CHANGES IN AIR AND BLOOD IN RESPIRATION. 601 



The Tension or Pressure of Gases in Solution or Combi- 

 nation. When a gas is held in solution the equilibrium is de- 

 stroyed if the pressure of this gas in the surrounding medium or 

 atmosphere is changed. If this pressure is increased the liquid 

 takes up more of the gas, and an equilibrium is established at a 

 higher level. If the pressure is decreased the liquid gives off 

 some of the gas. That pressure of the gas in the surrounding 

 atmosphere at which equilibrium is established measures the tension 

 of the gas in the liquid at that time. Thus, when a bowl of water is 

 exposed to the air the tension of the oxygen in solution is 152 mms. 

 Hg; that of the nitrogen is 608 mms. Hg. If the same water is 

 exposed to pure oxygen the tension of the oxygen in solution is 

 equal to 760 mms. Hg, while that of the nitrogen sinks to zero 

 if the gas that is given off from the water is removed. With 

 compounds such as oxyhemoglobin the tension under which the 

 oxygen is held is measured by the pressure of the gas in the sur- 

 rounding atmosphere at which the compound neither takes up nor 

 gives off oxygen. If, therefore, it is necessary to determine the 

 tension of any gas held in solution or in dissociable combination it is 

 sufficient to determine the percentage of that gas in the surrounding 

 atmosphere and thus ascertain the partial pressure that it exerts. 

 If the atmosphere contains 5 per cent, of the gas the partial pressure 

 is equal to 38 mms. Hg (760 X 0.05), and this figure expresses 

 the tension under which the gas .is held in solution or combination. 

 As regards the tension of the gases in arterial and venous blood, 

 this procedure is, of course, not possible, since the blood is sur- 

 rounded, not by an atmosphere whose composition can be analyzed, 

 but by the liquids of the body, the lymph and cell juices. To 

 determine the tension of the gases in the blood it is necessary to 

 remove the blood from the vessels and bring it into contact with an 

 atmosphere containing a known quantity of O, CO 2 , or N, according 

 to the gas to be measured. By trial an atmosphere can be obtained 

 in which this gas is contained in amounts such that there is no 

 marked increase or decrease in quantity after standing in diffusion 

 relations with the blood. The percentage of the gas in the atmos- 

 phere chosen will measure the tension of that gas in the blood. An 

 instrument which has been much used for such determinations is 

 represented diagrammatically in Fig. 245. It is known as an aero- 

 tonometer (Pfliiger). It consists of a tube (A) which can be con- 

 nected through b 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 experiment. 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 



