676 PHYSIOLOGY OF RESPIRATION. 



diagrammatically in Fig. 274. It is known as a tonometer or 

 aerotonometer (Pfliiger). It consists of a tube (A) which can be 

 connected through 6 directly with the blood-vessels. This tube A 

 is surrounded by a jacket (C) containing warm water, so that the 

 blood may be kept at the body temperature during the experi- 

 ment. A is first completely filled with mercury from the bulb M 

 to drive out the air. An atmosphere of known composition is 

 then sucked into A by dropping the bulb. Blood is allowed to 

 flow into A through 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 estab- 

 lished the gas is driven out through a into a convenient receiver 

 and analyzed. If two aerotonometers 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 obtained 

 which expresses with sufficient 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 in solution. 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 under 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 per- 

 centages of an atmosphere or in millimeters of mercury. Thus, the 

 tension of oxygen in arterial blood is found to be equal to about 

 13 per cent, of an atmosphere or 100 mms. Hg. (760 X 0.13). 



The Condition and Significance 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- 

 cesses. It is absorbed by the blood in proportion to its partial 

 pressure in the alveoli of the lungs and circulates in the blood in 

 small amounts without exerting any immediate influence upon the 

 tissues. 



Condition of Oxygen in the Blood. That the oxygen is not 

 held in the blood merely in solution is indicated, in the first place, 



* For description of more modern instruments, see Tigerstedt, "Handbuch 

 d. physiologischen Methodik," vol. ii., 1911. 



