages of an atmos- 

 phere or in millimeters of mercury. 

 Thus, the tension of oxygen in ar- 

 terial blood is found to be equal 

 to about 10 per cent, of an atmos- 

 phere or 76 mms. Hg. (760 X 0.10). 



The Condition and Signifi- 

 cance of the Nitrogen. We may 

 accept the view that the nitrogen 

 of the blood is held in physical 

 solution. The amount present 

 corresponds with this view, and, 

 moreover, it is found that the 

 quantity varies directly with the 

 pressure in accordance with the 

 law given above. If an animal 



is permitted to breathe an atmosphere of oxygen and hydrogen 

 the nitrogen disappears from the blood, and when ordinary air is 

 breathed the nitrogen contents of the arterial and venous bloods 

 exhibit no constant difference in quantity. It seems certain, there- 

 fore, that the nitrogen plays no direct role in the physiological pro- 

 It is absorbed by the blood in proportion to its partial 



Fig. 253. Diagram to show the 

 principle of the aerotonometer: A, The 

 tube containing a known mixture of 



for maintaining a constant body tem- 

 perature. When stopcock 6 is open 

 the blood trickles down the sides of A 

 and enters into diffusion relations with 

 the contained gases. After equilibrium 

 is reached the stopcock b is closed and 

 a is opened. By means of the mer- 

 cury bulb the gases can then be forced 

 out of A into a suitable receiver for 

 analysis. 



